Symposium on Size Selected Clusters 2007

Contents

Talks 1Monday - Cluster Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Tuesday - Clusters and Biomolecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Wednesday - Clusters and Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29Thursday - Cluster Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Posters - Session A 51Clusters and Biomolecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65Metal Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Methods and Machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93Molecular Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107Phase Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113Rare Gas Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

Posters - Session B 123Carbon and Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125Clusters and Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135Deposited Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161Reacted Clusters and Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

Index 191List of Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193

Talks

Monday - Cluster Physics

TALKS MONDAY - CLUSTER PHYSICS

Time-resolved ionic caging dynamics

W. Carl Lineberger

JILA and Department of Chemistry and Biochemistry University of Colorado, Boulder, Colorado 80309

Ultrafast pump-probe studies of recombination in partially solvated, size-selected dihalide clusteranions show long time coherent motions and the resulting non-statistical energy flow in the cluster.For photodissociated I−2 (CO2)n, we observe new type of recombination: a solvent asymmetry-drivenenergy transfer process without a condensed phase counterpart. Very short recombination timesare observed (∼10 ps) with the chromophore only partially solvated, and the time required forrecombination steadily decreases with additional solvation. Theoretical models point to the centralrole of the solvent electric field in the recombination process, but suggest electron transfer processesthat cannot be tested with a homonuclear dihalide chromophore. To further test these concepts,we investigate the time-resolved recombination of photodissociated IBr−(CO2)n clusters followingexcitation to the dissociative IBr− A’ 2Π1/2 state of the chromophore. In complete contrast toprevious studies involving solvated I−2 , the observed recombination times for IBr−(CO2)n increasedramatically with increasing cluster size, from 12 ps for n = 5 to 900 ps for n= 8,10. The basis forthis dramatic difference gives increased credence to the utility of a "solvent coordinate" descriptionof geminate recombination.

5

MONDAY - CLUSTER PHYSICS TALKS

Cage Clusters of Gold and Tin: Golden Buckyballs andStannaspherene

Lai-Sheng Wang

Department of Physics, Washington State University, 2710 University Drive, Richland, WA 99354, USAand Chemical & Materials Science Division, Pacific Northwest National Laboratory, P.O. Box 999,

Richland, WA 99352, USA.

Photoelectron spectroscopy (PES) yields direct electronic structure information for size-selectedclusters. Combining PES with theoretical calculations has become an effective approach to obtainstructural information for small and medium-sized clusters. We present recent discoveries of twoclasses of cage clusters in gold and tin. Negatively charged gold clusters (Au−n ) have been shownto exhibit a remarkable structural diversity from 2D structures for n = 4-12 and the pyramidalstructure for n = 20. Using PES and DFT calculations, we have found that gold clusters with n =16 and 17 possess unprecedented hollow cage structures [1]. We have been able to successfully dopea variety of foreign atoms into the empty spaces in the golden cages, confirming their structuralrobustness, as well as demonstrating chemical tuning of their electronic, magnetic, and catalyticproperties [2].Unlike carbon, the heavier congeners of the group 14 elements are not known to form hollow cagestructures similar to the fullerenes. In a recent PES study of tin clusters [3], we noted that thespectrum of Sn−12 is distinctly different from that of its neighbors or its Si/Ge counterpart. Thisobservation led to our discovery of a highly symmetric and stable icosahedral Sn2−

12 cage [4], forwhich we coined a name "stannaspherene" to describe its high symmetry and spherical p bonding.We have also shown that all transition metals including the f-block elements can be doped insideSn2−

12 to form a whole class of endohedral stannaspherenes [5], which may be used as potentialbuilding blocks for new cluster-assembled materials.

(a) (b) (c) (d)

Figure 1: (a) Au−16 (b) Cu@Au−16 (c) Sn2−12 (d) M@Sn12

[1]"Evidence of Hollow Golden Cages" (S. Bulusu, X. Li, L. S. Wang, and X. C. Zeng), Proc. Natl. Acad. Sci. (USA)103, 8326-8330 (2006).

[2]"Doping the Golden Buckyballs: Cu@Au−16 and Cu@Au−17" (L. M. Wang, S. Bulusu, H. J. Zhai, X. C. Zeng, andL. S. Wang), Angew. Chem. Int. Ed., in press (2007).

[3]"Evolution of the Electronic Properties of Snn- Clusters (n = 4 - 45) and the Semiconductor-to-Metal Transition"(L. F. Cui, L. M. Wang, and L. S. Wang), J. Chem. Phys. 126, 064505-1-8 (2007).

[4]"Sn122-: Stannaspherene" (L. F. Cui, X. Huang, L. M. Wang, D. Y. Zubarev, A. I. Boldyrev, J. Li, and L. S.Wang), J. Am. Chem. Soc. 128, 8390-8391 (2006).

[5]"Endohedral Stannaspherenes (M@Sn−12): A Rich Class of Stable Molecular Cage Clusters" (L. F. Cui, X. Huang,L. M. Wang, J. Li, and L. S. Wang), Angew. Chem. Int. Ed. 46, 742-745 (2007).

6


Transport Properties through Molecular Clusters byFirst-principles Calculations

Hiroshi Mizuseki1, Rodion V. Belosludov1, Amir A. Farajian1,2, Tomoki Uehara1, andYoshiyuki Kawazoe1

1. Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan2. Center for Nanoscale Science and Technology, Department of Mechanical Engineering and Materials

Science, Rice University, Houston, USA

Molecular devices are potential candidates for this next step, and they would make it possible torealize the most advantageous devices. However, source of expenditure is necessary that such a largenumber of organic molecules can be obtained by synthetic chemistry, so any means of exploring theirproperties and behavior in order to predict the relevant properties of a molecule in advance of itssynthesis would be extremely useful.Our group has covered a wide range of molecular clusters [1] which have potential application inmolecular devices using first-principles calculations and nonequilibrium Green’s function formalism.There are supramolecular wires [2], porphyrin [3], [4] and ferrocene [5] molecules and so on. Inthis presentation, we will present our recent study on the transport properties of a ferrocene-basedmolecule wire using the nonequilibrium Green’s function formalism for quantum transport and thedensity functional theory (DFT) of electronic structures using local orbital basis sets. The ferrocenehas high degree of chemical and thermal stability in different environments and a wealth of syntheticmethods for the construction of a variety of relatively complex ferrocene-based systems. Molecularwires based on ferrocene molecules are compact and linear, which may allow one to incorporatesuch wires into bulky molecules to create a shielded molecular wire. Iron substitution by differentmetals will also affect electron transport through the cyclopentadienyl ring. First, the transportproperties of two ferrocenedithiolate systems with different five-member ring connections have beenestimated and the results of the calculations reveal that the iron atom enhances the conductivityof the ferrocene molecule compared with all-organic molecules. Moreover, the conductance throughthe ferrocene molecule depends on the position of sulfur atoms. The molecule has a higher electricalconductivity at low bias when the same cyclopentadienyl ring is connected to an gold electrode bysulfur atoms. The I-V characteristics show that, in this case the transport properties of the moleculehave metallic features. The transmission coefficients of ferrocenedithiolate molecules changed withapplied bias. This is attributable to the shift of energy levels and the change of molecular orbitalshape by the electric field. The several structures of molecular wire based on ferrocene moleculeshave been proposed and their transport properties have been also estimated and analyzed.

[1]http://www-lab.imr.edu/ mizuseki/nanowire.html[2]R. V. Belosludov, A. A. Farajian, H. Mizuseki, K. Ichinoseki, and Y. Kawazoe, Jpn. J. Appl. Phys., 43, 2061

(2004).[3]R. V. Belosludov, A. A. Farajian, H. Baba, H. Mizuseki, and Y. Kawazoe, Jpn. J. Appl. Phys., 44, 2823 (2005).[4]A. A. Farajian, R. V. Belosludov, H. Mizuseki, and Y. Kawazoe, Thin Solid Films, 499, 269 (2006).[5]T. Uehara, R. V. Belosludov, A. A. Farajian, H. Mizuseki, and Y. Kawazoe, Jpn. J. Appl. Phys., 45, 3768 (2006).

7


Density functional modeling of free and supported clusters:on the road to an understanding of nanoscale catalysis

Michael Moseler

Fraunhofer Institute for Mechanics of Materials IWM, Wöhlerstr. 11, 79108 Freiburg, GermanyFreiburg Materials Research Center, Stefan-Meier-Str. 21, 79104 Freiburg, Germany

Understanding and predicting heterogeneous catalysis remains one of the main motivations under-lying the science of gasphase and supported nano-cluster. Gasphase and surface science experimentsalready provide important contributions to our knowledge how nano-particles catalyse reactions [1].Often however, this progress is only achieved in combination with quantum-chemical atomistic sim-ulations. Here we show how density functional theory can be used to understand experimental sizeevolutionary patterns in the activity of gasphase and metal oxide supported Pd clusters [2], [3]. Weprovide theoretical as well as experimental evidence that the reaction of supported PdN with molec-ular oxygen results in the formation of nano-oxides which are in epitaxy with the ceramic support.These oxide serve as a Mars-van-Krevelen oxygen reservoir and therefore play an important role inthe catalyzed combustion of carbon monoxide. The calculated low-temperature pathways for theoxide-formation and the CO oxidation are in perfect agreement with the experimentally observedreaction conditions.The theoretical explanation of the observed reactivity of gold cluster anion towards adsorption ofmolecular oxygen [4] provides a second example for the successful application of density functionaltheory to elucidate catalytic mechanisms [5]. Experimentally an odd-even oscillation in the O2

take-up of Au−n clusters is observed (a pattern which can be easily explained by open spin shells).We found an explanation for anomalies in this odd-even pattern occurring e.g. for Au−16 . Thereactive cluster states belong to a partial jellium model of the gold 6s electrons. In this modelthe hexadecamer anion is close to a shell closing and therefore behaves halogen-like. Consequently,electron donation to the oxygen dimer is strongly reduced resulting in the inertness of the cluster.

Figure 1: Majority-spin Kohn-Sham energies of the Au−16 and Au−18 cages and the Au−20 tetrahedron.The length of the energy-level bars represents the added weight of the atomic s and p orbitals formingthe wavefunctions. Blue bars denote high s+p weights; red bars represent states that are mostlyformed by atomic d orbitals (the "d band"). The majority-spin Fermi energy is depicted in green.Isosurfaces of representative jelliumlike orbitals of Au−20 are displayed on the right-hand side alongwith a classification into jellium-type states. Also included is an isosurface of a d state (in greenand red) illustrating localization about the atomic centers that form the tetrahedral skeleton of theAu−20 cluster.

[1]U. Heiz, E. L. Bullock, Mater. Chem. 14, 564 (2004).[2]B. Huber, P. Koskinen, H. Häkkinen and M. Moseler, Nature Materials 5, 44 (2006)[3]B. Huber, H. Häkkinen, U. Landman and M. Moseler, Comp. Mat. Sci. 35, 371 (2006)[4]Y. D. Kim, M. Fischer, G. Ganteför, Chem. Phys. Lett. 377, 170 (2003)[5]B. Yoon, P. Koskinen, B.H uber, O. Kostko, B. von Issendorff, H. Häkkinen, M.M oseler and U. Landman,

Chem.Phys.Chem. 8, 157 (2007)

8


Angle-resolved photoelectron spectroscopy of cold Na clusteranions

Christof Bartels, Christian Hock, Jan Huwer and Bernd v.Issendorff

Fakultät für Mathematik und Physik, Universität Freiburg, Stefan-Meier-Straße 19, 79104 Freiburg,Germany

Angle and energy-resolved photoelectron spectroscopy has been performed on cold (∼20 K) size-selected Na cluster anions in a wide size range using ns laser pulses with wavelengths from UV toIR.For the smallest clusters (2, 3, 4, 5, 7 atoms), the occupied molecular orbitals can be described aslinear combinations of the atomic valence orbitals. The observed transitions can be assigned to thesemolecular orbitals, and the evolution of the emission patterns with laser wavelength is in qualitativeagreement with what one expects from the symmetry of the orbitals.Bigger Na clusters, which have been investigated earlier by energy-resolved photoelectron spec-troscopy [1], can be described in the framework of the jellium model. This model neglects the ionicbackground and treats the cluster’s valence electrons as free electrons in effective single-particlepotentials, leading to an electronic shell structure similar to the well-known atomic shells, assigningenergy and angular momentum quantum numbers to the most loosely bound electrons of the cluster.For single-photon excitation with linearly polarized light, the angular distribution of photoelectronscan be described by a single anisotropy parameter. This parameter has been calculated for Naclusters using simple Woods-Saxon potentials; it shows variations with wavelength, which are char-acteristic for the different angular momentum eigenstates. We have measured the evolution of thisasymmetry parameter in the wavelength range 290. . . 755 nm for selected cluster sizes (19, 21, 33,34, 55, 147). The experimental results will be presented and compared to our calculations.

(a) (b) (c) (d)

(e) (f) (g) (h)

Figure 1: Experimental images of photoelectrons (first row) and the corresponding electron velocitydistributions (second row) for Na−3 at wavelengths 308, 348, 460 and 650 nm (from left to right).The electron velocity distributions were reconstructed using the pBasex program [2].

[1]G. Wrigge, M. Astruc Hoffmann and B. v.Issendorff, Phys. Rev. A 65, 063201 (2002).[2]G. A. Garcia, L. Nahon and I. Powis, Rev. Sci. Instr. 75 (11), 4989 (2004).

9


Where does the giant response in spin-dependent transportmeasurements of magnetic clusters in metallic matrices come

from?

G. Di Domenicantonio, M. Hillenkamp, C. Félix

EPFL, Lausanne, Switzerland

We have observed giant responses of up to several hundred percent in spin-dependent transportmeasurements of samples with well-defined Cobalt clusters embedded in Copper and Silver matrices[1]-[3]. These magnetic field dependencies are notably pronounced for smallest clusters of a fewatoms only.An analysis in terms of spin disorder scattering reflects our observations on a qualitative level andyields a mean magnetic moment per cluster. While not yet performed with truly mass-selectedclusters, we demonstrate the feasibility of the method with narrow size distributions between singleatom and several thousands. The possibility to derive information on size-dependent magneticmoments, the interaction between conduction electron spin and localized magnetic moment anddynamic effects like elastic spin mixing are discussed.

(a) (b)

Figure 1: Magneto-thermogalvanic voltage (MTGV) and GMR for Co clusters in Ag.

[1]S. Serrano-Guisan, G. Di Domenicantonio, M. Abid, J.-P. Abid, M. Hillenkamp, L. Gravier, J.-P. Ansermet and C.Félix, Nature Materials 5, 730 (2006)

[2]M. Hillenkamp, G. Di Domenicantonio, C. Félix, L. Gravier, S. Serrano-Guisan and J.-P. Ansermet, accepted forEur. Phys. J. B, (2007)

[3]L. Gravier, S. Serrano-Guisan, G. Di Domenicantonio, M. Abid, M. Hillenkamp, C. Félix and J.-P. Ansermet,Europhys. Lett., 77, 17002 (2007)

10


Room-Temperature Isolation of Organometallic FunctionalSandwiches via Soft-Landing into n-Alkanethiol

Self-Assembled Monolayers

Atsushi Nakajima

Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi,Kohoku-ku, Yokohama 223-8522, Japan

CREST, Japan Science and Technology Agency (JST), c/o Department of Chemistry, Keio University,Yokohama 223-8522, Japan

The adsorption state and thermal stability of organometallic sandwich clusters of vanadium (V)- benzene (Bz), V(Bz)2 and V2(Bz)3 , soft-landed onto a self-assembled monolayer of differentchain-length n-alkanethiols (Cn-SAM, n = 8, 12, 16, 18, and 22) were studied by means of in-frared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD).The IRAS measurement confirmed that V(Bz)2 clusters are molecularly adsorbed and maintain asandwich structure on all of the SAM substrates. In addition, the clusters supported on the SAMsubstrates are oriented with their molecular axes tilted 70-80◦ off the surface normal. An Arrheniusanalysis of the TPD spectra reveals that the activation energy for the desorption of the supportedclusters increases linearly with the chain-length of the SAMs. For the longest chain C22-SAM, theactivation energy reaches ∼150 kJ/mol, and the thermal desorption of the supported clusters can beconsiderably suppressed near room temperature. The clear chain-length-dependent thermal stabil-ity of the supported clusters observed here can be explained well in terms of the cluster penetrationinto the SAM matrixes. Besides, the vibrational assignment of the IRAS spectrum for the V2(Bz)3sandwich was based on a harmonic frequency analysis that employed density functional theory. Theclose similarity between the experimental and calculated results for the IR absorption frequenciesdemonstrates that the V2(Bz)3 complexes have a multidecker sandwich structure on the SAM sub-strate. Our thermal desorption study established that the sandwich complexes can be isolated onthe SAM substrate up to a high temperature of ∼350 K.

[1]S. Nagaoka, T. Matsumoto, K. Ikemoto, M. Mitsui, A.Nakajima, "Soft-landing isolation of multidecker V2(Bz)3complexes in an organic monolayer matrix: an infrared spectroscopy and thermal desorption study," J. Am. Chem.Soc. 129, 1528 (2007).

[2]S. Nagaoka, T. Matsumoto, E. Okada, M. Mitsui, A.Nakajima, "Room-temperature isolation of V(Bz)2 sandwichclusters via soft-landing into n-alkanethiol self-assembled monolayers," J. Phys. Chem. B 110, 16008 (2006).

[3]M. Mitsui, S. Nagaoka, T. Matsumoto, A. Nakajima, "Soft-landing isolation of vanadium- benzene sandwich clusterson a room-temperature substrate using n-alkanethiolate self-assembled monolayer matrices," J. Phys. Chem. B(Letter) 110, 2968 (2006).

[4]K. Miyajima, A. Nakajima, S. Yabushita, M. B. Knickelbein, K. Kaya "Ferromagnetism in one-dimensionalvanadium-benzene sandwich clusters," J. Am. Chem. Soc. 126, 13202 (2004).

11


Magic Clusters from Gold & Sulphur: Special Stability ofSelected Au-Cluster Thiolates, from Orange-Au25 to the SAM

Phases

Robert L. Whetten

Georgia Institute of Technology - Atlanta, USA

The gold-thiolate system is probably, after carbon, the most widely investigated and exploitedclass of cluster-assembled materials, offering also, through alloying & plating, an enhanced controlof diverse nano-metallic systems. Special ultra-stable i.e. ’magic’ compositions, or phases, havebeen successively identified and isolated. But what is the fundamental basis for their remarkablestability & utility? This question has proven to be surprisingly refractory to analysis, even thoughthe ultimate answers now appear quite simple. In retrospect, one problem was that the nature ofthe corresponding ’self-assembled monolayer’ (SAM) surface phases was profoundly misunderstood.Another problem was that the structural richness of the relevant naked gold clusters was profoundlyunderestimated. Finally, the mysterious ’aurophilic interaction’ between non-bonded gold atomsoffers an obscure source of interfacial order & stability. The resulting tragicomedy of errors offersboth entertainment & enlightenment.

12


From Designer Clusters to Synthetic Cluster Assemblies

S. N. Khanna1 and A. W. Castleman, Jr.2

1.Department of Physics, Virginia Commonwealth University, Richmond, VA 23284, USA.2.Department of Physics and Chemistry, Pennsylvania State University, PA 16802, U.S.A.

[email protected]

One of most promising developments in the field of clusters and nanoscience is the possibility ofsynthesizing nanoscale materials where size specific clusters serve as the elementary molecular build-ing blocks. As the physical, chemical, electronic, and magnetic properties of clusters can be tunedby size and composition, this may provide an unprecedented ability to design customized materi-als. The cluster materials, in addition, possess intra-cluster and inter-cluster length scales leading tonovel functionalities not available in conventional materials. While this thought has existed for morethan two decades, its realization has been delayed by the reality that clusters are usually metastableand coalesce when assembled. Indeed, the only working example is the alkali-doped-fullerides wherefullerenes marked by directional C-C bonds serve as the primary unit. Extending the fullerene ex-perience to other metal and semiconducting systems is likely to emerge as an important frontier inthe nanoscience.The talk will focus on our efforts in this direction. I will first talk about the possibility of form-ing cluster motifs, based on metals that are fairly stable and maintain their identity upon furthergrowth. These can be classified as superatoms forming a new dimension to the periodic table [1]-[3].I will then present a new protocol that combines gas phase investigations to examine feasible units,theoretical investigations of energy landscapes and geometrical shapes of feasible units to identifypotential motifs, and synthetic chemical approaches to identify and structurally characterize suchcluster assemblies in the solid state. Through this approach we have established selected arsenic-alkali cluster as a potential building block via gas phase molecular beam experiments. Employingthe idea that the particular species identified in the gas phase is a uniquely stable Zintl entity thatcould effect self-assembly, we report success in synthesizing and characterizing a lattice of analogoussuper-cluster assembled material. We demonstrate how the electronic properties of such assembliescan be fine tuned.

[1]D. E. Bergeron, A.W. Castleman, Jr., T. Morisato, and S.N. Khanna, Science 304, 84 (2004).[2]D. E. Bergeron, P. J. Roach, A. W. Castleman, Jr., N. O. Jones, and S. N. Khanna, Science 307, 231 (2005).[3]J. U. Reveles, S. N. Khanna, P. J. Roach, and A. W. Castleman, Jr., Proc. Nat. Acad. Sci. 103, 18405 (2006).

13


Evolution of magnetism in elemental clusters of transitionmetals

Murilo L. Tiago

University of Texas at Austin, USA

Clusters of ferromagnetic atoms (iron, cobalt, nickel) with a few nanometers in size are superpara-magnetic: their magnetic moment is unusually high compared to macroscopic samples. One reasonfor this behavior is the absence of grain boundaries. Another reason is a decrease in delocalizationof the "3d" atomic orbitals. Both mechanisms are typical of confined quantum systems. Directmeasurements have indicated a strong dependence of magnetic moment with the size of the cluster,especially in iron clusters. In this talk, I will discuss why the size and shape of a cluster affect itsmagnetic properties. This analysis is based on first-principles density functional theory. Numericalcalculations were done in iron clusters containing up to 400 atoms.

14

Tuesday - Clusters and Biomolecules

TALKS TUESDAY - CLUSTERS AND BIOMOLECULES

Size-Selected Carbon Cluster Materials

Daniel Löffler, Artur Böttcher, Stefan-Sven Jester, Patrick Weis, Sergei Lebedkin andManfred M. Kappes

Institute of Physical Chemistry, University of Karlsruhe and Institute of Nanotechnology, Research CenterKarlsruhe, Germany

Ion beam soft-landing has been used to generate multilayer films of size selected carbon clusters (Cx;x=50, 52, 54, 56, 58 and 60). We have studied their properties using a range of surface analyticalmethods including AFM, UPS, XPS, TDS and surface-enhanced Raman spectroscopy. Whereasdeposition of C+

60 generates the well-known v.d.Waals bound molecular solid, smaller fullerenes giverise to covalently linked networks. Interestingly, the major fraction of this deposited network materialcan be intactly desorbed at elevated temperatures. Experiments probing for dissipation of incidentkinetic energy and subsequent surface diffusion processes using periodically (pre-)nanostructuredpinning sites will also be discussed. These novel materials react with thermal energy atomic hydrogento make desorbable hydrofullerides [1]-[4].

[1]A. Böttcher, P. Weis, A. Bihlmeier and M. Kappes, Phys. Chem. Chem. Phys. 6, 5213 (2004).[2]A. Böttcher, P. Weis, S. Jester, D. Löffler, A. Bihlmeier, W. Klopper and M. Kappes, Phys. Chem. Chem. Phys.

7, 2816 (2005).[3]D. Löffler, S. Jester, P. Weis, A. Böttcher and M. Kappes, J. Chem. Phys. 124, 054705 (2006).[4]D. Löffler, S. Jester, P. Weis, A. Böttcher and M. Kappes, J. Chem. Phys. 125, 224705 (2006).

17

TUESDAY - CLUSTERS AND BIOMOLECULES TALKS

Carbon Clusters

Robert N. Compton

Department of Physics and of Chemistry, University of Tennessee, Knoxville, TN 37996, USA

Carbon represents the sixth most abundant element in the universe and is the "stuff of life", as weknow it. Carbon can bond with itself through single, double and triple bonds and, as a consequence,possesses three known allotropes (graphite, diamond and fullerenes). Carbon clusters can exist aslinear, branched or cyclic chains, graphene sheets, or fullerenes (including single or multi-walledcarbon nanotubes). This talk will attempt to summarize some of the important properties ofcarbon clusters and their potential applications in technology and medicine. In addition, the recentconcerns of the adverse environmental impacts of aqueous colloidal suspensions of so-called nano-C60, (or n-C60) will be briefly discussed.Fullerene molecules have unique negative ion properties, exhibiting a large electron attachment crosssection from ∼0 to 20 eV. A second electron can also be bound into a long-lived metastable (C−2

60 ,negative electron affinity) or stable (C−2

84 , positive electron affinity) state which is physically trappedby a Coulomb Barrier. Likewise, low intensity CW laser light can result in efficient ionization offullerenes through a thermionic emission process[1]. This ease of excitation/ionization makes possiblefor efficient means of detecting fullerenes as well as their therapeutic use for the thermal destructionof cancer cells.Fullerenes were accidentally discovered by Kroto, Curl and Smalley while sifting through the debrisof laser ablated graphite soot looking for carbon clusters which might prove responsible for the so-called Diffuse Interstellar Bands (DIBs). The discovery of fullerenes provided some initial excitementthat the DIBs might be attributed to the presence of fullerenes or fullerene ions, but this excitementhas waned. In another possibility, Prof. P. Sarre has suggested[2] that photo-absorption transitionsfrom valence-bound to dipole-bound anions of interstellar molecules such as CH2CN might carrythe signature of the illusive DIBs. Recently[3], the first anion (C6H−) in interstellar space has beenidentified using microwave spectroscopy. Such a valence bound anion will most certainly possessa dipole-bound state and this ion or others like it (CnH−) are possible candidates for the DIBsfeatures although the transitions are expected to be somewhat higher in energy. Some carbonclusters may possess quadrupole-bound anions states. In this connection, the possibility that theDIBs may be due to transitions from valence to quadrupole bound anion states of carbon clusterswill be considered.

[1]D. Ding et al., Phys. Rev. Lett. 73, 1084(1994).[2]P.J. Sarre, Monthly Notices of the Royal Astronomical Society 313, L14(2000).[3]M. C. McCarthy et al., The Astrophysical Journal, 652, L141(2006).

18


A novel nanocalorimeter device for free clusters

Fabien Chirot, Sébastien Zamith, Pierre Labastie, Jean-Marc L’Hermite

Laboratoire Collisions, Agrégats, Réactivité (UMR 5589, CNRS - Université Paul Sabatier),IRSAMC, 31062 Toulouse Cedex 9, France

Thermodynamics of small clusters has received a renewed interest since the pioneering work ofHaberland et al [1] who experimentally demonstrated that very small clusters could undergo aphase transition, by measuring caloric curves.Doing calorimetry (in any system) requires to be able to measure the variation of internal energy∆E corresponding to a variation of temperature ∆T. One possibility is to control the temperatureby a heat bath and to read a physical observable, say S, in one to one correspondance with theinternal energy. The exact correspondance is not relevant, since we only want to be able to measurewhen 2 processes end up in the same internal energy. Actually, a known temperature shift ∆T iscompensated by a known energy shift ∆E in order to observe the same signal S, and then deducethe heat capacity ∆E/∆T. In free clusters, the energy is brought either by a laser [1] or by collisions[2] and S is always somehow linked to the evaporation rate, which is very sensitive to the internalenergy.A third method measures the mobility of clusters in a drift tube, from which collision cross sectionsare deduced. The phase transition from liquid to solid is identified thanks to a variation in the crosssection [3]. The full caloric curve cannot be obtained in this way however.We propose here a novel method based on an original experimental setup [4] to measure the caloriccurve of clusters. The basic idea is to bring energy to the cluster by sticking atoms. After eachsticking, the internal energy of the cluster Mn is increased by a known energy. After a given numberof sticking collisions nmax the cluster is so heated that it dissociates before it can stick again. nmax

is related to the dissociation time, thus is a measure of the internal energy. As in the first twoexperiments mentioned above, we can construct the caloric curve by varying T.Our method does not require laser excitation, is easily transferable to many systems and it is modelfree. We shall present our first experimental results obtained with sodium clusters. They confirmthose of Haberland et al and extend them towards small sizes.

Figure 1: Molecular hydrogen adsorption curve on pure and Li-doped graphene layers.

[1]H. Haberland et al, Phys. Rev. Lett. 94, 035701 (2005).[2]G. A. Breaux et al, Phys. Rev. Lett. 91, 215508 (2003).[3]G. A. Breaux et al, J. Phys. Chem. B, 109, 16575 (2005).[4]F. Chirot et al, Rev. Sci. Instrum. 77, 063108 (2006).

19


Photoelectron spectroscopy on mass-selected neutral silverclusters in helium nanodroplets

Josef Tiggesbäumker, Andreas Przystawik, Sebastian Göde, Karl-Heinz Meiwes-Broer

Institut für Physik, Universität Rostock, Universitätsplatz 3, 18051 Rostock, Germany

Experiments on mass-selected neutral clusters are often hampered by the broad size distributiondelivered by common particle sources. Therefore most of the work concerning the properties ofclusters is performed with charged species; in particular most of the work on photoemission hasbeen performed using anionic clusters. We present a method which exploits absorption resonancesin order to select a certain cluster size for excitation and ionization from the neutral beam.Atomic Clusters are formed by using the helium nanodroplet pick-up technique. The resultingmolecular beam consists of neutral metal clusters embedded in ultracold and superfluid droplets.An advantage of helium pickup sources is to almost exclusively prepare clusters in high spin statesas already demonstrated with alkali clusters on the surface of droplets [1]. Despite the lack ofselectivity in favor of high spin states when growing the clusters inside the droplet we can clearlydetect signals from triplet silver dimers [2].Photoelectron spectra of certain silver clusters are recorded using resonant two-photon excitation forsize-selection and ionization. We derive informations on the electronic structure of metal clusters aswell as the interaction with the helium environment. An analysis of the spectra recorded at differentwavelengths shows that in the transition state all investigated systems undergo a rapid relaxationon a picosecond timescale to the lower edge of the absorption band, see the schematic view in Fig. 1.The use of a two-color pump-probe scheme enables for a measurement of the excited state lifetimeof the neutral clusters.

Figure 1: Schematic diagram of the ionization dynamics of Ag8 [3] . After excitation to the unoc-cupied band E∗ roughly 4.0 eV above the ground state, the cluster quickly relaxes to the lower edgeEL. In a resonant 2-photon ionization experiment, ionization occurs from this long-living level.

[1]P. Claas, D. Schumacher, F. Stienkemeier, Phys. Rev. Lett. 92, 013401 (2004).[2]A. Przystawik, P. Radcliffe, S. Göde, K.-H. Meiwes-Broer, J. Tiggesbäumker, J. Phys. B 39, S1183 (2006)[3]P. Radcliffe, A. Przystawik, Th. Diederich, T. Döppner, J. Tiggesbäumker, K.-H. Meiwes-Broer, Phys. Rev. Lett.

92, 173403 (2004).

20


Unexpected Stability of Al4H6: A Borane Analog?

Xiang Li1, A. Grubisic1, S. T. Stokes1, J. Cordes2, G. F. Ganteför2, K. H. Bowen1, B.Kiran3, M. Willis3, P. Jena3, R. Burgert4 and H. Schnöckel4

1. Depts. of Chemistry and Materials Science, Johns Hopkins University, Baltimore, MD 21218, USA2. Dept. of Physics, University of Konstanz, 78457 Konstanz, Germany

3. Department of Physics, Virginia Commonwealth University, Richmond, VA 23284, USA4. Institute of Inorganic Chemistry, University of Karlsruhe (TH), 76128 Karlsruhe, Germany

While boron has many hydrides, aluminum has been thought to exhibit relatively few. Anionphotoelectron spectroscopy and density functional theory were employed to study aluminum hydrideclusters, AlnH−

m (4 ≤ n ≤ 8, 0 ≤ m ≤ 10). Photoelectron spectra revealed that Al4H4, Al4H6 and afamily of species with general formula AlnHn+2 (5 ≤ n ≤ 8) have small adiabatic electron affinitiesand large HOMO-LUMO gaps (ranging from 0.5 to 1.9 eV) relative to those of their stoichiometricneighbors, implying their enhanced stability. Al4H6 takes on a distorted tetrahedral (D2d) structurewith two counter-positioned bridging hydrogen atoms and shows the largest HOMO-LUMO gap (1.9eV) of all studied alanes. (Supported by AFOSR)

21


Cavity ring-down spectroscopy of gold clusters on surfaces:From the atom to the bulk.

S. Gilb1, J. Peter1, A. Kartouzian1, J.-M. Antonietti1, K. Hartl1, M. Michalski1, U. Heiz1,A. Del Vitto2, G. Pacchioni2, K.H. Lim3, N. Rösch, H. Jones4

1. Department Chemie, Physikalische Chemie I, Technische Universität München, Germany2. Dipartimento di Scienza dei Materiali, Universitá di Milano-Bicocca, Italy

3. Department Chemie, Theoretische Chemie, Technische Universität München, Germany4. Abteilung Laseranwendungen in der Chemie, Universität Ulm, Germany

Cavity ringdown spectroscopy is a highly sensitive photoabsorption technique which in the lastdecade has been employed to perform trace detection of molecules or to study weak electronic tran-sitions in both the gas phase and in the liquid phase. We apply this technique in the solid phasein order to measure the absorption of gold nanoparticles on amorphous silica at low coverages. Inparticular, we explore the evolution of the optical properties of gold in the visible range going fromsingle atoms up to bulk materials.

Monodispersed samples of Aun=1,2,4,8,20 where prepared by cluster deposition. Comparison of thegold monomer and dimer spectra demonstrates that soft landing conditions are fulfilled. Molecular-like optical transitions can be identified up to the tetramer, while larger clusters and particlesshow characteristic surface plasmon peaks. In addition, the optical properties of the small clus-ter (n=1,2,4,8) are compared to time dependent TD-DFT calculation. Optically allowed electronictransitions were calculated, and comparisons with the experimental spectra show that silicon dan-gling bonds [≡Si•], nonbridging oxygen [≡Si-O•], and the silanolate group [≡Si-O-] act as trappingcenters for the gold particles.[1],[2]

Large gold particles with a diameter from 1.3 nm to 2.9 nm on SiO2 where also fabricated by thereversed micelle method [3], the samples were measured in air. The measured extinction spectracan be well described by the Mie-Drude model of absorption for small particles.

[1]Del Vitto, A.; Pacchioni, G.; Lim, K. H.; Rösch, N.; Antonietti, J. M.; Michalski, M.; Heiz, U.; Jones, H. Journalof Physical Chemistry B 2005, 109, 19876

[2]Antonietti, J. M.; Michalski, M.; Heiz, U.; Jones, H.; Lim, K. H.; Rösch, N.; Del Vitto, A.; Pacchioni, G. PhysicalReview Letters 2005, 94.

[3]Kästle, G.; Boyen, H. G.; Weigl, F.; Lengl, G.; Herzog, T.; Ziemann, P.; Riethmüller, S.; Mayer, O.; Hartmann,C.; Spatz, J. P.; Möller, M.; Ozawa, M.; Banhart, F.; Garnier, M. G.; Oelhafen, P. Advanced Functional Materials2003, 13, 853.

22


Spectroscopy of cold, gas-phase biological ions and theirclusters with solvent

Thomas R. Rizzo

Laboratoire de chimie physique moléculaire, Ecole Polytechique Fédérale de Lausanne, Station 6, CH-1015Lausanne, Switzerland

At physiological pH, most biological molecules exist as closed-shell molecular ions, where the compe-tition between charge solvation by water and by polar groups the molecule itself helps determine thesubtle energetic balance that leads to the stabilization of a particular conformation. To understandmore fully the interplay of these interactions, we measure both electronic and vibrational spectraof closed-shell biomolecular ions in the gas phase. We then use these results to test directly thepredictions of theory.We measure photofragment spectra of biomolecular ions as well as their clusters with solvent ina home-built tandem quadrupole mass spectrometer containing a 22-pole ion trap cooled to lessthan 10K. The ions of interest are produced in the gas-phase by electrospray, mass-selected in aquadrupole, and then injected into the trap where they are cooled via collisions with cold helium.After irradiating the ions with IR and/or UV laser pulses, the contents of the trap are ejected andsent through an analyzing quadrupole before being detected. Spectra are generated by monitoringthe appearance of a particular fragment ion mass as a function of the laser wave number.This talk will focus on UV and IR/UV photofragment spectroscopy of cold, protonated amino acids[1] as well as their clusters with a few solvent molecules [2]. Our results provide important insightinto the interplay of charge and solvent in controlling the photophysics of aromatic amino acidchromophores. I will also discuss our first steps in applying our techniques to larger peptides.

[1]O. V. Boyarkin, S. R. Mercier, A. Kamariotis, T. R. Rizzo, Journal of the American Chemical Society 128, 2816(2006).

[2]S. R. Mercier et al., Journal of the American Chemical Society 128, 16938 (Dec, 2006).

23


Infrared spectroscopic characterization of gas-phase clustersand cluster-adsorbate complexes

Gerard Meijer

Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4-6, D-14195 Berlin, Germanye-mail: [email protected]

One of the important issues in the study of gas-phase clusters and cluster-adsorbate complexes is todevelop experimental methods via which the geometric structure of the clusters and the nature of theadsorption-sites can be unambiguously determined. In principle, infrared absorption spectroscopy,i.e. directly probing the bonds that hold the atoms in the cluster together, is ideally suited for this.Vibrational spectroscopy of (mass-selected) gas-phase clusters remains a very challenging researcharea, however, due to the combination of low number densities attainable in the gas-phase and alack of commercially available intense and widely tunable infrared light sources.

In this presentation I will give an overview of the various experiments that we have performedover the last years to obtain structural information on gas-phase species via infrared spectroscopicmethods. For these experiments, the infrared radiation of the Free Electron Laser for InfraredeXperiments (FELIX) at the FOM-Institute "Rijnhuizen" in Nieuwegein (NL) is used as a source.Different experimental detection schemes have been developed – some of which are uniquely possiblewith the FEL – to obtain the desired vibrational spectroscopic information. The application ofthese experimental methods to systems that are of relevance in catalysis will be demonstrated anddiscussed.

[1]G. von Helden, I. Holleman, G.M.H. Knippels, A.F.G. van der Meer, and G. Meijer, Infrared resonance enhancedmultiphoton ionization of fullerenes, Phys. Rev. Lett. 79 (1997) 5234

[2]D. van Heijnsbergen, G. von Helden, M.A. Duncan, A.J.A. van Roij, and G. Meijer, Vibrational spectroscopy ofgas-phase metal-carbide clusters and nanocrystals, Phys. Rev. Lett. 83 (1999) 4983

[3]G. von Helden, D. van Heijnsbergen, and G. Meijer, Resonant ionization using IR light: A new tool to study thespectroscopy and dynamics of gas-phase molecules and clusters, J. Phys. Chem. A 107 (2003) 1671

[4]A. Fielicke, A. Kirilyuk, Ch. Ratsch, J. Behler, M. Scheffler, G. von Helden, and G. Meijer, Structure determinationof isolated metal clusters via far-infrared spectroscopy, Phys. Rev. Lett. 93 (2004) 023401

[5]A. Fielicke, G. von Helden, G. Meijer, D.B. Pedersen, B. Simard, and D.M. Rayner, Gold cluster carbonyls:Saturated adsorption of CO on gold cluster cations, vibrational spectroscopy and implications for their structures,J. Am. Chem. Soc. 127 (2005) 8416

24


Inelastic electron interaction (ionization/attachment) withbiomolecules embedded in superfluid helium droplets

S.Denifl, F. Zappa, I. Mähr, T.D. Märk, P. Scheier

Institut für Ionenphysik und Angewandte Physik, Leopold-Franzens-Universität Innsbruck,Technikerstrasse 25, A-6020 Innsbruck, Austria

The extensive number of spectroscopy studies with doped helium droplets shows impressively theability of superfluid helium droplets to be a perfect matrix for the preparation and study of coldtargets and the formation of complex fragile species [1]. In contrast, inelastic electron interactionwith doped helium droplets has been a much less studied subject. This is the more surprisingas clusters of biomolecules can be formed from the gas phase by embedding successively singlebiomolecules into a cold droplet. Moreover, it is known since recently that electrons can induceefficient DNA damage. This is important because in cells secondary electrons are produced withhigh abundance by ionizing radiation. Thus the underlying chemical and physical processes ofthe inelastic electron interaction with isolated and also solvated biomolecules is of relevance forthe investigation of DNA damage by ionizing radiation and moreover, of fundamental interest inphysical chemistry.We have recently constructed a helium cluster source which was initially used to study in detailthe properties of electron impact ionization of pure helium clusters [2], as well as metastable decaysof helium cluster ions produced [3]. Recently we modified our setup by adding a pick up chamberincluding molecular beam ovens, a pick up cell and external gas inlets which allow the embedmentof various molecules in cold superfluid helium droplets. The neutral mixed clusters thus producedare ionized in a Nier type electron impact ion source (with an energy range from about 0 to 150eV) and ensuing cations and anions are mass analyzed by a high resolution two sector field massspectrometer.First studies of the pick up process have been performed with DNA nucleobases adenine and thymineboth of which are well studied in the gas phase. Several interesting phenomena could be observed,e.g. in contrast to the gas phase situation electron attachment in this environment leads to theproduction of parent anions for adenine and thymine [4]. Moreover, site selectivity in the electronattachment process recently discovered in our laboratory for isolated nucleobases [5] is preserved inthis complex environment and in addition a novel two step reaction scheme has been proposed toexplain characteristic differences in the attachment spectra.These pick-up experiments have been recently extended by embedding other systems into the helium(and neon) droplets (e.g., chloroform, valine and fullerenes) and in some cases even in the additionalpresence of water molecules. In case of multiply charged neon droplets we found a serious failure ofthe previously accepted liquid drop model for vdW bound clusters [6].This work was partially supported by FWF, Wien, the European Commission, Brussels and theBrazilian agency CNPq.

[1]J. P. Toennies, A. F. Vilesov, Angew. Chem. Int. Ed. 43 2622 (2004).[2]S. Denifl, M. Stano, A. Stamatovic, P. Scheier, T. D. Märk, J. Chem. Phys. 124 054320 (2006).[3]S. Feil, K. Gluch, S. Denifl, F. Zappa, O. Echt, P. Scheier and T.D. Märk, Int. J. Mass. Spectr 252 166 (2006).[4]S. Denifl, F. Zappa, I. Mähr, J. Lecointre, M. Probst, T. D. Märk, P. Scheier, Phys. Rev. Lett., 97 043201 (2006).[5]S. Ptasinska et al., Angew.Chem.Int.Ed. 44 6941 and Phys.Rev.Lett. 95 093201 (2005).[6]I. Mähr et al., Phys.Rev.Lett. 98 023401 (2007).

25


Photoabsorption and Photofragmentation inNanoparticle-biomolecule Hybrid Systems

Roland Mitrić1, Jens Petersen1, Alexander Kulesza1, Vlasta Bonačić-Koutecký1, ThibaultTabarin2, Isabelle Compagnon2, Michel Broyer2, Philippe Dugourd2

1. Humboldt Universität zu Berlin, Institut für Chemie, Brook-Taylor-Strasse 2, 12489 Berlin, Germany2. Université Lyon 1; CNRS; LASIM UMR 5579, bât. A. Kastler, 43 Bvd. du 11 novembre 1918, F-69622

Villeurbanne, France.

We present a joint theoretical and experimental study of the size and structure selective opticalabsorption and dynamical properties of cationic silver cluster-tryptophan (Trp-Ag+

n n=2-5,9) hybridsystems. Our TD-DFT calculations and MD simulations together with experimentally measuredfragmentation channels provide insight into the nature of excitations in interacting nanoparticle-biomolecule subunits and allow to identify characteristic spectral features as fingerprints of twodifferent classes of structures: charge solvated and zwitterionic. Different types of charge transfersuch as from π-system of tryptophan to silver cluster or from silver cluster to NH−3+ group arecharacteristic for charge solvated and zwitterionic class of structures, respectively. Remarkably, weobserve a strong reduction of the photofragmentation yield in Trp-Ag+

9 in comparison with freeAg+

9 which may be attributed to energy dissipation by fluorescence. Interplay between internalvibrational energy redistribution and radiationless lifetimes will be addressed. Our findings providefundamental insight into the structure- and size-dependent mechanism responsible for the enhancedabsorption and emission in nanoparticle-biomolecular hybrid systems.

[1]R. Antoine, T. Tabarin, M. Broyer, P. Dugourd, V. Bonacic-Koutecký, R. Mitric, ChemPhysChem 7, 524 (2006).[2]I. Compagnon, T. Tabarin, M. Broyer, P. Dugourd, R. Mitric, J. Petersen, V. Bonacic-Koutecký, J. Chem. Phys..

125, 164326 (2006).

26


Phase Transition in Polypeptides: A Step towards theUnderstanding of Protein Folding

Alexander V. Yakubovich, Ilia A. Solov’yov, Andrey V. Solov’yov and Walter Greiner

Frankfurt Institute for Advanced Studies, Max-von-Laue str. 1, 60438 Frankfurt am Main, GermanyE-mail: [email protected]

The phase transitions in finite complex molecular systems, i.e. the transition from a stable 3Dmolecular structure to a random coil state or vice versa (also known as (un)folding process) occuror can be expected in many different complex molecular systems and in nano objects, such aspolypeptides, proteins, polymers, DNA, fullerenes, nanotubes.

Figure 1: The characteristic structural change of alanine polypeptide experiencing an α-helix ↔random coil phase transition.

We suggest a novel ab initio theoretical method [1] for the description of phase transitions in thementioned molecular systems. In particular, it was demonstrated that in polypeptides (chains ofamino acids) one can identify specific, so-called twisting degrees of freedom, responsible for thefolding dynamics of the amino acid chain, i.e. for the transition from a random coil state of thechain to its α-helix structure (see Fig. 1). The essential domain of the potential energy surface ofpolypeptides with respect to these twisting degrees of freedom can be calculated and thoroughlyanalysed on the basis of ab initio methods such as density functional theory (DFT) or Hartree-Fockmethod. It is shown [1] that this knowledge is sufficient for the construction of the partition functionof a polypeptide chain and thus for the development of its complete thermodynamic description,which includes calculation of all essential thermodynamic variables and characteristics, e.g. heatcapacity, phase transition temperature, free energy etc. The method has been proved to be applicablefor the description of the phase transition in polyalanine of different length by the comparison ofthe theory predictions with the results of several independent experiments and with the results ofmolecular dynamics simulations.This work was supported in part by INTAS (project No 03-51-6170) and by EU through the EXCELLproject.

[1]A. Yakubovich, I. Solov’yov, A. Solov’yov, and W. Greiner, Eur. Phys. J. D (Highlight paper), 40, 363 (2006);Europhys. News, in print (2007).

27


Ion Mobility as a Probe for Molecular Structure andOligomer States in Biological Assemblies

Michael T. Bowers

Department of Chemistry and Biochemistry, University of California Santa Barbara, California, USA

A brief description of the Ion Mobility Method will inaugurate the talk followed by several examplesof current systems of interest to our group. Peptide and protein systems of interest will be drawnfrom those responsible for several important neurological diseases. Recent evidence indicates thatAlzheimer’s disease, Parkinson’s disease and the prion diseases are caused by the early aggregationstates of misfolded peptides and proteins that eventually go on to form amyloid plaques. The focuswill be on the Alzheimer’s peptide, ABeta. The dominant ABeta peptide is the 40 amino acidfragment AB40 (90% in healthy brains) which is only very mildly neurotoxic. Addition of isoleucineand alanine to the C-terminal end of AB40 yields the strongly neurotoxic AB42 (9% in healthybrains). We have examined the distribution and structure of the early oligomer states of these twosystems and related alloforms. Major differences were found and a new paradigm for the etiology ofAlzheimer’s disease will be proposed. If time permits new data on the Parkinson’s protein, Alphasynuclein will be presented including results of two important familial mutants of the wild type.

A second part of the talk will deal with the formation and stabilization of G-quadraplexes in DNA.These structures are composed of multiple G-quartets connected by single strand DNA loops andare predicted to be formed by self assembly in G-rich DNA regions in the genome. These G-richstrands are ubiquitous with over 350,000 candidate segments in the human genome, mostly in generich regions. In addition, several thousand TTAGGG repeats comprise the telomeric capping regionsof all chromosomes whose reproduction is a critical element in cell mytosis. The ability to stabilizeG-quadraplexes with differing loop regions may well contribute to possible cures for many types ofcancers by selectively silencing gene expression. We will use Ion Mobility and high level moleculardynamics simulations to explore possible drug candidates for stabilizing the quadraplex structure.

28

Wednesday - Clusters and Hydrogen

TALKS WEDNESDAY - CLUSTERS AND HYDROGEN

Structure and Reactivity of Transition Metal Oxides: FromGas Phase Clusters to Solid Catalysts

Joachim Sauer

Institut für Chemie, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany

Transition metal oxides in general and vanadium oxides in particular are viable catalysts for theoxidation and oxygenation of hydrocarbons. Density functional theory (DFT) and other quantumchemical methods in concert with experiments are used to answer questions such as: What isthe structure of gas phase cluster ions and how do they differ from supported species and bulkmaterials [1]-[3]? How can IR spectroscopy and photoelectron spectroscopy in combination withDFT calculations be used to identify global minimum structures among many possible isomers?Can gas phase clusters model the reactivity of solid catalysts and to which extent [4],[5]?

[1]J. Sauer and J. Döbler, Dalton Trans., 2004, 19, 3116[2]K. R. Asmis, G. Santambrogio, M. Brümmer and J. Sauer, Angew. Chem., 2005, 117, 3182; Angew. Chem., Int.

Ed., 44, 3122-3125[3]E. Janssens, G. Santambrogio, M. Brümmer, L. Wöste, P. Lievens, J. Sauer, G. Meijer and K. R. Asmis, Phys.

Rev. Lett., 2006, 96, 233401[4]J. Döbler, M. Pritzsche and J. Sauer, J. Am. Chem. Soc., 2005, 127, 10861[5]S. Feyel, D. Schröder, X. Rozanska, J. Sauer and H. Schwarz, Angew. Chem., Int. Ed., 2006, 45, 4677; Angew.

Chem., 118, 4793-4797

31

WEDNESDAY - CLUSTERS AND HYDROGEN TALKS

Materials for Hydrogen Storage: What can Clusters do?

P. Jena

Physics Department, Virginia Commonwealth University, Richmond, Virginia 23284-2000, [email protected]

The success of a new hydrogen economy depends on our ability to find materials that can store hy-drogen with large gravimetric and volumetric densities and operate under ambient thermodynamicconditions. Although host materials consisting of light elements such as Li, Be, B, C, Na, Mg, andAl can meet the gravimetric and volumetric density requirements, the kinetics and thermodynamicsof hydrogen sorption are not favorable as the bonding of hydrogen in these materials is either strong(covalent or ionic) or very weak (van der Waals). Ways must, therefore, be found to either weakenor strengthen the hydrogen bond strength so that light metal complex hydrides and materials basedon carbon can be used as effective hydrogen storage materials. Two possible ways in which this canbe accomplished is to change the chemistry of the hydrogen bonding with host elements throughthe use of catalysts and/or nanostructures. This talk will discuss the progress that has been madein this direction.

In particular, I will discuss the role clusters can play in enhancing our understanding of the inter-action of hydrogen with nano-particles and how this interaction can be modified by doping withmetal atoms. The systems I will deal with include transition metal and Li coated carbon fullerenes,metal doped organic molecules such as CnHn (n=4, 5, 8), and metal decorated ethylene and cis-polyacetylene. The stability of these clusters as they are assembled to form bulk cluster assembledmaterials will also be discussed. The talk will highlight the importance of exploring a novel formof hydrogen bonding where hydrogen is bound to the host materials in nearly molecular form withbinding energies that lie between the physisorption and chemisorption states. I will illustrate howthis molecular chemisorption of hydrogen is ideal for the fast kinetics and desirable thermodynam-ics of hydrogen. It is hoped that the understanding gained here can be useful in designing bettercatalysts as well as hosts for hydrogen storage.

32


Chemistry of Transient Species in Water Clusters

Martin K. Beyer

Institut für Chemie, Sekr. C4, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin,Germany

Reactions of ionic water clusters - or hydrated ions - in a Fourier transform ion cyclotron resonance(FT-ICR) mass spectrometer, proceed analogous to solution phase reactions [1],[2]. This idea isapplied to study aqueous chemistry of species which are short-lived in aqueous solution. Examplesof such transient species are the hydrated electron[3] and monovalent transition metal ions [4]. Inthe gas phase experiment, individual reaction steps are identified unambiguously, since the natureof the reactive species is defined, in contrast to bulk solutions. Recent results suggest that also ther-mochemical information can be directly inferred from the experiment via the quantitative analysisof the number of evaporating water molecules. This approach is exemplified with the reactions ofcarbon dioxide with hydrated electrons (H2O)−n .Monovalent vanadium is metastable in the cluster, and is oxidized to V(II) or V(III) with forma-tion of atomic or molecular hydrogen, respectively [5]. These reactions are strongly size-dependent.Temperature resolved experiments with a nitrogen-cooled ICR cell, controlling the black-body radi-ation temperature experienced by the clusters between 86 - 300 K, give new insight into the reactionmechanism. At the lowest temperature studied, 86 K, H2 formation is still efficient, while loss ofwater molecules is exceedingly slow, at a rate of less than 1× 10−3s−1. This shows that the barrierfor the redox reaction is small, and that the V (H2O)+n clusters are only observable on the ICR timescale because the H2 formation pathway is very complicated, making the reaction highly improbable.Temperature resolved measurements of the rate constants of the black body radiation induced pro-cesses allow the extraction of activation energies via master equation modelling [6],[7]. It is shownthat the standard single-well approach is insufficient to describe the phase transitions occurring inthis process. Successful modeling is achieved by explicit consideration of multiple phases.

[1]G. Niedner-Schatteburg and V. E. Bondybey, Chem. Rev. 100, 4059 (2000).[2]V. E. Bondybey and M. K. Beyer, Int. Rev. Phys. Chem. 21, 277 (2002).[3]O. P. Balaj, C.-K. Siu, I. Balteanu, M. K. Beyer, and V. E. Bondybey, Int. J. Mass Spectrom. 238, 65 (2004).[4]B. S. Fox, O. P. Balaj, I. Balteanu, M. K. Beyer, and V. E. Bondybey, Chem. Eur. J. 8, 5534 (2002).[5]B. S. Fox, I. Balteanu, O. P. Balaj, H. C. Liu, M. K. Beyer and V. E. Bondybey, Phys. Chem. Chem. Phys. 4,

2224 (2002).[6]W. D. Price, P. D. Schnier, R. A. Jockusch, E. F. Strittmatter, and E. R. Williams, J. Phys. Chem. B 101, 664

-673 (1997).[7]R. C. Dunbar, Mass Spectrom. Rev. 23, 127-158 (2004).

33


Probing Stepwise Hydration by Gas Phase VibrationalSpectroscopy

Knut R. Asmis1, Daniel J. Goebbert1, Gabriele Santambrogio2, Jia Zhou3, EtienneGarand3, Jeffrey Headrick4, Mark A. Johnson4, Daniel M. Neumark3,5

1. Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany2. Institut für Experimentalphysik, Freie Universität Berlin, Berlin, Germany3. Department of Chemistry, University of California, Berkeley, CA, USA4. Sterling Chemistry Laboratory, Yale University, New Haven, CN, USA

5. Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

There is an ongoing effort, spanning all branches of the physical sciences, to obtain a detailed under-standing of how ions are solvated in aqueous media. Infrared spectroscopy has been an importanttool in this effort, not only for condensed phase studies, but also to investigate gas-phase species.Such studies help to elucidate the structural motifs that evolve as the number of solvent moleculeschanges. Here we report gas-phase infrared multiple photon photodissociation (IRMPD) spectrafor two different types of solute-solvent systems: the hydrated electron clusters (H2O)−15−50 andhydrated sulfate dianion clusters SO2−

4 (H2O)3−24. IRMPD spectra of these clusters, collisionallycooled to ∼20 K, were obtained using radiation from the Free Electron Laser for Infrared eXperi-ments, FELIX [1], from 575 - 1800 cm−1. Hydrated electrons play an important role in radiationchemistry and various biological processes. Finite water cluster anions [2] have drawn considerableattention, as they represent potential model systems for characterizing the water network accom-modating the excess electron under well controlled conditions and as a function of size. One ofthe central questions is related to the minimum cluster size required to accommodate the extraelectron within the water cluster. In small water clusters the extra electron predominantly resideson the outside of the cluster and is bound by mainly a single water molecule (AA-binding motif) [3].Our experiments suggest that the AA-binding motif survives in clusters containing up to 50 watermolecules and that the transition from external to internal solvation of the excess electron proceedsgradually.Hydrated sulfate dianions are of paramount importance in diverse branches of science ranging fromthe homogeneous nucleation of ice particles by sulfate aerosol in the upper troposphere to the es-sential function of sulfate ions in many metabolic and cellular processes in vivo. IRMPD spectraof SO2−

4 (H2O)n complexes were measured in the region of the stretching and bending modes of thesulfate core as well as characteristic intra- and intermolecular water modes, allowing an unprece-dented, atomic level insight into structure and bonding of these species. The antisymmetric stretchmode ν3 of the sulfate ion at ∼1100 cm−1 acts as sensitive probe for the overall solvation geometry.In a symmetric environment, like in solution, this threefold degenerate mode leads to a single bandin the IR spectrum, while asymmetric solvation, like for n=3, leads to symmetry lowering, liftingof the degeneracy and splitting of this band into its three components. On the other hand thewater bending motion (∼1700 cm−1) as well as intermolecular librational modes (∼800 cm−1) areparticular sensitive to the underlying nature and strength of the hydrogen bonding interaction [4].

[1]D. Oepts, A.F.G. van der Meer, P.W. van Amersfoort, Infrared Phys. Technol. 36, 297 (1995).[2]V. Coe,G.H. Lee, J.G.Eaton, S.T. Arnold, H.W. Sarkas, K.H. Bowen, C. Ludewigt, H. Haberland, D.R. Worsnop,

J. Chem. Phys. 92, 3980 (1990).[3]N. I. Hammer, J.-W. Shin, J.M. Headrick, E.G Diken, J.R. Roscioli, G.H. Weddle, M.A. Johnson, Science 306, 675

(2004).[4]J. Zhou, G. Santambrogio, M. Brümmer, D.T. Moore, L. Wöste, G. Meijer, D.M. Neumark, K.R. Asmis J. Chem.

Phys. 125 111102 (2006).

34


Interaction of hydrogen with graphitic nanostructures andhydrogen storage

Julio A. Alonso, Ivan Cabria, Maria J. Lopez

Department of Theoretical, Atomic and Optical Physics, University of Valladolid,47011 Valladolid, Spain

In the near future hydrogen could replace gasoline in cars, and prototypes using electric motorsthat obtain the energy from the reaction of hydrogen with atmospheric oxygen have already beendeveloped by most car manufacturers. The main remaining challenge is to develope an effective wayof storing the required amount of hydrogen in the tank of a car. A gravimetric hydrogen capacityof 6 % in weight and a volumetric capacity of 0.045 kg H2/L are the targets established for 2010by the Department of Energy of the USA. Another practical requirement is that hydrogen has tobe easily adsorbed and desorbed at room temperature and moderate pressures. One of the storagemethods that have been proposed is the physisorption of molecular hydrogen on graphitic materialswith high specific surface area. Thermodynamic estimations indicate that an adsorption energyof 300-400 meV per molecule should be necessary to obtain efficient cyclic adsorption/desorptionof hydrogen at room temperature and normal pressures. We have performed Density Functionalcalculations of the adsorption of molecular hydrogen on graphene layers and on the external surfaceof single-walled carbon nanotubes. In those cases, binding energies are close to 100 meV/molecule.The binding energies can be increased in two ways. One is by doping the nanotubes. We havefound (see Figure 1) that doping with lithium increases the binding energies by a factor of two. Theother is by adsorption of the hydrogen inside small nanopores. Models of nanopores are the internalpart of a carbon nanotube and the space between two parallel graphene layers with a separationsomehow increased with respect to the distance in graphite (slitpores). In that case, there is also anincrease of the binding energy by a factor of two. Consequently, the combination of these two effectsappears to offer a promising route for obtaining the required values of the adsorption energies. Astatistical mechanical model for adsorption inside slitpores will be presented, which can be comparedto experimental results for hydrogen storage on nanoporous carbon materials.

Figure 1: Molecular hydrogen adsorption curve on pure and Li-doped graphene layers.

35


Understanding proton and electron hydration at themolecular level through cluster spectroscopy

Mark Johnson

Department of Chemistry, Yale University, New Haven, USA

Two of the oldest problems in aqueous chemistry involve solvation of the elementary charges. Bothof these species are anomalous solute ions in the sense that they can become entirely or partiallyincorporated into the fabric of individual water molecules or delocalized into more extended networksof water molecules as the medium is introduced in a step-wise manner. We describe how size-selectedcluster spectroscopy provides detailed pictures of how these transformations occur. The excessproton problem is challenging because the extent of charge delocalization is network morphology-dependent, and these changes result in dramatic stepwise changes in the spectroscopic signaturesassociated with the embedded charged species. The excess electron, on the other hand, yields avery specific and unique spectroscopic signature in the bending region of the vibrational spectrumthat results from its accommodation largely by one water molecule. The challenge in the hydratedelectron case, therefore, becomes one of following how this robust motif in the small cluster regimeevolves into the bulk hydrated electron. The current status of the "surface vs internal" solvationmorphology will be discussed.

36


Abrupt and Gradual Transitions in Size DependentProperties of Clusters: Form Mercury to Protons.

Bernd von Issendorff1, Ori Cheshnovsky2

1. Fakultät für Mathematik und Physik, Universität Freiburg, Stefan-Meier-Straße 19, 79104 Freiburg,Germany

2. School of Chemistry, The Beverly and Raymond Faculty of Exact Sciences, Tel aviv University, 69978Tel Aviv, Israel

We critically review the issue of Metal to Insulator Transitions in metal clusters [1], in view ofour new photoelectron spectroscopy (PES) studies on bivalent Zn−n clusters [2]. We show that zincclusters in the size range of n=3-117 exhibit a distinct transition in their electronic structure char-acteristics as a function of their size. At small sizes up to n=18 the clusters follow the Bloch-Wilsonpicture of the development of a metal from closed-shell atoms, exhibiting a gradual decrease of thegap between the fully occupied s band and the empty p band. For large sizes (n>32 ) valenceelectrons probably fully delocalize. This leads to an almost perfect free-electron density of states, aspredicted by standard free-electron models and as supported by comparison to the PES obtained onsodium clusters. Based on these and other results we suggest a refined view on the gradual natureof Metal to Insulator Transitions in clusters.

In contrast we present new observations, showing critical size behavior of proton transfer from asolvent cluster to solute anion.

Figure 1: The size dependence of the band gap at the EF of different cluster families as obtainedfrom PES on cluster anions. The band gaps are plotted as a function of the number of valenceelectrons in the negatively charged clusters. The gaps have been normalized to the respective bulk EFof the materials and are shown as a function of the inverse cube root of the electron number, whichis proportional to the inverse cluster radius.

[1]B. von Issendorff, O. Cheshnovsky, Annu. Rev. Phys. Chem., 56, 549 (2005).[2]Kostko, O.; Wrigge, G.; Cheshnovsky, O.; von Issendorff, B. J. Chem. Phys. 123,221101 (2005).

37


Matter wave interferometry with massive systems:Foundations, applications and perspectives for cluster physics

Markus Arndt

Fakultät für Physik, Universität Wien, Wien, Austria

Matter wave interferometry with mesoscopic systems is driven by the desire to explore the exper-imental limits of quantum mechanics and also to test some new non-standard models predictingfundamental limits to the masses or times over which matter wave coherence may be observed.The talk gives a brief review over recent developments in matter wave interferometry, with largeneutral objects, presents new applications for molecule metrology and molecule lithography andpoints to open challenges in the manipulation of large biomolecules and massive clusters.

38


Chemistry and spectroscopy of gold and silver: X-rayactivated growth studies of small clusters in stable soda lime

silicate glasses

K. Rademann

Humboldt-Universität, Institut für Chemie Brook-Taylor-Straße 2, D-12489 BERLIN, Germany

The very early stages of cluster formation allow the control of nano-particle formation and depositionin different environments. Especially, nobel metal-clusters are important study objects for basicresearch, catalysis [1] and modern chemical and optical applications. Of particular interest is theoxidation and reduction of gold clusters on silica [2], and its optical properties [3]

Here we report on gold and silver clusters with narrow size distributions in soda lime silicate glasses.The clusters can be generated on demand by BESSY X-ray assisted activation of metal containingglass samples and a subsequent annealing process. This way, noble metal clusters even smaller thanone nanometer become accessible. These systems reveal extraordinary optical properties like highquantum yield photoluminescence [4]-[5].

[1]G. J. Hutchings, Catalytic Science Series, Vol. 6: Catalysis by Gold, ed. by G. C. Bond, C. Louis, D. T. Thompson,Imperial College Press, 2006

[2]D. C. Lim, R. Dietsche, M. Bubek, G. Ganteför, Y. D. Kim ChemPhysChem 2006, 7, 1909-1911[3]J.-M. Antonietti, M. Michalski, U. Heiz, H. Jones, K. H. Lim, N. Rösch, A. D. Vitto, G. Pacchioni; "Optical

Absorption Spectrum of Gold Atoms Deposited on SiO2 from CRDS" Phys. Rev. Lett. 94, 213402 (2005)[4]Goerner, W., Eichelbaum, M., Matschat, R., Rademann, K., Radtke, M., Reinholz, U., Riesemeier, H. "Non-

destructive investigation of composition, chemical properties and structure of materials by synchrotron radiation"INSIGHT 48 (9): 540-544 SEP 2006

[5]M. Eichelbaum, K. Rademann, R. Müller, M. Radtke, H. Riesemeier, W. Görner "Zur Chemie des Goldes inSilicatgläsern: Untersuchungen zum nicht-thermisch aktivierten Wachstum von Goldclustern" Angew. Chem., 117,8118-8122 (2005) "On the Chemistry of Gold in Silicate Glasses: Studies on a Nonthermally Activated Growth ofGold Nanoparticles" Angew. Chem. Int. Ed., 44, 7905-7909 (2005)

[6]M. Eichelbaum, K. Rademann, W. Weigel, B. Löchel, M. Radtke, R. Müller "Gold-Ruby Glass in a New Light: Onthe Microstructuring of Optical Glasses With Synchrotron Radiation" Gold Bulletin, 2007, accepted

39

Thursday - Cluster Chemistry

TALKS THURSDAY - CLUSTER CHEMISTRY

Catalytically active gold: from nano-particles to ultra-thinfilms

Mingshu Chen and D. Wayne Goodman

Department of Chemistry, Texas A&M University College Station, TX 77842-3012, USA

Ordered gold (Au) (1x1) mono- and (1x3) bi-layer films have been synthesized on a titania ultra-thinfilm grown on a Mo(112) surface [1]. The Au (1x3) bi-layer film has a significantly higher catalyticactivity for carbon monoxide oxidation than does the Au (1x1) mono-layer film. This discoveryis a key to understanding the nature of the active site and structure of supported Au catalysts.Furthermore, it is the first observation of Au completely wetting an oxide surface, and demonstratesthat ultra-thin Au films on an oxide surface have exceptionally high catalytic activity, comparableto the activity observed for Au nanoparticles [2].

[1]Chen, M. S.; Goodman, D. W., Accts. Chem. Res., 2006, 39, 739.[2]Chen, M. S.; Goodman, D. W., Science 2004, 306, 252; Valden, M.; Lai, X.; Goodman, D. W., Science 1998, 281,

1647.

43

THURSDAY - CLUSTER CHEMISTRY TALKS

Thermochemistry and Reactivity of Transition Metal andMetal Oxide Clusters

F. Liu, L. Tan, S. Liu, P. B. Armentrout

1. Department of Chemistry, University of Utah, 315 S. 1400 E. Rm 2020, Salt Lake City, UT 84108,USA

In our work, we have examined the kinetic energy dependences of the reactions of Fe+n , Co+

n , and Ni+n(n = 2 - 15) with D2, O2, N2, CD4, and ND3 in a guided ion beam tandem mass spectrometer overthe energy range of 0 - 10 eV. Metal cluster cations are formed in a laser vaporization/supersonicexpansion source and reactions are performed under single collision conditions. The kinetic energydependences are analyzed to determine thresholds for various primary and secondary reactions.From these thresholds, bond energies for iron, cobalt, and nickel cluster cation bonds to D, O, N, C,CD, CD2, CD3, ND, ND2, and ND3 have been determined. For the atomic systems, e.g., D and O,bond energies to modest size metal clusters (10 - 15 atoms) rapidly converge to values equivalent tobulk phase values. Thus, values for the molecular fragments provide some of the very FIRST datafor the thermochemistry of such species bound to surfaces, information that is largely non-existenteven though these are key intermediates in a variety of catalytic processes. Recent extensions ofthis work have examined the collision-induced dissociation of FexO+

y (x = 1 - 6) clusters, and haveelucidated the thermochemistry of several non-stoichiometric combinations.

44


Many Faces of Aromaticity in Clusters

Alexander I. Boldyrev

Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322, USA

Today there is no simple chemical bonding model allowing us to use the "paper and pencil" ap-proach for predicting global minima and low-lying isomers of homoatomic and heteroatomic clusters.However some progress in developing such a model has been made in recent years. In the currentpresentation we demonstrate, that structure, stability and other molecular properties of clusterscan be explained using multiple (σ−, π−, δ−, ψ−) aromaticity, conflicting aromaticity (simultane-ous presence of aromaticity and antiaromaticity), or (σ−, π−, δ−, ψ−) multiple antiaromaticity.Boron clusters, which are the most understood clusters in chemistry will be used as an example.The global aromaticity (or global antiaromaticity) can be assigned on the basis of the 4n+2 (or4n) electron counting rule for either π− or σ− electrons in the planar structures. We showed thatpure boron clusters can have double (σ− and π−) aromaticity (B−3 , B4, B+

5 , B+7 , B8, B2−

8 , B−9 ,B10, B+

11, and B+13), double (σ− and π−) antiaromaticity (B2−

6 , B15); and conflicting aromaticity(σ-antiaromatic and π-aromatic) (B−5 ) or (σ-aromatic and π-antiaromatic) (B14). On the basis ofour understanding of chemical bonding in all-boron clusters we propose which clusters could poten-tially be new ligands and building blocks in chemistry. In addition to boron clusters we will showhow aromaticity/antiaromaticity can be used to describe aluminum, silicon, and other main groupclusters.

45


Spectroscopy of biomolecules and cluster-biomoleculescomplex

M. Broyer

Université Lyon 1; CNRS; LASIM UMR 5579, bât. A. Kastler, 43 Bd du 11 Novembre 1918, F69622Villeurbanne Cedex, France

Various methods to study complex molecules and of biomolecules will be presented. Firstly wewill show how the conformation of complex molecules can be deduced from electric dipole measure-ments. This method will be illustrated by recent results on polypeptides and disubstitued benzenemolecules. Then we present recent photo-dissociation experiments performed on biomolecules storedin a quadrupole ion trap. Information on the molecular structure can be obtained from the deple-tion spectrum as well as from the fragmentation channels. In the case of Tryptophan silver complex(WAg+

n ), two classes of structure, zwitterionic and charge solvated, can be identified and are corre-lated to two different fragmentation spectra. Finally the photodetachement of peptides and DNApolyanions are investigated. The formation of radical anions and the mechanism of photodetache-ment are discussed.

46


Size Selected Metal Cluster Surface Chemistry

David M. Rayner

Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex Drive, Ottawa,Ontario, K1A 0R6 Canada

The structure-specific dynamics of chemical reactions taking place on the surface of isolated metalclusters have tantalized cluster science for a long time. From the early days we have been awareof dramatic size-variations in reactivity but have been largely blind to surface reactions that mightfollow the rate determining adsorption step. The promise of new insight into how particular ar-rangements of metal atoms can be highly effective and highly specific in promoting chemical changein nanoparticle based catalysis has been tempered by the limited structural information providedby traditional mass spectrometry.

Our approach is to use free-electron laser infrared photodepletion spectroscopy to probe the structureof mass-selected metal cluster complexes. Vibrational spectroscopy allows us not only to establishstructure but to determine the influence of cluster constitution, size and charge on the adsorbate(s),to follow the effects of co-adsorption and to determine the extent of any chemistry (e.g. isomerization,dissociative adsorption, reaction with a co-adsorbed ligand) taking place on the surface of thecluster. By carrying out these experiments as a function of the cluster reactor temperature we aimnot only to establish if we can control reactions on cluster surfaces but also to obtain quantitativethermodynamic information. Progress in this area will be discussed with specific emphasis on COas both a probe and as a reactant.

47


Size dependent reactivity of transition metal clusters

Stephanie Jaberg, Britta Pfeffer, Tanja Walter, and Gereon Niedner-Schatteburg

Fachbereich Chemie, TU Kaiserslautern, [email protected]

We report on recent progress in the ongoing research on transition metal clusters as pursuit by ourFourier-Transform-Ion-Cyclotron-Resonance (FT-ICR) apparatus. When stored within its ion trapthe clusters may be exposed to rarefied gases such as e.g. various hydrocarbon derivatives. Thethen persisting single collision conditions allow for the recording of reaction kinetics and for thequantitative determination of elementary reaction steps in operation.

We found naked iron clusters to reveal a surprisingly size dependent lack of bimolecular reactivitytowards aromatic hydrocarbons as well as towards some prospectively σ-directing ligands. We shalldiscuss these findings in the light of possible interpretations.

Further experiments on related clusters and molecules shall help to put forward an increasinglysystematic understanding of the prevailing effects.

48


Electron Spectroscopy of mass selected clusters: From fslasers to the FEL

Wolfgang Eberhardt

BESSY GmbH, Albert Einstein Str. 15, 12489 Berlin, Germany

Clusters exhibit unique materials properties which differ largely from the corresponding bulk solids.Furthermore, these properties also show strong and non-monotonous variations with the size of theparticles. Most widely investigated properties are the chemical reactivity as well as the electronicand magnetic properties, since all of these offer a unique potential for applications. Another uniquefeature of clusters is that the energy relaxation mechanisms and the dynamics are also quite differentthan in bulk solids. Partly this is due to the fact that an excited electronic state can distributethe excitation energy only over a very limited number of degrees of freedom. Furthermore, evenif the excitation energy is delocalized over various degrees of freedom, it still remains localized tothe spatially confined region of the cluster itself. Accordingly unusual relaxation mechanisms suchas the evaporation of atoms or the delayed emission of electrons are quite common in clusters,whereas they are rarely observed in solids. Using fs pump-probe electron spectroscopy of massselected clusters, we have explored these relaxation processes of electronically excited states inAu clusters. The phenomena observed depend strongly on the size of the cluster and range fromelectron-electron to electron-phonon scattering and from vibrational wavepacket oscillations to aquasi ’melting’ of the cluster. Apart from these electronic relaxation processes, fragmentation of thecluster is also observed. This opens the path to the field of femtochemistry. So far however onlyindirect dissociation processes have been observed. Presently this type of spectroscopy is limitedby the photon energies available from laboratory laser sources. However the new soft X-ray lightsources such as the VUV-FEL at DESY and the planned BESSY-FEL offer a unique potential toexpand these studies to include not only the complete valence electronic structure but also to corelevel spectroscopy, which is a sensitive probe of the local geometry and site specific chemistry.

49

Posters - Session A

Clusters and Biomolecules

POSTERS - SESSION A A1

Ionization potential of sodium and imidazole in small waterclusters - ab-initio molecular dynamics study

Lukasz Cwiklik, Pavel Jungwirth

Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic and Centerfor Biomolecules and Complex Molecular Systems, Flemingovo n. 2, Prague, 16610, Czech Republic

We studied water clusters of sodium and imidazole employing Born-Oppenheimer ab-initio molec-ular dynamics simulations. In the case of sodium we investigated clusters consisting of 32 watermolecules. In these simulations collisions of neutral sodium atom with water clusters were studiedin order to investigate the process of ionization of sodium atom and delocalization of its valenceelectron. In the trajectories we observed ionization process, however, the resulting ’excess’ electrondensity was not fully delocalized. Calculated vertical ionization potential was changing from thevalue corresponding to the ionization of sodium atom in the gas phase (for initial stages of sim-ulations, i.e., before the collision with water cluster) to the values comparable with experimentalionization potential of sodium in water clusters.

In the studies concerning imidazole we investigated clusters with changing number of water molecules.We started with systems consisting of one imidazole molecule and one up to five solvent molecules.We performed ab-initio molecular dynamics simulations in order to identify minimal energy con-figurations. Then the minima were found and vertical ionization potential values were calculated.We also studied clusters of imidazole and 20 water molecules in order to use snapshots from thesetrajectories in higher-level ab-initio calculations of ionization potential.

55

A2 POSTERS - SESSION A

Imaging for molecular beam deflection experiments:Asymmetric rotors in electric field-influence for internal

torsions.

M. Abd El Rahim1, R. Antoine2, D. Rayane2, Ph. Dugourd2 and M. Broyer2

1.Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, D-14195, Berlin, Germany2. Laboratoire de Spectrométrie Ionique et Moléculaire, UMR 5579 (Université Lyon I et CNRS), Bt. A.

Kastler, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France

We have developed a position sensitive detection coupled to a high resolution time of flight massspectrometer and to an electrostatic deflector. This experimental setup allows us to measure theelectric dipole moment of molecules in the gas phase. With this new type of detection, we succeededto uncouple the two measurements: mass-position of beam’s molecules. Performance of this setupwas demonstrated over a large mass range with cesium bromide clusters.Using this direct imaging, electric beam deflection experiments on aminobenzonitrile and (dimethy-lamino) benzonitrile molecules have been performed. They are used as prototypes to study theinfluence of the asymmetry and rotation-vibration couplings in deflection experiments. Experimen-tal results were compared to those of ab initio calculations in the frame of the rigid rotor Starkeffect and of the statistical linear response. This comparison show that the change in symmetry andthe introduction of methyl groups lead to a transition from the rigid rotor response to the linearresponse.

Figure 1: MDMABN beam profiles (along the axis of electric field) obtained respectively for a nulland non null electric field. The tensions above indicate the potential differences across the deflector

[1]M. Abd El Rahim et al, Review of scientific instruments, 75, 12, 5221 (2004).[2]M. Abd El Rahim et al, Journal of Physical Chemistry A, 109, 8507 (2005).[3]R. Antoine et al, Journal of Physical Chemistry A, 110, 10006 (2006).

56


Isomer specific studies of the Zundel cation (H2O·H+·OH2)

Ben M. Elliott, J. Robert Roscioli, Joseph C. Bopp, Jeffrey M. Headrick, Laura McCunn,Nathan Hammer2, Mark A. Johnson1

1. Department of Chemistry, Yale University, 225 Prospect St., New Haven, Ct 06520, USA2. Department of Chemistry, University of Massachusetts, Amherst, MA 01003, USA

Although the vibrational spectrum of the Zundel cation (H5O+2 ) is simple, consisting of a pair of

doublet transitions at 1000 and 1700 cm−1, the assignment of the mechanical origin of this patternhas proven remarkably difficult. One problem certainly arises from the strong anharmonicitiespresent owing to the coupling of the shared proton motion to the soft modes nominally associatedwith wagging and twisting of the flanking water molecules.In order to address these issues, we report progress on a new generation of experiments where wefollow how the bands evolve upon H/D substitution in such a way that the single hetero-isotopelies in the shared position, while the dominant isotope occupies the four free OH positions on thetwo water molecules. Because the relevant singly substituted target species occur in two isomericforms depending on whether this atom lies in the shared or free position, we carry out isomer-selective spectroscopy using population modulation with two IR lasers in a tandem time-of-flightarrangement. Lines of the spectra relating to specific positions of the unique isotope are selected bythe initial IR laser, and the selected isomer is then interrogated by a second IR laser pulse.

Figure 1: Calculated structure of the Zundel cation.

57


Laser desorption source for large molecules

Markus Marksteiner, Lucia Hackermüller, Gregor Kiesewetter, Hendrik Ulbricht andMarkus Arndt

Fakultät für Physik, Universität Wien, Boltzmanngasse 5, A - 1090 Wien, Austria

The experimental proof of the wave nature for massive particles is an important demonstrationfor the validity of the foundation of physics. With a Talbot-Lau interferometer consisting of threemechanical gratings we could show the wave like behaviour of fullerenes and special fluorinatedfullerenes C60F48 with a mass of 1630u (Hackermüller et al. PRL 91, 90408 (2003)).

In order to do interferometry of high mass biomolecules a laser desorption source was built andtested with the two biomolecules tryptophan (204u) and gramicidin (1880u). The molecules aredesorbed by a nanosecond laser pulse and cooled in a mixing channel via collisions with a supersonicexpanding gasbeam, resulting in a beam of neutral molecules, that can be photopostionized for timeof flight detection. Experiments showed a sufficient signal for tryptophan and gramicidin, whichshould allow the combination of the source with the interferometer. Photo-ionization has beentested at two different wavelengths, namely at 266 nm and 157 nm. In the case of tryptophan theionization signal was saturated in both cases, whereas for gramicidin the signal recorded at 157 nmphotoionization wavelength is 5 times higher than that at 266 nm.

We study the dependence of the ionization signal both on the desorption wavelength, for differentcarrier gases and different cooling modes, and the functional dependence on the ionization intensity.Experiments with molecules at even higher mass, such as insulin (5700 u), showed, that theseparticles generally strongly fragment during desorption and/or ionization and they did not revealany significant intact mother ion.Metal clusters are an interesting alternative to biomolecules for matter wave experiments. Theyhave a significantly lower ionization energy and it is known that they may undergo single-photonionization.

58


Formation of a Covalent Bond upon Electron Attachment tovan der Waals Complex

Sang Hak Lee, Nam Doo Kim, Dong Gyun Ha, Ki Wook Hwang, and Seong Keun Kim

School of Chemistry, Seoul National University, Seoul 151-747, Korea

Following our earlier finding of the covalent bond formation in the van der Waals complex of pyridineand CO2 upon electron attachment, we extended our study to diazines and s-triazine. In the anioncomplexes between pyrazine and CO2, the 1:1 and 1:2 complexes showed a drastic increase invertical detachment energy (VDE) of 2.0 and 4.3 eV, respectively, apparently because an extendedπ-network is formed through the aforementioned covalent bond bridge between CO2 and pyrazine.In pyrimidine and pyridazine, the VDE shifts by 2.0 eV upon covalent bond formation with the firstCO2 and by 0.2 eV when simply solvated by the second CO2. s-triazine showed a similar behaviorwith the first two CO2’s, but the third CO2 increased the VDE by another 2.0 eV. It appears thatthe covalent bond formation is sterically hindered in the 1:2 complexes pyrimidine and pyridazineunlike the case of pyrazine, while such steric hindrance is overcome in the 1:3 complexes of s-triazineby "forced" binding of the third CO2. Optimized geometry of the anion complexes obtained at theB3LYP/6-31++G∗∗ and MP2/6-31++G∗∗ level supports the above experimental results, showingthat the interaction between azabenzene and CO2 undergoes a drastic change in its bonding natureupon electron attachment.

[1]Michael J. DeLuca et al., J. Chem. Phys, 88, 5857(1988).[2]S. Y. Han et al. J. Chem. Phys. 113, 596 (2000).[3]Holger Schneider et al., J. Chem. Phys, 123, 074316(2005).

59


Quantum Building Blocks Concept for Biomacromolecules

O. I. Obolensky, I. A. Solov’yov, A. V. Solov’yov, W. Greiner

Frankfurt Institute for Advanced Studies, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, Germany

The idea of representing complex molecular structures as sets of building blocks is not new. Itallows one to reduce drastically the volume of the conformational space, since fine details are inte-grated out producing renormalized effective interactions between the surviving collective degrees offreedom (the building blocks). The number of the degrees of freedom and accuracy of the effectiveinteractions range widely depending on the system under consideration and on simplicity of themodel. This strategy has led to significant breakthroughs in understanding structural properties ofcomplex systems. For example, in the protein folding problem such phenomena as nucleation of thehydrophobic core or existence of a molten globule state can be explained with the building blocksconcept [1]. The shortcoming of this approach is in the lack of predictive power for the de novostructure determination stemming from the excessive simplicity of the interaction potentials used.We present here the formalism and the first results of quantum building blocks calculations inwhich the interaction potentials are calculated at an ab initio quantum mechanical level. We haveperformed density functional theory calculations of multidimensional potential energy surface foralanine-alanine interactions. In our approach the potential energy of the interaction depends onsix variables, three of which describe the relative position and the other three describe the mutualorientation of the coordinate systems associated with each amino acid, see the figure. Using thequantum building blocks approach we found the global energy minimum structures of several Alaoligopeptides. The found structures are in a good agreement with our previous complete quantummechanical analysis [2].

We acknowledge partial financial support from the European Commission (project EXCELL).

[1]A. Kolinski, J. Skolnick, Polymer, 45, 511 (2004).[2]I. A. Solov’yov, A. V. Yakubovich, A. V. Solov’yov, W. Greiner, Phys. Rev. E, 73, 021916 (2006)

60


Solvation of amino acids: From the isolated molecule to thesolution.

M. N. Blom1, I. Compagnon1, N. Polfer1, G. v. Helden2, G. Meijer2, S. Suhai3, B. Paizs3

and J. Oomens1

1. FOM Institute "Rijnhuizen", Edisonbaan 14, 3439 MN Nieuwegein, The Netherlands.2. Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany.3. German Cancer Research Center, Im Neuenheimer Feld 580, 69120 Heidelberg, Germany.

In contrast to aqueous solutions, amino acids adopt a non-zwitterionic structure in the gas phase.To answer the question on how many solvent molecules are required to stabilize the zwitterionicstructure, neutral tryptophan-solvent complexes (Trp(H2O)1−6, Trp(D2O)1−6 and Trp(MeOH)1−9)are probed by infrared spectroscopy and density functional theory (B3LYP/6-31+G∗∗). The struc-tural identification is performed based on diagnostic vibrations, such as the C=O stretch, the C-O-H bending mode, the asymmetric COO− stretch and the weaker NH+

3 bending mode (1300-1850cm−1). Small solvated complexes (n=1-4) are found to be non-zwitterionic and exhibit two dis-tinctive conformeric motifs. Starting with 5 solvent molecules a well separated COO− band wasfound for Trp(MeOH)5−9 and a shift of the CO stretch frequency was detected for Trp(H2O)5−6

and Trp(D2O)5−6 displaying the appearance of zwitterionic structures. Moreover, the convergencetoward the FTIR spectrum of tryptophan in methanol is clearly observed.

61


Inelastic electron interaction (ionization/attachment) withbiomolecules embedded in superfluid helium droplets

S.Denifl, F. Zappa, I. Mähr, T.D. Märk, P. Scheier

1. Institut für Ionenphysik und Angewandte Physik, Leopold-Franzens-Universität Innsbruck,Technikerstrasse 25, A-6020 Innsbruck, Austria

The extensive number of spectroscopy studies with doped helium droplets shows impressively theability of superfluid helium droplets to be a perfect matrix for the preparation of cold targets andthe formation of complex fragile species [1]. In contrast, the inelastic electron interaction withdoped helium droplets is much less studied. This fact is rather surprising as clusters of biomoleculescan be formed from the gas phase by embedding single biomolecules into the cold droplets. It isknown that electrons can induce efficient DNA damage and in cells secondary electrons are producedwith high abundance by ionizing radiation. Thus the underlying chemical and physical processesof the inelastic electron interaction with isolated and also solvated biomolecules is of relevance forthe investigation of DNA damage by ionizing radiation and moreover, of fundamental interest inphysical chemistry.Our group has constructed a helium cluster source which was initially used to study the electronimpact ionization of pure helium clusters [2], as well as their metastable decays [3]. Recentlywe modified our setup by adding a pick up chamber with oven, pick up cell and external gasinlet which allows the embedment of various molecules in cold superfluid helium droplets. Theneutral mixed clusters are ionized in a Nier type ion source and mass analyzed by a two sectorfield mass spectrometer. First studies of the pick up process have been performed with DNAnucleobases adenine and thymine which are well studied in the gas phase. Several interestingphenomena occurred, e.g. we could observe parent anions for adenine and thymine which is not thecase in the gas phase [4]. In this contribution we will present in detail the experimental setup for ourpick-up experiments and summarize our results concerning the inelastic electron interaction withDNA nucleobases embedded in cold helium droplets. The pick-up experiments have been continuedby embedding the amino acid valine, water and very recently even C60 in the helium droplets. Fromthese measurements we can characterize the ionization/attachment process for the different specieswhen inside the He droplets.

This work was partially supported by FWF, Wien, the European Commission, Brussels (ITS-LEIF)and the Brazilian agency CNPq.

[1]J. P. Toennies, A. F. Vilesov, Angew. Chem. Int. Ed. 43 2622 (2004).[2]S. Denifl, M. Stano, A. Stamatovic, P. Scheier, T. D. Märk, J. Chem. Phys. 124 054320 (2006).[3]S. Feil, K. Gluch, S. Denifl, F. Zappa, O. Echt, P. Scheier and T.D. Märk, Int. J. Mass. Spectr 252 166 (2006).[4]S. Denifl, F. Zappa, I. Mähr, J. Lecointre, M. Probst, T. D. Märk, P. Scheier, Phys. Rev. Lett., 97 043201 (2006)

62


Enantioselective Nickel Mediated Gas-Phase Dehydrogenationof Secondary Alcohols

Francesca Novara1, Detlef Schröder 1,2, Helmut Schwarz1

1. Institute of Chemistry, Technical University Berlin, Strasse des 17. Juni 135, D-10623 Berlin,Germany

2. Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences, Flemingovo nam. 2,16610 Prague 6, Czech Republic

Enantioselectivity remains an intriguing and challenging topic of investigation due to its importancein asymmetric synthesis and for its distinct role in biological systems. Gas-phase studies can providean understanding of the intrinsic properties underlying the mechanisms of chiral discrimination.[1]-[3] Here, we report on enantioselective bond activations observed upon collision-induced dissociation(CID) of chiral Ni complexes of the generic composition [(BINOLate)Ni(CH3(CHOH)R)]+, whereBINOLate corresponds to singly deprotonated R- or S-bis-2-naphthol and R = n-C2H5, n-C3H7,n-C4H9, n-C5H11, n-C6H13 and C6H5, respectively. Upon CID, the mass-selected nickel complexeslose either the entire alcohol ligand or undergo bond activation followed by loss of the correspond-ing ketone. When enantiomeric BINOLate ligands are combined with chiral secondary alcohols,the complexes become diastereomeric. Differences in the branching ratios between the two chan-nels for different diastereomeric complexes are observed and can serve as a measure for the chiraldiscrimination operative in the C−H− and O−H−bond activation processes.

Figure 1: Breakdown graph of the CID fragments of mass-selected (R-BINOL-H)Ni(R-1-phenyl-ethanol)+ as a function of collision energy in the center- of-.mass frame.

The chiral BINOLate ligand has been chosen as a stereochemical discriminator by analogy to well-known asymmetric hydrogenation catalysts.[4] Thus, by reference to the principle of miscroscopicreversibility, the proposed system can be considered a model for the transition-metal catalyzedenantioselective hydrogenation of ketones.

[1]M. Speranza Adv. Phys. Org. Chem. 39, 147 (2004).[2]A. Filippi, A. Giardini, S. Piccirillo and M. Speranza Int. J. Mass Spectrom. 198, 137 (2000).[3]D. Schröder and H. Schwarz Int. J. Mass Spectrom. 231, 139 (2004).[4]R. Noyori and T. Okhuma Angew. Chem. Int. Ed. 40, 40 (2001).

63

Magnetism


Magnetic and structural properties of mass-filtered 3dtransition metal nanoparticles

Armin Kleibert1, Johannes Passig1, Joachim Bansmann2, Renate K. Gebhardt3, FurkanBulut3, Mathias Getzlaff3, Karl-Heinz Meiwes-Broer1

1. Institute of Physics, Rostock University,Universitätsplatz 3, D-18051 Rostock, Germany2. Institute of Surface Chemistry and Catalysis , Ulm University, Albert-Einstein-Allee 47, D-89069 Ulm,

Germany3. Institute of Applied Physics, Düsseldorf University, Universitätsstrasse, Düsseldorf, D-40225

Düsseldorf, Germany

Free clusters and nanoparticles are not just small pieces of material with physical properties nearlyidentical to the bulk. Their electronic, optical and magnetic properties are clearly size-dependentwith a non-linear behaviour between the two general limits given by the atomic and the bulk-like behaviour [1]. Recently, it was shown that even large iron nanoparticles with diameters ofabout 10 nm show magnetic orbital moments that significantly deviate from the respective bulkproperties [1],[2]. In this contribution we compare the magnetic properties of mass-filtered Fe, Co andFeCo nanoparticles being deposited onto different epitaxially grown thin films. The crystallographicstructure and morphology of the particles have been determined independently by means of electrondiffraction and high resolution transmission electron microscopy (HRTEM), respectively (cf. Fig.1). The magnetic moments of the clusters have been measured in situ via X-ray magnetic circulardichroism (XMCD) after deposition onto ferromagnetic films.

Figure 1: High resolution transmission electron microscopy images of individual Fe, Co, and FeConanoparticles with diameters of about 15 nm.

[1]J. Bansmann et al., Surface Science Reports 56, 189 (2005).[2]J. Bansmann and A. Kleibert, Appl. Phys. A 80, 957 (2005).

67


Structure and Magnetism of La Clusters

Andrey Lyalin, Andrey Solov’yov, and Walter Greiner

Frankfurt Institute for Advanced Studies, Max von Laue Str. 1, 60438 Frankfurt am Main, Germany

We have performed a systematic theoretical investigation of optimized ionic structure, electronic andmagnetic properties of La clusters within the size range N < 14 [1]. We found a giant enhancementof magnetism in La4, La6, and La13 clusters. We also found that the ground states of La2, La3,La5, La7, La9-La11, and La14 clusters possess nonzero magnetic moments, that ranged from 0.1 µBto 1.0 µB per atom. This clearly indicates that small La clusters display magnetic behavior, eventhough bulk La has no magnetic ordering. We show that magnetism in La clusters is governed byunpaired valence electrons, in contrast to the local-moment magnetism in clusters of heavy rare-earth elements. In addition to the ground state isomers of La clusters we found an ensemble ofenergetically low-lying spin isomers. We predict an increase of the average magnetic moments forensembles of La2, La3, La5, La8, La9, La11, La12, and La14 clusters with temperature due to thethermal population of the spin isomers. For ensembles of La4, La7, and La13 clusters, the averagemagnetic moment decreases with temperature. Such an anomalous behavior of the magnetic momentwith temperature can be detected in Stern-Gerlach deflection experiments.

Figure 1: Binding energy per atom for the most stable La clusters (left); magnetic moments peratom for La clusters as a function of cluster size. Open circles present the results of experiment byKnickelbein [2] (b).

This work is partially supported by the European Commission within the Network of Excellenceproject EXCELL, and by INTAS under the Grant No. 03-51-6170.

[1]A. Lyalin, A. V. Solov’yov and W. Greiner, Phys. Rev. A 74, 043201 (2006).[2]M. B. Knickelbein, Phys. Rev. B 71, 184442 (2005).

68


Ferromagnetism in Finite Mn Linear Chains

Andrey Lyalin, Andrey Solov’yov, and Walter Greiner


The study of evolution of magnetic properties from atoms to the bulk is important for the devel-opment of magnetic nanomaterials with specific properties and for understanding the fundamentalprinciples of spin coupling in finite and low dimensional systems. In this work we report the resultsof a systematic theoretical investigation of optimized structure, electronic and magnetic propertiesof linear chains of Mn atoms and organometallic sandwich clusters MnN (C6H6)N+1 within the sizerange N ≤ 6 [1]. The choice of Mn is stipulated by the fact that the Mn atom possesses a large mag-netic moment due to the half-filled 3d electron shell, and thus, manganese is a good candidate forstrong nanomagnets. Our calculations are based on ab initio theoretical methods invoking density-functional theory with the gradient-corrected exchange-correlation functional of Perdew, Burke andErnzerhof (PBEPBE). The standard LANL2MB basis set of primitive Gaussians have been usedto expand the electronic orbitals formed by the 3s23p63d54s2 outer electrons of Mn (15 electronsper atom). We show that the finite linear chains of Mn atoms exhibit novel magnetic propertiesthat differ from those of the corresponding Mn bulk and one-dimensional infinite chains. Thus,we demonstrate that finite one-dimensional monoatomic chains of Mn atoms possess ferromagneticorder in spite of the fact that the most stable crystal structure of Mn exhibits antiferromagneticbehaviour. We demonstrate that magnetic ordering in finite linear chains depends on its lengthand therefore can be controlled by the fixing chain’s geometry. We also predict enhancement ofmagnetism in organometallic sandwich clusters MnN (C6H6)N+1.

Figure 1: Binding energy per atom for Mn6 linear chain as a function of multiplicity 2S+1 (right).Mulliken atomic spin densities for different spin isomers of Mn6 chain (left). Interatomic distancesare given in angstroms.


[1]A. Lyalin, A.V. Solov’yov and W. Greiner, in preparation for Phys. Rev. A (2007).

69


Magnetic properties of mass selected, deposited alloy clusters

Michael Martins, Leif Glaser, Michael Wellhöfer, Wilfried Wurth

Institut for Experimental Physics, University of Hamburg, Luruper Chaussee 149, D-22761 Hamburg,Germany

Nano particles are promising new materials for ultra small magnetic storage devices. In particularmass selected clusters are good candidates for such applications, because their properties can betailored by just changing the cluster size. In recent work properties as a function of the numberof atoms per cluster have been studied [1],[2]. However, by changing also the composition of theclusters a much larger functional space will be available. For magnetic storage devices in particulara huge magnetic anisotropy is mandatory, to fix the magnetization direction in space. This canpotentially be realized by systems with a large orbital magnetic moment and a large spin-orbitcoupling. Therefore, we have started experiments on small mass selected, deposited ConPtm clusterson different magnetized substrates to study their orbital and spin moments size dependent. To probethe magnetic properties of the clusters the x-ray magnetic circular dichroism (XMCD) method isapplied using synchrotron radiation. In figure 1 the XMCD spectra for several CoPtm clusterdeposited on a thin magnetized Fe film are shown. All spectra have been normalized to the sameL2 dichroism intensity, so that the L3 dichroism signal reflects directly the ratio of orbital/spinmagnetic moment. By adding a single Pt atom to a Co atom the orbital/spin ratio is increasingby 60%, whereas addition of further Pt atoms decreases the ratio again. This shows, that tailoringa special property might be possible by choosing the right size and composition of a cluster. Thiswork is performed within the framework of the Sonderforschungsbereich 668.

Figure 1: XMCD spectra of deposited CoPtm cluster at the Co L2 and L3 edges on a magnetizedFe film. The spectra have been normalized to the same intensity at the L2 edge.

[1]J.T. Lau et al, Phys. Rev. Lett. 89, 57201 (2002).[2]M. Reif et al., Phys. Rev. B 72, 155405 (2005).

70

Metal Clusters


Theoretical Study of Thermodynamical Properties and PhaseChanges in Krypton Clusters

Ales Vitek

Department of Physics, University of Ostrava, Ostrava, Czech Republic

The subject of our research has been a theoretical study of thermodynamical properties and phasechanges in medium-size krypton cluster cations Kr+12, Kr+13 and Kr+14, by means of Monte Carlosimulations. The simulations have been performed for a broad range of cluster temperatures orinternal energies. Single ionized rare gas clusters are heterogeneous systems. The positive chargeis located on a small subunit (ionic core), involving 2-4 atoms, surrounded by a cloud of almostneutral atoms. The Kr+13 cluster is extremely stable due to its closed-shell configuration. For adeeper insight into the structural changes in Kr+N , it turned out to be interesting to focus on Kr+12and Kr+14 cluster, in which one atom is absent from or added to the perfect symmetry configurationof Kr+13. Single structural change has been detected prior to the evaporation. It has been proved thatthe ionic core melts at higher temperatures than the neutral envelope. The intra-cluster interactionhas been described by extended diatomics-in-molecules models with the inclusion of the spin-orbitalcoupling. Further, the diatomics-in-molecules models have been extended by inclusion of three-bodypolarization forces acting between two induced dipoles and three-body van der Waals forces.

73


Electronic photodissociation spectroscopy in comparison withTDDFT calculations in the case of Au+

m·Arn (m=2-9,n=0-3)

Alexia N. Gloess1, Holger Schneider2, J. Mathias Weber2, Manfred M. Kappes1

1. Institut für Physikalische Chemie, Universität Karlsruhe, Germany2. JILA, University of Colorado, Boulder, USA

We report on electronic photodissociation spectroscopy of Au+m·Arn clusters (m = 4, 7-9; n = 0-3).

The spectra were obtained in the photon energy range between 2.14 eV and 3.35 eV in the case ofAu+

4 and Au+4 ·Ar and in the range between 2.14 eV and 3.02 eV for all other clusters. Photodissoci-

ation spectra were recorded by monitoring ion depletion upon photon absorption, yielding absolutephotodissociation cross sections. The interpretation of the experimental data was performed bycomparing the results with calculations of the electronic absorption spectra using time-dependentdensity functional theory based on cluster structures calculated with density functional theory withthe functional B3-LYP as well as with ab-initio calculations at the MP2 level. These calculationshave been extended to a cluster size of Au+

2 - Au+9 and Au+

m·Arn (m = 4, 7; n = 1-3).The experimentally obtained electronic photodissociation spectra show a qualitative change betweenthe size of m = 7 and m = 8. We compare the photodissociation spectra with the quantum me-chanically calculated electronic absorption spectra. The observed change can be explained by thetransition from two- to three-dimensionality of the cluster’s underlying structures.

74


Far-infrared spectroscopy of small neutral silver clusters

André Fielicke, Ira Rabin, and Gerard Meijer

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany

The determination of the geometric structures of metal clusters is still one of the central goals ofexperimental cluster research. Infrared absorption spectroscopy as a fairly direct method to obtainstructural information is very difficult, since the vibrational bands of metal clusters are in the far-infrared and have notoriously low IR intensities.Here, we report on the vibrational spectra of the neutral silver trimer and tetramer in the gas-phase. IR spectra are obtained via Multiple Photon Dissociation Spectroscopy of their complexeswith argon atoms. The focus is in particular on Ag3, which has been studied intensively up tonow, but for which the available data on the vibrational properties is rather controversial.[1] Ourinvestigations on the isolated clusters provide direct and mass selective measurements of the infraredspectra and form the basis for the evaluation of the data from earlier studies. Further, by increasingthe argon coverage we can obtain conditions similar to matrix embedding that is then reflected inshifts of the vibrational frequencies towards the matrix values.In the present study, complexes of small neutral silver clusters with one or a few argon atoms aregenerated using a pick-up cluster source.[2] The vibrational spectra of Ag3 and Ag4 are recordedin the far-infrared between 100-220 cm−1using multiple photon dissociation spectroscopy of theircomplexes with Ar atoms. For Ag3-Ar two IR active bands are found at 113 cm−1 and 183 cm−1,for Ag4-Ar one band at 163 cm−1 and very weak IR activity at 193 cm−1 are observed. This,together with recent theoretical studies, allows for a reassignment of the controversial vibrationaldata reported earlier for the bare Ag3 cluster. The influence of the number of Ar atoms in thecomplexes on the frequency of the IR active modes is found to be minor. However, the low-frequencyIR-active band of Ag3 shifts with increasing Ar coverage from 113 cm−1 for Ag3-Ar to about 120cm−1 for Ag3-Ar4, the value known for Ag3 embedded in rare gas matrices.[3]

[1]J. R. Lombardi and B. Davis, Chem. Rev., 102, 2431 (2002).[2]D. Ievlev, I. Rabin, W. Schulze, and G. Ertl, Chem. Phys. Lett., 328, 142 (2000); D. Ievlev, I. Rabin, W. Schulze,

and G. Ertl, Eur. Phys. J. D, 16, 157 (2001).[3]A. Fielicke, I. Rabin, and G. Meijer, J. Phys. Chem. A, 110, 8060 (2006).

75


Electric Deflection Studies of Rhodium Clusters

Martin K. Beyer1, Mark B. Knickelbein2

1. Institut für Chemie, Sekr. C4, Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin,Germany

2. Chemistry Division, Argonne National Laboratory, Argonne, IL 60439, USA

The static electric dipole polarizabilities of rhodium clusters Rhn, n=5-28, have been measured [1]via a molecular beam deflection method. Uniform high-field beam deflections, indicative of inducedpolarization, were observed for all Rhn except Rh7 and Rh10 which by contrast exhibited beambroadening and anomalously high effective polarizabilities. Analysis of the beam deflection profileof Rh7 indicates that it possesses a permanent dipole moment of 0.24±0.02 D. Unlike the otherclusters in the n=5-28 size range, the polarizability of Rh10 is observed to decrease with increasingsource temperature. We attribute this temperature dependence to paraelectric behavior, suggestingthat Rh10 is a fluxional molecule possessing a dipole moment that spatially fluctuates, uncorrelatedwith overall rotation.

[1]M. K. Beyer and M. B. Knickelbein, J. Chem. Phys. in print.

76


Liquid-Liquid Coexistence in Gold Clusters: 2D or not 2D?

Pekka Koskinen1,2, Hannu Häkkinen3, Bernd Huber2, Bernd von Issendorff2, and MichaelMoseler1,2

1. Fraunhofer-Institut für Werkstoffmechanik IWM, Wöhlerstraße 11, 79108 Freiburg, Germany2. Fakultät für Physik, Hermann-Herder Strasse 3, 79104 Freiburg, Germany

3. Department of Physics, NanoScience Center, University of Jyväskylä, 40014 Jyväskylä, Finland

The thermodynamics of gold cluster anions (AuN , N=11...14) is investigated using quantum molec-ular dynamics. Our simulations suggest that AuN may exhibit a novel, free-standing planar liquidphase which dynamically coexists with a normal three-dimensional liquid. Upon cooling with ex-perimentally realizable cooling rates, the entropy-favored three-dimensional liquid clusters oftensupercool and solidify into the ”wrong” dimensionality. This indicates that experimental validationof theoretically predicted AuN ground states might be more complicated than hitherto expected.

[1]P. Koskinen, H. Häkkinen, B. Huber, B. v.Issendorff, and M. Moseler, Liquid-liquid coexistence in gold clusters:2D or not 2D? Phys. Rev. Lett. 98, 015701 (2007).

77


Mass-resolved Soft X-ray Ion Yield Spectroscopy of FreeTransition Metal Clusters

J. T. Lau1, M. Vogel1, J. Rittmann1, V. Zamudio-Bayer1, T. Schadow1, B. v. Issendorff2,and T. Möller1

1. Institut für Optik und Atomare Physik, PN 3-1, Technische Universität Berlin, Hardenbergstraße 36,D-10623 Berlin, Germany

2. Fakultät für Physik/FMF, Albert-Ludwigs-Universität Freiburg, Stefan-Meier-Straße 21, D-79104Freiburg, Germany

Core level X-ray absorption spectroscopy is a valuable tool to investigate the unoccupied localelectronic density of states with element specificity [1],[2]. Because of the local nature of inner shellexcitation, the chemical environment and bonding of individual chemical elements can be studied.This is a great advantage, offering the exciting prospect of studying the electronic properties ofindividual constituents in doped or binary clusters. In a first step towards this aim, mass resolvedsoft X-ray absorption of neutral, pure transition metal clusters was measured recently, using ion yieldspectroscopy at the Berlin synchrotron radiation source BESSY. Neutral transition metal clustersare produced in a magnetron source, ionized by soft X-ray radiation, and detected by time-of-flightmass spectroscopy. High mass resolution up to m/∆m = 1500 is used to record transition metalcluster mass spectra while scanning the photon energy across the L2,3 absorption edges.Relaxation of the core excited state via Auger decay leads to multiply charged cluster ions. Thecharge state distribution of cluster ions after soft X-ray absorption is analyzed as function of clustersize and excitation energy in the vicinity of the L2,3 edges. Charge states extend from 1+ to 6+ witha maximum around triply and quadruply charged cluster ions, in line with experimental results ontransition metal atoms. Photon energy dependent fragmentation of the cluster ions leads to apparentchanges in the observed cluster size distribution with varying photon energy.X-ray absorption lineshapes and branching ratios are almost bulk-like for medium sized clusters, whereas smaller clustersshow a richer structure in their absorption lines. The size evolution of L2,3 X-ray absorption will bediscussed for Tin, Vn, Con, and Nin clusters.

Figure 1: . Detail of a soft X-ray ion yield spectrum of neutral cobalt clusters with n/z =20-21, recorded at 780 eV photon energy. Contributions of clusters with different z state are clearlydiscernible. Dashed line/markers: data; solid line: fit to the data. All charge states contribute ton/z =20 and 21.

[1]O. Björneholm, F. Federmann, F. Fössing, and T. Möller, Phys. Rev. Lett. 74, 3017 (1995).[2]P. Piseri et al., New J. Phys. 8, 136 (2006).

78


Interplay of Electronic and Geometry Shell Closures forNeutral and Charged Strontium Clusters

Andrey Lyalin, Ilia Solov’yov , Andrey Solov’yov, and Walter Greiner


The optimized structure and electronic properties of neutral, singly and doubly charged strontiumclusters have been investigated using ab initio theoretical methods based on density-functionaltheory. We have systematically calculated the optimized geometries of those clusters consisting ofup to 14 atoms, their average bonding distances, electronic shell closures, binding energies per atom,dissociation energies and the gap between the highest occupied and the lowest unoccupied molecularorbitals [1]. It is demonstrated that the size-evolution of structural and electronic properties ofstrontium clusters is governed by an interplay of the electronic and geometry shell closures. It isshown that the excessive charge affects the optimized geometry of strontium clusters. Ionizationof small strontium clusters results in the alteration of the magic numbers. Stability of positivelycharged strontium clusters towards fission is analysed [1],[2]. The obtained results are comparedwith the available experimental and theoretical data.

(a) (b)

Figure 1: (a): binding energy per atom for the most stable neutral (filled squares), singly charged(filled circles) and doubly charged (filled triangles) Sr clusters; (b): monomer dissociation energiesand fission barriers for doubly charged strontium clusters.


[1]A. Lyalin, I. A. Solov’yov , A. V. Solov’yov and W. Greiner, submitted to Phys. Rev. A (2007).[2]A. Lyalin, A.V. Solov’yov, C. Bréchignac, and W. Greiner, J. Phys. B: At. Mol. Opt. Phys., 38, L129 (2005).[3]G.M. Wang, E. Blaisten-Barojas, A.E. Roitberg, and T.P. Martin, J. Chem. Phys. 115, 3640 (2001).

79


Calculations on AuN Cage Structures

Min Zhang

York University, Toronto, Canada

Recently, a lot of effort went into the study of the structures of AuN (neutral cluster and anion).There is evidence, from both experiment and theory, that gold clusters possess hollow cage-likestructures. Au16 and Au32 appear to have cage structure. In our research, we did global optimizationto find most favored structures for Au16 and Au32, including neutral clusters and anions. We usedplane-wave basis sets and PBE exchange-correlation functional implemented by VASP to evaluateenergies. Two optimization algorithms were used in our calculations:1. Simulated Annealing algorithm supplied by VASP;2. Tabu Search in Descriptor Space (TSDS) which was developed in our group. According to resultsobtained so far, the most stable structure for Au16 neutral cluster is flat-disc shaped, shown inpictures below. The current energy low-lying structures from our calculations already showed usvery interesting structures, some cage-like and others compact. We will calculate the photoelectronspectra for the optimized structures and compare them with the experimental results.

(a) (b)

Figure 1: (left) Top view of best structure of Au16 neutral cluster. (right) Side view of best structureof Au16 neutral cluster

80


Interplay between Theory and Experiment in StructureDetermination of Gas Phase Metal Oxide Clusters

Marek Sierka, Jens Döbler, Joachim Sauer

Institut für Chemie, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany

The interest in gas phase metal oxides clusters ranges from astrophysics to studies of elementarysteps in catalysis. Prerequisite is a detailed knowledge of the atomic structure of the systems,but its determination is a nontrivial task due to a large number of possible configurations. Veryoften structural models of gas phase clusters are found by intuition, but only in a few simple casesdefinitive structure assignments have been made. Here we present an approach for fully automaticdetermination of the atomic structure of gas phase clusters. This hybrid ab initio genetic algorithm(HAGA) is based on a global energy minimization using genetic algorithm in combination withdensity functional theory [1]. The efficiency of the approach is demonstrated for known systemssuch as gold and boron clusters containing up to 20 atoms. We also study the Al8O+

12 ion for whichan infrared spectrum by multi-photon dissociation was recently measured [1]. This spectrum showsbands up to 1050 cm−1, which is unusually high for aluminum oxide. Calculated spectra using thestructures for Al8O12 available in the literature are not compatible with the measured spectrum.Global energy minimization using HAGA approach yields a structure that was so far unknown andwhich does not resemble any of the known solid Al2O3 phases. Its calculated vibrational spectrum isin excellent agreement with the experimental data. In addition to the cation we have also calculatedthe global minimum of the neutral Al8O12 cluster. The structure is distinctively different from thecation, showing that the charge can have considerable impact on the structure of gas phase clusters.It is also noteworthy that the structure of the neutral cluster also differs from any of the knownsolid Al2O3 phases, so that the a-alumina cluster cutout often employed in theoretical studies doesnot correspond to the global minimum.

[1]M. Sierka, J. Döbler, J. Sauer, G. Santambrogio, M. Brümmer, L. Wöste, E. Janssens, G. Meijer, K. R. Asmis,Angew. Chem. Int. Ed. (2007) in press.

81


Core-Level Photoelectron Spectroscopy on Mass-SelectedLead Clusters at the VUV Free-Electron-Laser FLASH

Volkmar Senz1, Tim Fischer2, Patrice Oelßner1, John Neville3, Markus Schöffler4, JörgStanzel5, Heiko Thomas6, Matthias Neeb5, Josef Tiggesbäumker1, Michael Martins7, EckartRühl3, Christoph Bostedt6, Wolfgang Eberhardt5, Gerd Ganteför2, Thomas Möller6, Horst

Schmidt-Böcking4, Reinhard Dörner4, Wilfried Wurth7, Karl-Heinz Meiwes-Broer1

1. Insitut für Physik, Universität Rostock, Germany 2. Fachbereich Physik, Universität Kontanz,Germany 3. Physikalische und Theoretische Chemie, Freie Universität Berlin, Germany 4. Institut für

Kernphysik, Universität Frankfurt, Germany 5. BESSY Berlin, Germany 6. Institut für Optik undAtomare Physik, Technische Universität Berlin, Germany 7. Institut für Experimentalphysik, Universität

Hamburg, Germany

The electronic structure forms the basis for understanding and tailoring the optical, magnetic andchemical properties of clusters and has therefore attracted a lot of interest. The confirmation of theshell structure of simple metal clusters or the size-dependent chemical reactivity of metal clustersare some of the prominent issues in this field. Of particular importance is the correlation of theelectronic with the geometrical structure, which is hardly accessible by any other experimentaltechnique. A promising method to study this issue is core-level photoelectron spectroscopy of mass-selected clusters. However, for investigation of the complete valence band and shallow core levels,no photon source except the VUV free-electron-laser FLASH at HASYLAB/DESY is available atthe moment. Only this source provides the appropriate radiation of up to 95 eV with sufficient highphoton flux [1].First promising results, featuring a size-dependent lead 5d core-level shift, have been obtained duringthe last year and will be discussed.

[1]V. Ayvazyan et al., Eur. Phys. J. D 37, 297 (2006)

82


Geometric and Electronic Structure of Closed-shell BimetallicA4B12 clusters

Yan Sun1, Rene Fournier2

1. Department of Physics, York University, 4700 Keele Street, Toronto ,ON,M3J1P3 Canada2. Department of Chemistry, York University, 4700 Keele Street, Toronto ,ON,M3J1P3 Canada

We studied a group of 16-atom metallic clusters having general formula A4B12 by density functionaltheory, where A is divalent and B is monovalent. Global optimization [1] was done in each casefollowed by calculation of energy second derivatives and vibrational frequencies. The clusters havelarge HOMO-LUMO gaps ranging 1.2 to 2.6eV and other features suggesting special stability. Thisis consistent with jellium model and 20 electron count. A Td symmetry cage structure is foundas the putative global minimum for Mg4Ag12, Mg4Au12, Cd4Au12 and Ca4Na12. It is also a lowenergy isomer for Zn4Au12, Be4Au12 and Be4Ag12. The Td cage structure has ions arranged inshells, with charge +8 and +12, and the importance of symmetry and ion shells is shown by therelative stability of homotops of Mg4Ag12.

[1]J.Chen and R. Fournier, Theor. Chem. Acc. 112, 7(2004).

83


Density Functional Study of 13-atom metal Clusters

Yan Sun1, Rene Fournier2

1. Department of Physics, York University, 4700 Keele Street, Toronto ,ON,M3J1P3 Canada2. Department of Chemistry, York University, 4700 Keele Street, Toronto ,ON,M3J1P3 Canada

The lowest-energy structures of 4d transition-metal (from Y to Cd) clusters are studied at thesize n=13. The global optimization is performed by TSDS (Tabu Search in Descriptor space [1])with energy evaluated with Gaussian03 and VWN/B3LYP functional. We also studied the energydifference between our lowest-energy structures and the stable structures previously found such asthe buckled biplanar structure [2] and the close-packed icosahedral or cuboctahedral structures.

[1]J. Chen and R. Fournier, Theor. Chem. Acc. 112, 7(2004).[2]C. M. Chang and M. Y. Chou, Phys. Rev. Lett. 93, 133401(2004)

84


Binding energy and preferred adsorption sites of CO onsilver-gold cluster cations (AgmAu+

n , m+n<7): Kinetic studyand quantum-chemical calculations

Marco Neumaiera, Florian Weigenda, Oliver Hampea,b, and Manfred M. Kappesa,b

a Institut für Nanotechnologie,b Institut für Physikalische Chemie, Universität und Forschungszentrum Karlsruhe, D-76128 Karlsruhe,

Germany

Room temperature reactivity of carbon monoxide on trapped silver-gold alloy cluster cations (AgmAu+n ,

m+n<7) is studied in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer asa function of size and composition. Cluster sizes (m+n)<4 were found unreactive towards CO whilethe tetramers undergo loss of a neutral a gold or silver atom depending on composition. Thesereaction channels are in excellent agreement with exothermicities calculated by densitiy-functionaltheory methods leading to the most stable isomers. Cluster sizes 4<(m+n)<7 adsorb CO and themeasured absolute bimolecular rate constants are analyzed by means of a radiative association model(as recently described [1]) to give adsorption energies for the first CO binding to the alloy clustercations. These energies are found to decrease strongly with increasing silver content [2] in accordwith concomittant quantum-chemical calculations which also predict that CO prefers a gold atomas adsorption site. The general possibility of tuning the adsorption propensities (binding site andenergy) of typical σ-donor molecules to silver-gold nanoalloy clusters is discussed on the basis ofmolecular orbital theory and contrasted to previous calculations [3].

Figure 1: Binding energies of CO to silver-gold nanoalloy cluster cations (left); relative energies oflowest unoccupied molecular orbitals for penta-atomic AgmAu+

n as a function of silver content.

[1]M. Neumaier, F. Weigend, O. Hampe, and M. M. Kappes, J. Chem. Phys. 122, 104702 (2005); ibid. 123, 04990(2005).

[2]M. Neumaier, F. Weigend, O. Hampe, and M. M. Kappes, J.Chem. Phys. 125, 104308 (2006).[3]V. Bonačić-Koutecký, J. Burda, R. Mitrić, M. Ge, G. Zampella, and P. Fantucci, J. Chem. Phys. 117, 3120 (2002).

85


Far-infrared spectra of cobalt clusters

Philipp Gruene, Gerard Meijer, André Fielicke

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany

Metal clusters often exhibit physical and chemical properties that change substantially with clustersize and often differ from those of the bulk phase. One of the important ingredients for understand-ing this size-dependence is the knowledge of the geometrical and electronic structure of the clusters.Unfortunately, there has been a lack of experimental methods to address this problem. Recently,it has been shown that vibrational spectroscopy in combination with density functional theory cal-culations can provide information on the geometric structures of free metal clusters containing 3to more than 20 atoms.[1]-[3] In these studies, the experimental far-infrared spectra of free metalclusters are obtained by multiple photon dissociation (MPD) spectroscopy of their complexes withrare gas atoms.Cobalt clusters are chosen as a topic of investigation since they play an important role in the fieldof catalysis, being used for example in the Fischer-Tropsch synthesis, i.e. the catalytic process forobtaining hydrocarbons from CO and H2.The binding energy of argon to cobalt clusters Co+

n is strongly dependent on the cluster size n. Mea-surements of the equilibrium constant between the bare metal clusters and their rare-gas complexesas a function of temperature allow for an estimation of the bond dissociation energies (BDE). Valuesof 0.17 - 0.22 eV are found for n = 3 - 5, while the BDEs for larger clusters are significantly lower.The high BDEs for Co3−5Ar+ make IR-MPD spectroscopy possible even at elevated temperaturesof 70 °C. Spectra in the range between 100 cm−1 and 250 cm−1 are presented.At a temperature of -180 °C also bigger clusters form metal-rare gas complexes, while in the lowermass regime the mass spectrum is dominated by small cluster complexes containing up to 6 argonatoms. The IR-spectra for each cluster size are unique and clearly distinguishable from each other.With increasing argon coverage the relative absorption cross sections of the complexes increase atthe cost of a decreased spectral resolution. The spectral range covered for these systems extendsfrom 80 cm−1 to 400 cm−1.

Figure 1: IR-MPD spectra of Co5Ar+n (n = 1, 4, 5)

[1]A. Fielicke, A. Kirilyuk, C. Ratsch, et al., Phys. Rev. Let. 93, 023401 (2004).[2]A. Fielicke, C. Ratsch, G. von Helden, et al., J. Chem. Phys. 122, 091105 (2005).[3]A. Fielicke, G. von Helden and G. Meijer, Eur. Phys. J. D. 34, 83 (2005).

86


Spectroscopy on magnesium in superfluid helium droplets

Sebastian Göde, Andreas Przystawik, Josef Tiggesbäumker, K.-H. Meiwes-Broer

Institute of physics, University of Rostock, Universitätsplatz 3, 18051 Rostock, Germany

Superfluid helium droplets serve for controlled and selective cluster growth and allow spectroscopicmeasurements near absolute zero temperature, i.e. at 0.4 K [1]. The interaction of the heliumenvironment with a foreign particle is quite sensitive to its electronic properties and might definethe structure of the complex.We have measured the excitation spectra of the 3s3p←3s2 transition of Mg atoms solvated in heliumnanodroplets with resonant two-photon-ionization under various source conditions. The signal ofMgHe50 snowballs are analyzed in order to extract the optical properties. Figure 1 shows the blueshift and line broadening of the atomic absorption line which mirror the recent LIF experiments innanodroplets and liquids [2], [3]. In [2] the apparent splitting in the spectra was attributed to aquadrupole-like deformation of the cavity around the solute atom after excitation.In contradiction to this model our measurements show that the intensity of the two componentsstrongly correlates with the pick-up density in the scattering cell, indicating that a more complexmechanism is involved. For highly doped droplets the high energy component in the absorptionspectra is strongly suppressed whereas for a size distribution with a mean number of one atom perdroplet the low energy one is reduced. The latter can be attributed to a resonant excitation ofsmall weakly bound magnesium clusters. The experimental findings can be explained by a novelformation process of these complexes assisted by the superfluid properties of the ultra cold matrix.

Figure 1: Absorption spectrum of the 3s3p←3s2 transition of magnesium measured at the MgHe50

ion signal. The spectrum shows two distinct maxima whereas there relative intensities stronglydepend on the pick-up density in the scattering cell.

[1]M. Hartmann, R. E. Miller, J. P. Toennies, and A. Vilesov, Phys. Rev. Lett. 75, 1566 (1995).[2]Reho et al., J. Phys. Chem. 104, 3620-3626 (2000).[3]Y. Moriwaki and N. Morita, Eur. Phys. J. D 5, 53-57 (1999)

87


Characterization of the Dimerization Effect in Small CrNClusters by Theoretical Photoabsorption Spectroscopy

J. I. Martínez1,2, J. A. Alonso1

1. Departamento de Física Teórica, Atómica y Optica, Universidad de Valladolid, E-47001 Valladolid,Spain

2. Institut für Theoretische Physik, Freie Universität Berlin, D-14195 Berlin, Germany

The photoabsorption spectrum of CrN (N = 2-11) clusters have been calculated using time-dependentDensity Functional Theory (TDDFT) with a real-space and real-time scheme [1]. Two different ap-proximations have been employed for exchange and correlation effects: the adiabatic local densityapproximation (ALDA) and the self-interaction correction (SIC). The small chromium clusters showpeculiarities that make them special compared to clusters of other 3d metals [2], in particular adimerization effect that controls the initial growth of CrN clusters up to N = 11. This effect is dueto the formation of robust closed-shell Cr2 dimers with a strong bond and an unusually short bondlength.

The calculated spectra in the optical region (0 - 5 eV) for a few representative clusters in this sizerange are shown in the Figure. The corresponding geometrical structures are also included. Thehigh energy part of the absorption spectra (not shown in the Figure) characterizes the dimerizationeffect: an excitation peak appears at around 20 eV, and its intensity increases each time a new Crdimer forms in the structure as the cluster grows. This effect is not tied to any other clusteringeffect and is only associated to Cr2.

Figure 1: Calculated photoabsorption cross sections for representative CrN clusters in the range N= 2 - 11.

[1]M. A. L. Marques, A. Castro, G. F. Bertsch and A. Rubio, Comp. Phys. Comm. 151, 60 (2003).[2]J. A. Alonso, Chem. Rev. 100, 637 (2000).

88


Theoretical Study of Coalescence of Ni nanoparticles

Won Ha Moon, Hee Su Kim, Chul Tack Lim and Chang Hwan Choi

Central R & D Institute, Samsung Electro-Mechanics Co., Ltd., Suwon, 443-743 KOREA

The present work investigates the coalescence process of nickel nanoparticles for a host of initialtemperatures and starting radii in vacuum with the help of molecular-dynamics (MD) simulationsbased on the embedded atom potential method [1]. The shell-closed icosahedron nickel nanoparticlescould only be preferred until 923 atoms at temperatures no higher than 1380 K, which is in agreementwith the experiments. Figure 1 shows the optimized structure of the nickel nanoparticle with 1415atoms.

Figure 2 shows that the structural evolution of Ni nanoparticles during coalescence at 800 K. Themelting temperature of larger clusters would depend on their starting structures, which can be at-tributed to surface premelting. Dependence of total energy per atom and cluster size with differentatoms on temperature is shown in Fig. 3. With no exception for larger particles, the melting evo-lution undergoes a surface premelting stage and an overall melting stage before the transformationto liquid phase, and the melting points are significantly lower than that of bulk Ni.Diverse mechanisms of the first sintering stage, characterized by a growing neck region, were found.

[1]D. Schebarchov and S. C. Hendy, Phys. Rev. Lett., 95, 116101, (2005).

89


Structures of small noble metal cluster ions

A. Lechtken1, D. Schooss1, J. R. Stairs1, F. Furche2, O. Kostko3, N. Morgner3, B. v.Issendorff3, and M. M. Kappes1,2

1. Institute of Nanotechnology, Forschungszentrum Karlsruhe,D-76021 Karlsruhe, Germany2. Institut für Physikalische Chemie, Universität Karlsruhe, D-76128 Karlsruhe, Germany

3. Fakultät für Physik, Universität Freiburg, D-79104 Freiburg, Germany

The structures of small noble metal cluster ions have been studied by the recently developed tech-nique of trapped ion electron diffraction (TIED) [1]. The assignment of the structures or the struc-tural motif is done via comparison of the experimental and simulated scattering function, calculatedfrom density functional theory structure calculations. We present a comparison of the structures ofCu±20, Ag±20 and Au±20. Our findings show unambiguously that the structure of Au±20 is predominantlygiven by a tetrahedron in agreement with the results of Wang et al. [2]. In contrast, structures ofAg±20and Cu±20 based on the icosahedral motif agree best with the experimental data. The struc-tures of gold clusters ions Au±n in the size range n = 11-19 have been investigated. While most ofthe anionic data is in agreement with structures found before [3], a slightly different structure issuggested for Au−14.

Figure 1: The chiral (C3) structure of Au−34(side view left, top view right). The darker atoms areinternal atoms

Finally the structure of Au34- is presented (see Fig. 1). Based on TIED and photoelectron data achiral structure with C3 point group symmetry is found in agreement with (time dependent) densityfunctional calculations [4].

[1]M. Maier-Borst, D. B. Cameron, M. Rokni, and J. H. Parks, Physical Review A 59, R3162 (1999), D. Schooss,M.N. Blom, B. v. Issendorff, J. H. Parks, and M.M. Kappes, Nano Letters 5, 1972 (2005).

[2]J. Li, X. Li, H. J. Zhai, and L. S. Wang, Science 299, 864 (2003).[3]F. Furche, R. Ahlrichs, P. Weis, C. Jacob, S. Gilb, T. Bierweiler, M. M. and Kappes, J. Chem. Phys. 117, 6982

(2002); S. Bulusu, X.Li, L.C.Wang, and X. C. Zeng PNAS, 103, 8326 (2006); X. Xing, B. Yoon, U. Landman, andJ. Parks, Phys. Rev. B 74, 165423 (2006); B. Yoon, P. Koskinnen, B. Huber, O. Kostko, B. v. Issendorff, H.Häkkinen, M. Mosler, and U. Landman, ChemPhysChem 8, 157 (2007).

[4]A. Lechtken, D. Schooss, J. R. Stairs, M. N. Blom, F. Furche, N. Morgner, O. Kostko, B. v. Issendorff, and M. M.Kappes, Angew. Chem., Int. Edt. (in press)

90


Structures of small platinum clusters

E. Oger, Th. Bierweiler, P. Weis, M. M. Kappes

Institut für Physikalische Chemie, Universität Karlsruhe, Germany

The cross sections of small cationic platinum clusters Pt+n (n = 1..9) have been studied using ionmobility measurements [1].The clusters are generated by pulsed laser vaporization and mass separated by means of time-of-flight (TOF) mass spectrometry. All unwanted ions are removed by a pulsed mass gate. The massselected clusters are decelerated and injected into a helium filled drift cell. Their cross section isdetermined by the time it takes for them to drift through the cell guided by a static electrical field.The clusters are analyzed by a quadrupole mass filter to preclude registration of any fragment ionsand detected by a channeltron secondary electron multiplier. To determine the geometry of the clus-ters, we compare the experimental cross sections with values derived from model structures from abinitio theory (density functional theory (DFT) and Møller-Plesset perturbation theory (MP2)).Three-dimensional structures have been found for platinum clusters as small as n ≥ 4 . For com-parison cationic clusters of gold, the neighbour in the periodic table of elements, are planar up ton = 7 [2]. In previous work Weis et al. found that the structure of silver cations are planar only forsizes 3 and 4 [3].

[1]Patrick Weis, Stefan Gilb, Philip Gerhardt, and Manfred M. Kappes, Int. J. Mass Spectrom., 216, 59-73 (2002).[2]Stefan Gilb, Patrick Weis, Filipp Furche, Reinhart Ahlrichs, and Manfred M. Kappes, J. Chem. Phys., 116,

4094-4101 (2002).[3]Patrick Weis, Thomas Bierweiler, Stefan Gilb, and Manfred M. Kappes, Chem. Phys. Lett, 355, 355-364 (2002).

91

Methods and Machines


Photon-Trap Spectroscopy for High-Sensitivity OpticalMeasurements of Trapped Ions and Surface Adsorbates

Akira Terasaki1, Takuya Majima2, Kazuhiro Egashira2, Tamotsu Kondow1

1. Cluster Research Laboratory, Toyota Technological Institute, in East Tokyo Laboratory, GenesisResearch Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan

2. East Tokyo Laboratory, Genesis Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001,Japan

We present ’photon-trap spectroscopy’ developed for high-sensitivity optical absorption and polar-ization measurements of a trace amount of atoms, molecules, clusters, and their ions in the gas,liquid, and solid phases. The absorption measurement is based on a storage lifetime of photons ina high-finesse optical cavity; the lifetime is free from intensity fluctuation of a light source, whichlimits the sensitivity in conventional transmission spectroscopy. It is operated not only by a pulsedlight, known as cavity-ring-down spectroscopy [1], but also by a cw light; the former is describedby the time-domain picture with a light pulse traveling back and forth in the cavity, whereas thefrequency-domain picture is more appropriate in the latter to deduce a lifetime from the frequencywidth of the cavity-resonance profile, which enables the analysis even for a cavity containing a solidsubstrate [2]. The measurement is sensitive to polarization as well [3], allowing studies of magneto-optical phenomena. The photon-trap spectroscopy thus unifies all these features of the techniqueusing photons trapped in a cavity.Application to mass-selected trapped ions We have developed an apparatus combining an ion trapwith an optical cavity. It is designed so that cross sections of ∼ 10−16 cm2 (relatively strong elec-tronic transitions) are detectable with 1ppm absorption sensitivity. Mass-selected cluster ions wereadmitted to a 40-cm linear octopole ion trap, which stored ∼109 ions and was located in a 1.6-mcavity. Absorption spectra are presented for the manganese ion, Mn+, in the vicinity of 260 nm,showing 7P2,3,4 ←7S3 transitions with hyperfine structures. The measurements were performedunder a magnetic field up to 3 T as well to observe the Zeeman splitting and the Faraday rotation.Cluster experiments are in progress to achieve ’direct’ measurements of absorption without usingphotodissociation/depletion processes. Small manganese cluster ions are to be explored under amagnetic field to elucidate their ferromagnetism [4].Application to solids The present technique is applicable to solids, provided that a substrate-formsample with optically flat surfaces is placed exactly normal to the cavity axis. Frequency-domainanalyses of the properties of this cavity revealed the importance of optical phases in optimizing thesignal intensity and the noise level of lifetime measurements [2]. A vibrational spectrum is presentedof an alkylsiloxane monolayer adsorbed on a transparent silicon substrate as measured by using atunable mid-infrared cw light source. It exhibited a clear optical-phase effect; the absorbance wasmaximized when the monolayer was adjusted at the anti-nodes of a standing wave, while it waspractically zero at the nodes. This experimental technique is to be applied to studies of reactionprocesses of molecules on deposited clusters.

[1]A. O’Keefe and D. A. G. Deacon, Rev. Sci. Instrum. 59, 2544 (1988).[2]A. Terasaki, T. Kondow, and K. Egashira, J. Opt. Soc. Am. B 22, 675 (2005).[3]R. Engeln, G. Berden, E. van den Berg, and G. Meijer, J. Chem. Phys. 107, 4458 (1997).[4]A. Terasaki et al., J. Chem. Phys. 114, 9367 (2001); ibid. 118, 2180 (2003).

95


A Novel Method for Measuring Clusters Caloric Curves

Fabien Chirot, Sébastien Zamith, Pierre Labastie, Jean-Marc L’Hermite

Laboratoire Collisions, Agrégats, Réactivité (UMR 5589, CNRS - Université Paul Sabatier), IRSAMC,31062 Toulouse Cedex 9, France

Melting of small clusters has received a considerable interest since the pioneer experimental workof Haberland et al who found that the melting temperature of very small particle show a non-monotonical behaviour. This temperature is found either lower (for the majority of clusters, forinstance sodium [1]) or higher (for tin and gallium [2]) than in the bulk. Moreover, both the meltingtemperature and the latent heat of fusion can vary considerably from one size to another.To our knowledge, only three experimental methods have been used successfully up to now tomeasure the caloric curve of clusters. In all the experiments, clusters are first thermalized in a heatbath.In the first two methods [1], [2] a known temperature shift ∆T is compensated by a known energyshift E in order to observe the same decay rate (i.e. the same internal energy), and then deduce theheat capacity ∆E/∆T. The energy is brought either by a laser [1] or by collisions [2].The third method measures the mobility of clusters in a drift tube, from which collision cross sectionsare deduced. The phase transition from liquid to solid is identified thanks to a variation in the crosssection [3].We propose here a novel method based on an original experimental setup [4] to measure the caloriccurve of clusters. The basic idea is to count the number of atoms that can be stuck onto a clusteras a function of its initial temperature. At each sticking, the internal energy of the cluster Mn isincreased by a known energy. After a given number of sticking collisions nmax the cluster is soheated that it dissociates before it can stick again. nmax is related to the dissociation time, thus is ameasure of the internal energy. As in the first two experiments mentioned above, we can constructthe caloric curve by varying T.Our method does not require laser excitation, is easily transferable to many systems and it is modelfree.We present here our first experimental results obtained with sodium clusters. Our results confirmthe ones of Haberland et al and extend them towards small sizes.

Figure 1: Comparison of our results (squares) with the one of Haberland et al (circles) [1].

[1]H. Haberland et al, Phys. Rev. Lett. 94, 035701 (2005).[2]G.A. Breaux et al, Phys. Rev. Lett. 91, 215508 (2003).[3]G.A. Breaux et al, J. Phys. Chem. B, 109, 16575 (2005).[4]F. Chirot et al, Rev. Sci. Instrum. 77, 063108 (2006).

96


An experimental setup for fluorescence spectroscopy oftrapped ions

Mattias Kordel1, Detlef Schooss1, Lars Walter1, and Manfred M. Kappes1,2

1. Institute of Nanotechnology, Forschungszentrum Karlsruhe,D-76021 Karlsruhe, Germany2. Institut für Physikalische Chemie, Universität Karlsruhe, D-76128 Karlsruhe, Germany

We have completed the setup of a new experiment to study the fluorescence properties of trappedclusters and nanoparticles in gas phase. Gas phase measurements can help to reveal the dependenceof optical properties on size, charge, surface passivation and chemistry. The main advantage offluorescence experiments in gas phase is the total control about environmental effects which otherwisemight superimpose the intrinsic qualities of the system under investigation. In contrast to molecularbeam methods the trapping of charged particles in a Paul trap allows for a long sampling time, whichis critical at low signal levels.

Figure 1: Schematic setup of the TILIF apparatus.

Our apparatus comprises an electrospray ionization source (ESI), a quadrupole bender to changethe direction of the ion beam, a time of flight mass spectrometer (TOF), and the main chamberwith the Paul trap and the optics for inducing and detecting the fluorescence. The cluster ionsare excited by an Argon Ion Laser perpendicular to the axis of light detection. Resulting fluores-cence photons are collected in a photon counting photomultiplier tube (PMT), or dispersed using amonochromator/CCD camera.As proof of principle, dispersed fluorescence measurements of trapped dye ions (Rhodamine 6G,Rhodamine B, and Rhodamine 101) are presented and compared to solution spectra.

97


Phase, amplitude, and polarization shaped pulses for optimalcontrol on molecules

Fabian Weise, Stefan M. Weber, Mateusz Plewicki and Albrecht Lindinger

Institut für Experimentalphysik, Freie Universität Berlin, Arnimalle 14, 14195 Berlin, Germany

We present two novel pulse shaper designs, which are capable of manipulating the phase, the ampli-tude and the polarization of femtosecond laser pulses independently and simultaneously [1] [2]. Thecapabilities of the polarization modulation with the serial and the parallel setup are demonstratedand the limitations are discussed.Furthermore, we transform the three dimensional electrical field into the obvious parameters subpulse energy, position in time, zero order phase, chirps as well as the polarization states ellipticity,major axis angle, and helicity. These parameters are the basis of the shown tailored example pulses,which are retrieved by measuring a set of cross-correlations and spectra [3]. Implementing the serialpulse shaper in a feedback loop raises the level of adaption in coherent control experiments. As a testof this method it is applied on the second harmonic generation. The advantage of manipulating allthree parameters of the electrical field is shown in the optimization of the multi-photon ionization ofNaK in a molecular beam. The obtained pulse shape enhances the ionization yield greatly in com-parison to traditional optimizations where only the scalar parameters phase and amplitude havebeen employed. Moreover, the recorded optimized pulseform reveals the particular optimizationpath via the selected exited states [4].

Figure 1: Serial pulse shaper setup, which is able to shape pulses in phase, amplitude, and polar-ization. Double pulse with a linear and circular sub-pulse with an amplitude modulation patterncorresponding to the sum-intensity.

[1]M. Plewicki, S. M. Weber, F. Weise, and A. Lindinger, Appl. Phys. B. 86, 259 (2007).[2]M. Plewicki, F. Weise, S. M. Weber, and A. Lindinger, Appl. Opt. 45, 8354 (2006).[3]S. M. Weber, M. Plewicki, F. Weise, and A. Lindinger, Phys. Rev. A., submitted[4]F. Weise, S. M. Weber, M. Plewicki, and A. Lindinger, Chem. Phys., in press (2007).

98


New setup for photoelectron spectroscopy of mass selectedclusters: high mass range and resolution, high photon energy

and high energy resolution

Tim Fischer, Matthias Götz, Jörn Cordes, Karsten Vetter, and Gerd Ganteför

Department of Physics, University of Konstanz, 78457 Konstanz, Germany

New materials for catalysis and hydrogen storage attract more and more attention. For instance,experiments on reactions of gold clusters depict the diverse chemistry for clusters consisting of upto 70-80 atoms [1]. Studies on metal sulfide clusters shed light on the underlying mechanisms oftheir catalytic activity [2]. Furthermore, hydrogen is considered to be one of the most importantfuture energy sources. The main technological problem of a viable hydrogen economy is its storage;clusters may represent a possible answer [3].To reach the fundamentals of chemistry concerning clusters itself, photoelectron spectroscopy (PES)in the gas phase provides an excellent approach. To get a deeper insight into the clusters the completeconduction band should be investigated [4]. Moreover, bridging the gap towards nanoparticles isstill a prospective ambition for cluster research. Investigating adsorbates like hydrogen and oxygenon relatively heavy clusters is only possible, using an intense ion beam and a mass resolution beyondm/δm = 1000.In this contribution, we present an improved setup (fig. 1) consisting of a pulsed arc cluster ionsource, a reflectron, an interaction chamber and a F2-Laser (7.9 eV). Due to the relation m/δm= s/δs for a time-of-flight spectrometer with reflectron the resolution rises with increasing length.Accordingly, the prolongation of the drift length to 12 m enables a higher mass resolution. Firstresults reach a resolution of m/∆m = 1400 (fig. 2). In addition, the interaction chamber wasimproved to achieve background-free PES measurements even using a F2-Laser.

Figure 1: (a) New experimental setup: (I) PACIS; (II) TOF tubes; (III) Reflectron; (IV) Interactionchamber; (V) F2 Laser Small picture: Improved interaction chamber→ should allow background-freemeasurements. (b) Mass spectrum after the improvements. The mass resolution advanced from 400to 1400.

[1]H.-G. Boyen, G. Kästle, F. Weigl, B. Koslowski, C. Dietrich, P. Ziemann, J. P. Spatz, S. Riethmüller, C. Hartmann,M. Möller, G. Schmid, M. G. Garnier, P. Oelhafen, Science 297, 1533 (2002)

[2]J. Kibsgaard, J. V. Lauritsen, E. Lægsgaard, B. S. Clausen, H. Topsøe, and F. Besenbacher, JACS 128, 13950(2006)

[3]Q. Sun, Q. Wang, P. Jena und Y. Kawazoe, JACS 127, 14582 (2005)[4]S. Gemming, J. Tamuliene, G. Seifert, N. Bertram, Y. D. Kim, G. Ganteför, Appl. Phys. A 82, 162 (2005)

99


Metastable Impact Electron Spectroscopy (MIES) of sizeselected metal clusters

C.J. Harding, V. Teslenko, M. Röttgen, M. Arenz and U. Heiz

Lehrstuhl für Physikalische Chemie 1, Technische Universität München, Lichtenbergstr. 4, D-85748Garching, Germany

The formation of a beam of helium metastable atoms can be used as an extremely sensitive andnon-destructive probe of surfaces and concomitant adsorbates. This type of spectroscopy (MIES) isperformed through the detection of energy resolved electrons released through Auger-like processescaused by the interaction of the metastables with surface based systems. MIES was used, in parallelwith FTIR and pulsed molecular beam methods (p-MBRS) to probe model catalysts. The modelcatalysts were made by depositing mass selected and mass distributed Pd or Au clusters onto thinfilms of MgO. Firstly, MIES was used to characterise the MgO substrate. Following this, thedetection limit was tested using various mass distributed gold cluster coverages. Finally, MIES wasused to investigate the presence of an oxide intermediate in CO oxidation reactions involving massselected Pd13 clusters, in parallel with other surface based techniques.

100


Strong Field Excitation of Metal Clusters and Fullerenes withEUV Free Electron Laser Pulses

Johannes Passig1, Tilo Döppner1, Paul Radcliffe2, Volkmar Senz1, Slawomir Skruszewicz1,Hubertus Wabnitz2, Josef Tiggesbäumker1 and Karl-Heinz Meiwes-Broer1

1. Institut für Physik, Universität Rostock, Universitätsplatz 3,18051 Rostock, Germany2. HASYLAB at DESY, Notkestrasse 85, 22607 Hamburg, Germany

We describe in detail an experimental system which has been designed to study the strong fieldinteraction of extreme ultra-violet (EUV) light with nanoparticles at the free electron laser facilityFLASH (DESY, Hamburg). In addition to the direct photoionization of the atoms, the high intensityof the FEL pulses may lead to cooperative effects in the ionization process, e.g. field-assistedionization and enhancement by many body effects [1],[2]. Here we present first results of time-of-flight experiments on ions emitted by lead clusters and fullerenes exposed to the EUV-FEL pulsesat a fundamental wavelength of 32 nm (38.5 eV). For lead, the spectra exhibit a broadening andsplitting of the ion signal indicating a Coulomb explosion of the clusters. The results for C60 showsignals of highly charged fullerenes and carbon fragments.

Figure 1: Time-of-flight-spectrum of lead ions and small clusters emitted by lead nanoparticles(N∼2000) exposed to intense EUV laser light. The spectrum is averaged over 2000 FEL pulses.Singly charged fragments of the particles can be detected up to N=8. No atomic ions with chargestates larger than two can be identified. The signals of Pb+ and Pb++ show a clear splitting indicatingCoulomb explosion of the particles.

[1]U. Saalmann and J.M. Rost, Phys. Rev. Lett. 89, 143401 (2002).[2]T. Döppner et al., Phys. Rev. Lett. 94, 013401 (2005).

101


High Yield Cluster Source Combining Magnetron Sputteringand Aggregation Techniques

Juri Demuth, Ewald Janssens, Shaohui Li, Torsten Siebert, Ludger Wöste

Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany

High particle density is generally favorable for optical spectroscopy of clusters. In order to meetthis requirement, a cluster source combining magnetron sputtering and gas aggregation has beenconstructed for the application in femtosecond time-resolved NeNePo (negative ion-to-neutral-to-positive ion) spectroscopy with a design inspired by previous work [1],[2]. The advantage of thiscluster source in comparison with alternative methods such as laser vaporization and electrosprayis a high and continuous yield of ionized clusters in the size range of interest (n = 1-10). Generally,the method of magnetron sputtering is an advanced sputtering technique, which is prevalent in thinfilm production. The ionization of the sputtered clusters takes place through sputtered secondaryelectrons, which are forced on spiral trajectories by a magnetic field. Inside a liquid nitrogen cooledand helium filled chamber, cluster growth takes place through three-particle- and cluster-cluster-collisions. The general design of the constructed magnetron sputter source is presented in thiswork, together with a discussion of various parameters such as voltage and discharge currency of thesputter source, Argon / Helium - partial pressure, temperature and nozzle size of the aggregationchamber for attaining clusters with the desired attributes. Furthermore, the application of thissource to the study of nuclear dynamics in silver oxides and the controlled optical preparation ofreactive structures by means of femtosecond time-resolved NeNePo will be discussed [3].

Figure 1: Design drawing of the constructed magnetron sputter source. Part A Torus sputter source;Part B source mount; Part 1 flange with feedthrough for liquid nitrogen, Helium and Argon; Part2 vacuum chamber; Part 4 cooled aggregation chamber; Part 6 front plate with nozzle at the end ofthe aggregation zone.

[1]H. Haberland, M. Karrais, M. Mall, Z.Phys.D 20, 413-415 (1991).[2]S. Pratontep, S.J. Carroll, C. Xirouchaki, M. Streun R.E. Palmer, Rev. Sci. Instrum. 76, 045103 (2005).[3]T. Leisner, S. Vajda, S. Wolf, L. Wöste, J. Chem. Phys., 111, 1017-1019 (1999).

102


Structure of Medium-Sized Sodium Clusters

Oleg Kostko1, Bernd Huber2,3, Michael Moseler2,3, Bernd von Issendorff1

1. Fakultät für Physik, Universität Freiburg, Hermann-Herder-Straße 3, 79104 Freiburg, Germany2. Fraunhofer Institut für Werkstoffmechanik, Wöhlerstraße 11, 79108 Freiburg, Germany3. Freiburg Materials Research Center, Stefan-Meier-Strasse 21, 79104 Freiburg, Germany

The properties of nanoparticles and clusters are dominated by electronic and geometric finite size ef-fects, that is by the discretization of the electronic density of states and the surface induced tendencytowards nonbulklike crystalline structures. Therefore, the knowledge of electronic and geometricalstructures of the clusters is the basis for an understanding of more complex effects in small parti-cles. But the information about structures in the intermediate size range (1-10 nm) is scarce. Forsodium clusters, there exists some information about small clusters, obtained from photoelectronspectroscopy and DFT calculations [1], and about large clusters, obtained from mass spectrometry[2]. Here, by a combined photoemission and density functional theory study we are assigning forthe first time building principles of medium-sized sodium clusters.

The growth pattern of all clusters studied here favor icosahedron-based geometry. Surprisingly,the assigned geometrical structures of small (20<N<60) sodium cluster anions follow quite welljellium model predictions, as is evident from a calculation of the cluster’s radii of inertia. In thisrespect the electronically magic clusters Na−39 and Na−57 are close to perfectly spherical, whereasintermediate clusters exhibit a change of geometrical shape from prolate (which appears immediatelyafter electronic magic number) to oblate (appearing closer to the next electronic magic number).Bigger sodium clusters between complete Mackay icosahedra (Na−55, Na−147, and Na−309) are formedby addition of twisted Mackay overlayers as predicted by Murrell-Mottram potential calculations[3], leading to prominent subshell closings at sizes 71, 92, 178, 206, and 216 [4].

[1]M. Moseler, B. Huber, H. Häkkinen, U. Landman, G. Wrigge, M. Astruc Hoffman, B.v. Issendorff, Phys. Rev. B68, 165413 (2003).

[2]T.P. Martin, T. Bergmann, H. Göhlich, T. Lange, Z. Phys. D 19, 25 (1991).[3]E.G. Noya, J.P.K. Doye, D.J. Wales, arXiv :cond-mat/0506329.[4]O. Kostko, B. Huber, M. Moseler, B.v. Issendorff, Phys. Rev. Lett. 98, 043401 (2007).

103


Molecular Dynamics for Fission Processes

O. I. Obolensky, A. Lyalin, I. A. Solov’yov, A. Yakubovich, E. Henriques, A. V. Solov’yov,W. Greiner


Fission is a ubiquitous process playing important roles in many physical phenomena. In fission, theinitial and final states of a system are separated by a potential barrier, the so-called fission barrier.The height and the shape of this barrier determine most of the experimental observables of theprocess, such as characteristic fission times and branching ratios between different fission channels.We discuss relative advantages and disadvantages of two general approaches which are used for car-rying out a molecular dynamics analysis of fission processes [1]. The goal of both approaches consistsin finding the optimum fission pathway which minimizes the fission barrier. The optimum fissionpathway frequently involves complicated rearrangements of the ionic structure including formationof intermediate metastable configurations [2] which makes this problem rather difficult.

The first approach to the molecular dynamics analysis is the conventional molecular dynamics sim-ulations which emulate the evolution of a system with time [3],[4]. This approach relies on theassumption that the system explores sufficiently large volumes of its phase space (thus coveringsufficiently large areas of the potential energy surface) if a sufficiently large number of runs of thesimulations are made and each run is sufficiently long. In order to shorten the required CPU timeone usually assigns rather high initial temperatures to the system. In such a case, however, thesystem tends to fission along the shortest path rather than along the path with the lowest potentialbarrier. Therefore, one has to find the optimum balance between the reliability and the feasibilityof simulations.

The second approach consists in a direct (manually- or algorithmically-driven) exploration of thepotential energy surface of the system [5]. This approach allows one to perform a systematic analysisof the various fission scenarios, reducing the chance of an accidental omission of an important fissionpathway. We review one of the possible algorithms for exploration of the potential energy surface[2],[5] and give examples of its applications to fission processes in atomic clusters, bio- and macro-molecules.We acknowledge partial financial support from INTAS (contract 03-51-6170) and from EuropeanCommission (project EXCELL).

[1]A. Lyalin, O.I. Obolensky, A.V. Solov’yov, W. Greiner, Int. J. Mod. Phys. E, 15, 153 (2006)[2]O.I. Obolensky, A.G. Lyalin, A.V. Solov’yov, W. Greiner, Physical Review B, 72, 085433 (2005) and references

therein[3]P.-G. Reinhard and E. Suraud, Introduction to Cluster Dynamics (Wiley-VCH, Weinheim, 2004).[4]F. Baletto and R. Ferrando, Rev. Mod. Phys., 77, 371 (2005)[5]O.I. Obolensky, I.A. Solov’yov, A.V. Solov’yov, W. Greiner, Computing Letters, 1, 313 (2005)

104


Noble Metal Cluster Reaction Kinetics in a New Ion Trap -Reflectron ToF-Mass Spectrometer Setup

Sandra M. Lang, Thorsten M. Bernhardt

1. Insitut für Oberflächenchemie und Katalyse, Universität Ulm, 89069 Ulm, Germany

About two decades ago the fascinating and unexpected catalytic properties of supported clustersof gold and silver have been discovered. In the meantime it was shown that not only dispersedgold nanoparticles deposited on oxide supports but also small gold particles in the gas phase exhibitcatalytic properties in the CO oxidation reaction. However, the reaction mechanism and the devel-opment of catalytic cycles is not always understood in detail. For a detailed study of the mechanismof molecular processes as well as the influence of different parameters as the charge state and thesize of the cluster, humidity, and temperature isolated clusters in the gas phase represent an idealmodel system.So far the clusters are produced by an ion sputtering source (CORDIS), transferred and thermal-ized by a helium filled quadrupole and mass selected by a further quadrupole. The clusters arethen transferred to an octopole ion trap, which can be cooled down to 40K. Here the cluster beamis in contact with reactive gases. The reaction products are then analyzed in a last quadrupolemass spectrometer. For a fast and high resolution detection of the reaction products an additionaltime-of-flight mass spectrometer was recently inserted. The combination of an octopole ion trap anda quadrupole mass analyzer with the new time-of-flight mass spectrometer is discussed in detail.Furthermore, the setup of a new magnetron cluster source to obtain clusters with up to 4000 amuis shown.

105


Configuration interaction for large scale vibrationalcalculations

David Benoit and Yohann Scribano

Theory Group -SFB 569, University of Ulm Albert-Einstein-Allee 11, Ulm, D-89081 Germany

The computation of the thermal properties of nanosystems (e.g. heat conduction) or the simulationof infrared spectra of polyatomic molecules both require an accurate theoretical description of thevibrational excited states of the system. However, the most widespread theoretical method for theprediction of vibrational frequencies - the harmonic approximation - is not able to accurately repro-duce the dynamics of polyatomic molecules far from their equilibrium geometry. In our approach,we use a direct vibrational self-consistent field (VSCF) scheme to obtain the vibrational wave func-tion. This scheme is also able to treat larger molecular systems for which exact theoretical methodscannot be used.

We present a description of vibrational excited states based on a configuration-interaction (VCI)approach within the VSCF technique. We also introduce a new approach that reduces the compu-tational cost of VCI calculations for excited states of large systems and examine the convergencebehaviour of VCI-based methods for molecular systems such as H2O, NH3, CH4 and HF on graphite.

106

Molecular Clusters


Evidence for mode specific fragmentation in the infraredspectrum of NH+

4 (H2O)

Anita Lagutschenkov1, Tobias Pankewitz1, Sotiris S. Xantheas2, Yuan-Tseh Lee3, GereonNiedner-Schatteburg1

1. Physikalische Chemie, Technische Universität Kaiserslautern, Erwin-Schrödinger-Straße, 67663Kaiserslautern, Germany

2. Chemical Science Division, Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box999, MS K1-83, Richland, Washington 99352, USA

3. Institute of Atomic and Molecular Science, Academia Sinica, P.O. Box 23-166, Taipei, Taiwan 106,Republic of China

We present the gas phase infrared (IR) spectrum of the singly hydrated ammonium ion, NH+4 (H2O)

in the spectral region from 3250 to 3810 cm−1 [1]. The spectrum was recorded by photofragmen-tation of isolated, mass selected ions. The experimental results are interpreted using high level abinitio calculations, including anharmonic calculations of vibrational spectra.

The four bands obtained are assigned to N-H and O-H stretching modes. The recorded relative bandintensities compare favorably with the predictions of ab initio calculations - except for the ν3(H2O)band, where the experimentally observed value is about 20 times weaker than calculated. A similarbehavior has previously been reported for several action spectra of other hydrated cations, in whichthe ν3(H2O)/ν1(H2O) intensity ratios become smaller the stronger the complexes are bound. Therecorded ratios vary, in particular among the data collected from action spectra that were recordedwith and without rare gas tagging [2]-[5].Inspection of calculated anharmonic coupling constants in NH+

4 (H2O) reveals that the coupling ofthe ν3(H2O) mode to other cluster modes is indeed weaker than that of the ν1(H2O) mode byorders of magnitude. This leads to a mode specific fragmentation dynamic in the action spectra ofNH+

4 (H2O) in particular and of most monohydrated cations in general.

[1]T. Pankewitz, A. Lagutschenkov, S. S. Xantheas, Y.-T. Lee and G. Niedner-Schatteburg, J. Chem. Phys. -accepted.

[2]T. D. Vaden, C. J. Weinheimer and J. M. Lisy, J. Chem. Phys. 121 (7), 3102 (2004).[3]J. H. Choi, K. T. Kuwata, B. M. Haas, Y. B. Cao, M. S. Johnson and M. Okumura, J. Chem. Phys. 100 (10),

7153 (1994).[4]C. J. Weinheimer and J. M. Lisy, J. Chem. Phys. 105 (7), 2938 (1996).[5]N. Solca and O. Dopfer, J. Phys. Chem. A. 107 (20), 4046 (2003).

109


Microsolvation of NaSO−4 and MgNO+3 in water clusters

Barbara Jagoda-Cwiklik

Institute of Organic Chemistry and Biochemistry Academy of Sciences of the Czech Republic and Centerfor Biomolecules and Complex Molecular Systems Flemingovo n. 2, Prague,16610, Czech Republic

This study presents results of our investigation of microhydration of sodium sulfate and magnesiumnitrate molecular ions. Up till now, there exist studies concerning microsolvation of atomic ions andtheir ion pairs. In this work we focus on complex ions pairs and their hydration. In particular, weare interested in the process of conversion of contact ion pairs (CIP) to solvent separated ion pairs(SSIP). We aim at getting a microscopic picture of interactions of water molecules with ion pairsand the mechanism of their separation, especially to answer the question how many water moleculesare needed to separate the investigated CIPs.To explore and to understand how water interacts with NaSO−

4 and MgNO+3 we used ab initio MP2

calculations (or DFT method for the biggest clusters) with aug-cc-pvDZ basis sets and we comparedour results with available experimental data.In case of sodium sulfate anion, according to our calculations, the first water molecule is locatedaside of NaSO−

4 ion-pair. It binds both to the sodium cation and oxygen atom in SO−4 . The second

and the third water behave in a similar way, each forms one hydrogen bond with one sulfate oxygenatom and interacts with Na+. In cluster with 3 water molecules water forms a solvation ring betweensodium cation and sulfate anion and this ring begins to separate the cation from anion. Calculatedvertical detachment energies are in good agreement with experimental ones. Clusters containing upto 15 water molecules were also studied and in the system with 13 solvent molecules we eventuallyfound SSIP.In the studies of MgNO+

3 ion pair, we located a new energeticaly optimal structure of the 5-watercluster where one of water molecules is located in the second solvation shell. An addition of sixthwater molecule causes that the solvent separated ion pair is formed where the magnesium cation issurrounded by six water molecules and the nitrate anion is located in second solvation shell.

110


Structure of Coronene Clusters

O. I. Obolensky, A. V. Solov’yov, W. Greiner


Clusters of polycyclic aromatic hydrocarbons (PAH) and their ions are believed to be one of themost probable species in the interstellar space responsible for converting the UV radiation of thestars into the intensive IR radiation, known as unidentified infrared bands [1], [2]. The currentknowledge of structure and properties of PAH clusters is very limited. Laboratory studies have juststarted to appear while reliability of the theoretical results is limited due to the subtle interplaybetween the van der Waals polarization forces and the electrostatic interaction [3].

We have recently performed density functional theory (DFT) calculations of energies for four plau-sible configurations of dimer of coronene (C24H12) [3] (shown on the left). The stack configurations(superimposed stack (SS), twisted stack (TS) and parallel-displaced (PD) stack) are favoured by thevan der Waals forces while the T-shaped configuration is favoured by the electrostatic interaction.In order to get a better description of the van der Waals interaction, the DFT results were correctedby a phenomenological London-type 1/r6 term. Depending on the magnitude of this term, both theT-shaped and the PD stack configurations can be the global minimum configuration of the system.It is quite surprising that the T-shaped configuration is energetically competitive for such a largesystem for which a graphite-like stacked structure had been a priori expected. On the basis of theDFT calculations we developed a model in which each coronene molecule is represented by a set offour charged rings [3]. Adjusting the parameters of these rings both the T-shaped and the stackconfigurations can be made energetically preferable configurations.We found the global energy minimum structures of the coronene clusters in the situation whenthe electrostatic interaction dominates and in the situation when the van der Waals forces prevail(also studied previously [4]). Rotational constants and the normal modes in the obtained sets ofstructures are distinctively different making possible to distinguish experimentally between the twosituations.We acknowledge partial financial support from the European Commission (project EXCELL).

[1]W. W. Duley, S. Lazarev, Astrophysical Journal, 612, L33 (2004)[2]M. Rapicioli, C. Joblin, P. Boissel, Astronomy and Astrophysics, 429, 193 (2005)[3]O. I. Obolensky, V. Semenikhina, A. V. Solov’yov, W. Greiner, in print (available on-line), Int. J. of Quantum

Chemistry (2007)[4]M. Rapacioli, F. Calvo, F. Spiegelman, C. Joblin, D. Wales, J. Phys. Chem. A, 109, 2487 (2005)

111

Phase Transitions


Supersonic Jets at High Stagnation Pressures:Cooling, Nucleation, Thermodynamics

Wolfgang Christen, Tim Krause, Sibylle Rabeus, Klaus Rademann

Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, 12489 Berlin, Germany

Motivated by the possibility to study molecular behavior at very low temperatures the interest incold molecules has grown enormously in recent years [1]. Among the various techniques approachingthe unexplored millikelvin regime are supersonic beams, providing, most notably, superfluid heliumdroplets [2].While an increase in stagnation pressure of the gas increases the number of collisions taking placeduring expansion and thus significantly enhances the cooling efficiency of supersonic jets, conden-sation and cluster formation is considered as a limiting factor [3]. Surprisingly, at high stagnationpressures the influence of thermodynamic parameters on the relative molecular velocities in su-personic beams is virtually unexplored. At such conditions new, more complex phenomena dueto the presence of phase transitions must be taken into account. Therefore we have investigatedpulsed, supersonic jet expansions of neat CO and CO2 at high stagnation pressures by time-of-flightmeasurements. In fact, unprecedented low translational temperatures have been observed, if themolecules are expanded from their supercritical state [4]. The remarkably low translational temper-atures of less than 100 mK are rationalized by the large heat capacity in the vicinity of the criticalpoint, allowing condensation and cluster formation without significant increase in temperature.

[1]Special Issues on Cold Molecules, Eur. Phys. J. D 31 (2004), J. Phys. B 39 (2006).[2]J. P. Toennies, A. F. Vilesov, Annu. Rev. Phys. Chem. 49, 1 (1998). F. Stienkemeier, K. K. Lehmann, J. Phys.

B 39, R127 (2006).[3]M. Hillenkamp, S. Keinan, U. Even, J. Chem. Phys. 118, 8699 (2003).[4]W. Christen, K. Rademann, U. Even, J. Chem. Phys. 125, 174307 (2006).

115


Experimental characterization of nucleation center formationand growth sequence in the clustering of unsupported

nanoparticles

P. Feiden, J. Leygnier, Ph. Cahuzac, C. Bréchignac

Laboratoire Aimé Cotton, C.N.R.S. UPR 3321, Bât. 505 Université Paris Sud, F91405 Orsay Cedex,France

The condensation of a nano droplet in a supersaturating vapor decomposes in two steps: the forma-tion of a nucleation center, also called critical nuclei or nucleation seed, and the growth sequence,by accretion of further atoms on the nucleation center. These two steps were investigated separatelythrough the clustering of homogeneous Nan and Heterogeneous NanX particles in a helium buffergas, with X = (NaOH)2 or (Na2O)2. Heterogeneous NanX clusters are formed by the growth se-quence of atoms sticking on X molecules diffusing in a supersaturing vapor of sodium. The meannumber of atoms condensed on the nucleation center increases with the sodium concentration. Forgrowth sequence, the temperature of helium buffer gas is essential. When the helium gas is coldenough, it prevents nascent clusters from evaporation and all the cluster-atom collisions are stickingones, leading to large clusters. Turning the buffer gas to a higher temperature modifies the balancebetween atom accretion and cluster evaporation, and thus the mean cluster size of the distributiondecreases. The cluster size distribution against sodium concentration and Helium temperature iswell interpreted by a monte carlo simulation assuming growth by sequential accretion of atoms withhard sphere cross section and an energy dissipation function of the buffer gas temperature.

Figure 1: Time of Flight mass spectra for Nan and Nan(NaOH)2 clusters produced without (tracea) and with (trace b) injection of preformed (NaOH)2 molecules in a supersaturating sodium vapor.

116

Rare Gas Clusters


Multiply charged neon cluster ions: critical size and Coulombexplosion

I. Mähr1, F. Zappa1, S. Denifl1, D. Kubala2, O. Echt3, T. D. Märk1,2 and P. Scheier1

1. Institut für Ionenphysik und Angewandte Physik, Leopold Franzens Universität Innsbruck,Technikerstr. 25, A-6020 Innsbruck, Austria.

2. Department of Experimental Physics, Comenius University, SK-84248 Bratislava, Slovak Republic3. Department of Physics, University of New Hampshire, Durham, NH 03824, USA

Multiply charged neon cluster ions are formed upon electron impact ionization of large neutralclusters and analyzed utilizing a two sector field mass spectrometer applying techniques developedrecently [1]-[3].The critical size for doubly charged neon cluster ions is determined experimentally with isotopicallypure 20Ne to be 287. In spectra of natural neon, the presence of doubly charged clusters can beobserved as an increase of the ion yield at the same cluster size confirming the above result. Thismethod allows in addition an approximate determination of the critical sizes for triply chargedcluster ions.Utilizing a beam deflection method [4] it is possible to observe (for the first time) directly theasymmetric fission of multiply charged neon clusters right after the ionization event. The mostabundant low mass fragment ion from this reaction process is the dimer ion. The yield of largerfragment ions decreases exponentially with the fragment size. The kinetic energy of the emitted lowmass fragment ions is of the order of 200 meV which is surprisingly low when comparing the resultswith a simple point charge model taking into account the critical cluster sizes presently determined.

Figure 1: Part of a mass spectrum for a Nen cluster beam consisting of isotopically pure 20Ne.

This work was supported in part by the FWF, Wien and the European Commission, Brussels. F.Z.acknowledges gratefully support by the Brazilian agency CNPq.

[1]I. Mähr et al., Phys. Rev. Lett. 98 (2007) 023401.[2]S. Denifl et al., J. Chem. Phys. 124 (2006) 054320.[3]S. Feil et al., Int. J. Mass Spectrom. 252 (2006) 166.[4]S. Feil et al., Int. J. Mass Spectrom. 233 (2004) 325.

119


Appearance Sizes of Multiply-Charged Van der WaalsClusters

Masato Nakamura1, Paul-Antoine Hervieux2

1. College of Science & Tech, Nihon University, Narashinodai, Funabashi, 274-8501, Japan2. Institut de Physique et Chimie des Materiaux de Strasbourg, UMR 7504 ULP-CNRS, 23 Rue du Loess,

BP 43, F-67034 Strasbourg, France

Multiply-charged clusters are stable if the energy barrier exists for every fission channel. About20 years ago, Echt et al. [1] calculated the energy barrier using the liquid drop model with anassumption that the two fragment clusters are in contact at the transition state (the so-calledcontact sphere model) and estimated the appearance sizes for multiply-charged Van der Waalsclusters. Their calculations well explained the experimental measurements on the appearance sizesof many multiply-charged clusters. However, some discrepancies still exist for some clusters suchas that of Ar. Very recently, the appearance sizes of doubly- and triply- charged Ne clusters havebeen measured [2]. The appearance sizes are found to be much smaller than those estimated by themodel described in [1]. To reveal these discrepancies, we recalculate the appearance size of multiply-charged Van der Waals clusters. In [1], the parameters for the liquid drop model were those given byBrian and Burton (BB) [3]. Recently, some groups [4] have calculated the energy of global minimumof Lenard-Jones clusters up to size n = 1000. We find that the liquid drop energy using the BB’sparameters, deviates largely from the value obtained in [4] if n exceeds 100. Thus, we propose anew set of parameters which reproduces these recent calculations. Using this new expression, wecalculate the appearance sizes of Van der Waals clusters. Some of them are listed in Table I. As forAr clusters, our predictions are in better agreement with the experimental measurements. For Neclusters, the discrepancy is diminished although it is still insufficient to explain the new experimentalfindings. We also study the role of the geometrical shell effects on the stability of multiply-chargedVan der Waals clusters. In some cases, it is plausible that the appearance size varies due to shelleffects [5]. Detailed report will be given elsewhere [6].

Figure 1: Appearance Sizes of Multiply-Charged Van der Waals Clusters.

[1]O. Echt et al., Phys. Rev. A 38, 3236 (1988).[2]I. Maehr et al., Phys. Rev. Lett. 98, 023401 (2007).[3]C. L. Briant and J. J. Burton, J. Chem. Phys. 63, 2045 (1975).[4]D. Wales and J. Doye, J. Phys. Chem. A 101, 5111 (1997), Y. Xiang et al., J. Phys. Chem. A, 108, 3586 (2004).[5]M. Nakamura and P.-A. Hervieux, Chem. Phys. Lett. 428, 138 (2006).[6]M. Nakamura and P.-A. Hervieux, Preparation for publication

120


Stable Structures of the Small and Medium-Size SinglyIonized Helium Clusters.

Daniel Hrivňák, Karel Oleksy, René Kalus, František Karlický

Department of Physics, University of Ostrava, 30. dubna 22, Ostrava, 701 03, Czech Republic

The semiempirical triatomics-in-molecules (TRIM) method has been applied to study singly ionizedhelium clusters. Stable structures for the clusters of sizes 3 – 20 have been found through the use ofthe special optimization method based on the genetic algorithm. A comparison with results obtainedby the method of conjugated gradients has been done. The basic principle of the TRIM method aswell as an overview about He+

3 potential energy surfaces used is presented.

121

Posters - Session B

Carbon and Silicon

POSTERS - SESSION B B1

Phase Transitions in Fullerenes

Adilah Hussien, Ilia A. Solov’yov, Andrey V. Solov’yov and Walter Greiner

Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe University, Max-von-Laue Str.1,Frankfurt am Main, Germany

Email: [email protected]

We investigate the various fragmentation and formation channels of fullerenes using a statisticaldescription first applied to polypeptide folding [1]. In this formalism, the growth and fragmentationof fullerenes are treated as phase transitions, and we present a scheme of constructing a parameter-free partition function describing such a process. Thus, we develop a theory describing all essentialthermodynamical properties such as heat capacity, free energy and the phase transition temperature.The predictions of our theory are compared with available experimental measurements on the meltingof fullerenes [2] and with results from molecular dynamics simulations [3].

Two examples of possible fragmentation channels of C60 are shown in Figure 1. Channel (a) corre-sponds to the transition of C60 in the gas phase of 30 C2 molecules, while channel (b) correspondsto the polymerization of the fullerene.

Figure 1: Possible fragmentation channels of C60 into a gas or polymer phase

[1]A. V. Yakubovich, I. A. Solov’yov, A. V. Solov’yov and W. Greiner, Eur. Phys. J. D (Highlight paper) 40, 363(2006); Europhysics News, in print (2007)

[2]Y. Jin, J. Cheng, M. Varma-Nai, G. Liang, Y. Fu, B. Wunderlich, X. Xiang, R. Mostovoy and A. Zetti, J. Phys.Chem. 96, 5151 (1992)

[3]S. G. Kim and D. Tom?nek, Phys. Rev. Lett. 72, 2418 (1994)

127

B2 POSTERS - SESSION B

Deuteration of solid fullerene films versus the structure of Cn

cages

D. Löffler, S.-S. Jester, P. Weis, A. Böttcher, M. M. Kappes

Lehrstuhl für die Physikalische Chemie Mikroskopischer Systeme, Institut für Physikalische Chemie,Universität Karlsruhe (TH), D-76128 Karlsruhe, Germany

The deuteration of solid fullerene films has been investigated by means of thermal desorption spec-troscopy, TDS, quadrupole mass spectrometry, ultraviolet photioionization spectroscopy, UPS, andatomic force microscopy, AFM. We compare here the monodisperse films consisting of small non-IPR carbon cages, Cn 50 ≥ n ≥ 58, with the classical IPR fullerenes as represented by C60. Thefilms have been created under ultra high vacuum conditions by depositing low-energy mass-selectedC+

n ions onto HOPG [1]. The stability of the non-IPR Cn films is governed by pairs of adjacentpentagons in Cn cages, 2AP, which are responsible for the formation of covalently stabilized [Cn]moligomers [1]-[3]. In contrast, C60 films as molecular crystals are stabilized by weak van der Waalsinteractions.Mass spectra of particles sublimating from deuterated C58 films reveal broad distributions of C58Dn

cages with recognizable maxima at n=6, 26 and 30. Thermal decomposition of such films is markedby considerably lower sublimation temperatures than that found for bare C58 films. The progressingdeuteration process manifests itself also by a gradual lowering of the work function (down to 3.6 eV)and a significant increase of the ionization potential (up to 7.2 eV for C58Dn cages). AFM images ofdeuterated films reveal a thermally induced transition from dendritic islands of C58 oligomers intosmooth-rim islands composed of deuterated cages. This transition results from a gradual replace-ment of the 2AP-2AP bonds by deuterium atoms tied at cage periphery what leads to the formationof molecular solid stabilized by only weakly interacting C58Dx cages [4].In contrast to non-IPR films (Cn , 50 ≥ n ≥ 58) the deuteration of C60 films raises the binding en-ergy of adjacent cages. The initial deuteration stages are associated with the formation of polymericchains likely comprising covalently interlinked cages which exhibit higher thermal stability than pris-tine C60 films. The corresponding sublimation heat increases with the deuterium dose applied andlevels off at a value of 2.15 eV/cage. For comparison, the higher deuterofullerenes, C60D36, exhibitsublimation energy of 1.39 eV/cage, considerably lower than the polymeric chains but comparableto pristine C60 films. The polymer-based capping layers are air resistant and amenable to fabricatestable multi layered [C60/C60Dx]m films [5].The evident difference in the deuteration routes found here for the IPR and non-IPR fullerene classescan be assigned to the structure-specific reaction centers terminating the non-IPR cages.

[1]A. Böttcher, P. Weis, A. Bihlmeier, M.M. Kappes, PCCP 6 (2004) 5213[2]A. Böttcher, P. Weis, S.-S. Jester, D. Löffler, A. Bihlmeier, W. Klopper, M.M. Kappes, PCCP, 7 (2005) 2816[3]D. Löffler, S. S. Jester, P. Weis, A. Böttcher and M. M. Kappes, J. Chem. Phys. 124 (2005) 054705[4]D. Löffler, S. S. Jester, P. Weis, A. Böttcher and M. M. Kappes, J. Chem. Phys. 125 (2006) 224705[5]D. Löffler, P. Weis, A. Böttcher and M. M. Kappes, J. Phys. Chem. , submitted, 2007

128


Modeling Carbon Nanotube Double-Layer Capacitors

Lars Pastewka1, Michael Moseler1,2

1. Fraunhofer Institut für Werkstoffmechanik, Wöhlerstraße 11, 79108 Freiburg, Germany2. Freiburger Materialforschungszentrum, Stefan-Meier-Straße 21, 79104 Freiburg, Germany

Capacitors with capacities of several Farads, sometimes termed "Supercaps", have been in scientificfocus for some time now. In this context, carbon nanotubes (CNTs) have been proposed for use asan electrode material for electric double-layer capacitors (EDLC). The advantage of CNTs is thelarge surface area exposed to the electrolyte and their inertness with respect to chemical reactions[1]. Except for a few experimental [2] and theoretical [3] studies on gated CNTs not much is knownabout the detailed capacitive response of these systems. For CNTs immersed in an electrolyte asused in EDLCs the microscopic mechanisms are still unclear [4].Here, we present a large scale study on the capacitive response of CNTs immersed in an electrolyte.Our study is based on density-functional based tight-binding at the self-consistent charge level oftheory [5]. In our calculations, the electrolyte is included within a mean-field approach. This isachieved by smearing out the double-layer charges on an idealized cylinder. All calculations areperformed with full relaxation of the atomic positions because part of the stored energy is of elasticorigin. For a water based electrolyte, the quantum and classical contributions to the capacity aresimilar in magnitude. We also find that the total differential capacity per double-layer surfacedoes strongly depend on the charging level, or bias potential, of the nanotube and increases withincreasing tube diameter. The capacities for different tubes in a charge neutral environment aregiven in figure 1.

Figure 1: Differential capacity per double-layer surface as a function of tube radius for a water basedelectrolyte. Labels give the chirality of the respective CNT. The 10x10-5x5 tube is double-walled witha 10x10 outer and a 5x5 inner wall.

[1]D. S. Futaba et al., Nat. Mat. 5, 987 (2006); B.-J. Yoon et al., Chem. Phys. Lett. 388, 170 (2004); K. H. An etal., Adv. Mat. 13, 497 (2001)

[2]S. Ilani et al., Nat. Phys. 2, 687 (2006)[3]L. Latessa et al., Phys. Rev. B 72, 035455 (2005); P. Pomorski et al., Phys. Rev. B 69, 115418 (2004); P. Pomorski

et al., Phys. Rev. B 67, 161404 (2003); J. Luo et al., Phys. Rev. B 66, 115415 (2002)[4]I. Heller et al., J. Am. Soc. Chem. 128, 7353 (2006)[5]See: P. Koskinen, H. Häkkinen, G. Seifert, S. Sanna, Th. Frauenheim and M. Moseler, New J. Phys. 8, 9 (2006)

and references therein.

129


Chirality of Single Walled Carbon Nanotubes and itsInfluence on the Energetics and Stability

Maneesh Mathew, Ilia A. Solov’yov, Andrey V. Solov’yov and Walter Greiner

Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe University Max-von-Laue-Str. 1,60438, Frankfurt am Main, GermanyE-mail:[email protected]

We systematically study carbon nanotubes of various chiralities and their stabilities to understandhow the chirality influences the binding energy of nanotubes. Both ab initio and molecular mechanicsmethods are used for the investigation. The ab initio density functional theory (DFT) calculationsare performed with the use of Gaussian 03 software package [1] and the molecular mechanics calcula-tions are performed with the use of NAMD software package [2]. The results obtained through thesestudies are used for a better understanding of the growth mechanism of carbon nanotubes [3], [4].The mechanism which controls the growth of a nanotube of desired chirality is poorly understood.Our studies can be used for the theoretical investigation of the dependence of growth of carbonnanotube on its chirality. The following figure explains the concept of the chirality of a nanotube bydepicting how a nanotube of certain chirality, characterized by two integer numbers n and m, canbe constructed from a graphite sheet. The figure below shows the structure of a chiral nanotube,corresponding to n=7 and m=4, being a typical conductive nanotube.

Figure 1: : (a) Chirality vector shown in a graphite sheet; (b) The nanotube formed after foldingthe graphite sheet in figure-(a) along the chirality vector.

This work was supported in part by INTAS (project No 03-51-6170), by EU through the EXCELLproject and by EU through PECU project.

[1]Gaussian 03, Revision C.02, M. J. Frisch, et al, Gaussian, Inc., Wallingford CT (2004)[2]J.C. Phillips, et al, J. of Comp. Chem., 26, 1781 (2005)[3]Feng Ding, Kim Bolton and Arne Rosen, J. Phys. Chem. B 108, 17369 (2004)[4]J. Gavillet, et al, Phys. Rev. Lett., 87, 27 (2001)

130


Photoelectron spectroscopy of fullerene dianions C2−n (86 ≤ n

≤ 98)

K. Matheis, O. T. Ehrler, M. M. Kappes

Institut für Physikalische Chemie, Universität Karlsruhe (TH), Germany

Several doubly negatively charged fullerenes were examined with laser photoelectron spectroscopyat photon energies of 2.33, 3.49, 4.66, and 6.42 eV [1]. By determining the vertical detachmentenergies and their dependence upon the cage size, the range of experimentally determined electronaffinities of the higher homologous of the carbon clusters was extended [2],[3]. Furthermore repulsiveCoulomb barriers have been extracted from the spectra and discussed in consideration of the existingresults on smaller fullerenes and in terms of an electrostatic model. For the 19 IPR-isomers of C2−

86

semiempirical calculations of the second electron affinities have been made.

Gas phase dianions were generated by electrospray ionization from a fullerene solution in o-dichlorobenzenecontaining TDAE as an electron donor [4]. After leaving the heated desolvation capillary, the ionswere transmitted through an electrodynamic ion funnel and accumulated in a hexapole ion trapfor 33 ms interface the cw ion source to a pulsed reflectron time-of-flight mass spectrometer. Afterpassing the reflectron, mass selected fullerene dianions were decelerated before entering the detach-ment region of a magnetic bottle photoelectron spectrometer. Here they were irradiated with higherharmonics of the Nd:YAG or the fundamental wavelength of an ArF-laser. Arrival time distributionsof the emitted electrons was measured and converted to energy spectra.

The obtained data of the second electron affinities are in good agreement with results from a classicalelectrostatic model, although this was compiled for spherical objects. Calculations consideringellipsoidal deformations show that differences in the potential energies are negligible with respect tothe resolution of our experimental setup. The repulsive Coulomb barrier heights are in agreementwith predictions from electrostatic theory.

[1]K. Matheis, O. T. Ehrler and M. M. Kappes, in preparation[2]O. T. Ehrler, J. M. Weber, F. Furche, and M. M. Kappes, Phys. Rev. Lett. 91, 113006 (2003).[3]O. T. Ehrler, J. M. Weber, F. Furche, and M. M. Kappes, J. Chem. Phys. 122, 094321 (2005).[4]O. Hampe, M. Neumaier, M. N. Blom, and M.M. Kappes, Chem. Phys. Lett. 354, 303 (2002).

131


Kinetic Model for Nanotubes Growth

O. I. Obolensky, I. A. Solov’yov, A. V. Solov’yov, W. Greiner


Size selected nanoclusters of such metals as Ni, Fe, Co are widely used as catalysts in the chemicalvapor deposition (CVD) techniques of production of carbon nanotubes [1]. The characteristics ofthe nanotubes as well as the parameters of their growth are closely related to the size and propertiesof the catalytic particles [1]. Despite of intensive studies the physical mechanism responsible for thecatalytic action of the nanoclusters leading to the nanotubes growth is not yet known.

We have developed a kinetic model describing the process of catalytically assisted growth of carbonnanotubes. The model describes the flow of carbon atoms starting from the catalytic decompositionof the feedstock molecules at the surface of the nanocluster and ending at their embedding intothe nanotube. In the figure we show a cross section of the nanocluster and the nanotube with theschematic representation of the carbon flows which are accounted for by the model.Feedstock molecules are decomposed on the catalytically active part of the nanocluster surface,which is exposed to the environment and not screened by any additional inactive layers (region"c"). The carbon atoms from this region diffuse (either along the surface or through the bulk ofthe cluster) into the other parts of the cluster. We single out the growth region from which theatoms embed themselves into the nanotube walls (region "g") and the near-surface region insidethe nanotube (region "b"). From the region "b" carbon atoms can reach the growth region bysurface or bulk diffusion or they can evaporate. Some of the evaporated atoms are deposited on thenanotube walls and reach the growth region by surface diffusion. We also account for the possibilityof decomposition of the feedstock molecules directly by the nanotube walls. Thus, we present amodel comprehensively describing the kinetics of the nanotube growth. The model allows one topredict with a reasonable accuracy the distribution of carbon within the catalytic nanoparticle, therelative importance of various growth mechanisms, the nanotube growth rate, etc. However, theresults are rather sensitive to the input parameters (e.g., diffusion coefficients, activation energiesfor embedding into the nanotube) which are to be obtained from accurate quantum mechanicalcalculations or experiments.We acknowledge partial financial support from the European Commission (project EXCELL).

[1]M. Chhowalla et al., J. Appl. Phys., 90, 5308 (2001) and references therein.

132


UV-vis and IR multi-photon electron detachmentspectroscopy of C2−

76

O. Hampe1,4 , M. Neumaier1, Bruno Concina1,5 A.D. Boese1, J. Lemaire2, P. Maître2, G.Niedner-Schatteburg3, and M. M. Kappes1,4

1 Institut für Nanotechnologie, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany;2 Laboratoire de Chimie Physique (CNRS-UMR-8000), Université Paris XI, Orsay Cedex 91405, France;

3 Fachbereich Chemie, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany;4 Institut für Physikalische Chemie, Universität Karlsruhe, 76128 Karlsruhe, Germany;

5 present address: Laboratoire de Spectrométrie Ionique et Moléculaire, CNRS and Université Lyon I,69622 Villeurbanne cedex, France.

Doubly negatively charged C2−76 may serve as a model system of a molecular multianion for inves-

tigating the electron-electron interaction ("Coulomb barrier") as well as the effect of the excesscharges on the molecular structure (electron-lattice interaction). We present results from two spec-troscopic probes on electrosprayed C2−

76 using two experimental ion trap setups:(i) Laser-photoexcitation of C2−

76 and C2−84 in the Uv-vis region (using 2nd and 3rd harmonics of a

ns-pulsed Nd:YAG laser coupled to the Karlsruhe FT-ICR mass spectrometer) is studied in detailat different wavelengths as a function of laser fluence to give information on the degree of internalexcitation required to induce electron loss on the experimental time scale and the mechanism of elec-tron emission (statistical vs. direct detachment). Electron loss response is modelled by thermionicemission kinetics leading to an effective Coulomb barrier height. [1](ii) IR vibrational spectra of C2−

76 (as the first gas-phase IR spectrum of an isolated multianion)are obtained at the free-electron laser facility CLIO on a ESI-Q-trap mass-spectrometer setup. Thespectrum is contrasted to computed harmonic vibrational frequencies based on density-functionaltheory. They show that the excess charges lead to a fairly small distortion of the fullerene cagewhich maintains its D2 symmetry prevailing for neutral C76.

Figure 1: left: Fluence dependence (data points, at 355 nm) of electron loss efficiency modelledwith a one and two-photon absorption law; inset: thermionic emission lifetime after one-photonabsorption bracketing an effective Coulomb barrier (exp. time scale as horizontal lines); right: FEL-IR-multi-photon detachment spectrum of C2−

76 (solid line) contrasted to DFT computations employingtwo different functionals/basis sets shown as stick plots.

[1]B. Concina, M. Neumaier, O. Hampe, M. M. Kappes, Int. J. Mass Spectrom. 252, 110 (2006).[2]M. Neumaier, A.D. Boese, J. Lemaire, P. Maître, G. Niedner-Schatteburg, M. M. Kappes, and O. Hampe, in prep.

(2007).

133


Screening of a Confined Atom by a Fullerene of FiniteThickness

S. Lo1, , A.V. Korol1,2 and A.V. Solov’yov1

1. Frankfurt Institute of Advanced Studies, Johann Wolfgang Goethe-Universität, Max-von-Laue-Str. 1,60438 Frankfurt am Main, Germany

2. Department of Physics, St. Petersburg State Maritime Technical University, Leninskii prospect 101, St.Petersburg 198262, Russia

Email: [email protected]

An atom confined within a fullerene is dynamically screened from the external electromagnetic field- it experiences a field that is amplified - leading to an enhancement of the photoabsorption of theconfined atom. The amplification occurs in a certain frequency range determined by the plasmonexcitation in the confining fullerene. This enhancement factor was discussed by Connerade andSolov’yov in [1] for the case of the infinitely thin fullerene.

The dynamical enhancement factor due to a fullerene of finite thickness is presented here. Theexistence of a second surface of this fullerene is significant as this allows for the presence of twosurface plasmon modes [2], in contrast to the single plasmon considered in [1]. The figure is a plotof the enhancement factor for the infinitely thin fullerene and the fullerene of finite thickness. Theprofile of the enhancement factor for the thick fullerene has an additional feature, which is due tothe manifestation of the second plasmon mode, as indicated by the arrow in the figure.

Figure 1: A plot comparing the enhancement factor of the infinitely thin fullerene and the fullereneof finite thickness.

This work was supported in part by INTAS (project number 03-51-6170) and by EU through theEXCELL project.

[1]J.-P. Connerade and A.V. Solov’yov, J. Phys. B: At. Mol. Opt. Phys., 38, 807 (2005)[2]A.V. Korol and A.V. Solov’yov, Phys. Rev. Lett. (to be published in 2007)

134

Clusters and Hydrogen


Optical Excitations in CdS Nanoparticles

Johannes Frenzel, Jan-Ole Joswig, and Gotthard Seifert

Physikalische Chemie, Technische Universität Dresden, Bergstr. 66b, D-01062 Dresden, Germany

The absorption spectra of cadmium sulfide nanoparticles with up to 2000 atoms have been calculatedusing a density-functional tight-binding method and linear-response theory. We have considereddifferent stoichiometries, underlying crystal structures (zincblende, wurtzite, rocksalt), and particleshapes (spherical, cuboctahedral, tetrahedral). Moreover we have investigated the influence ofsaturation with hydrogen atoms on the spectra.

Saturated particles show a decreasing lowest excitation energy with increasing particle size due tothe quantum-confinement effect. Dangling bonds at unsaturated surface atoms introduce trappedsurface states which lie below the lowest excitations of the completely saturated particles and, thus,close the gap between highest occupied and lowest unoccupied molecular orbital.

Moreover, we find strong excitonic onset excitations arising from the molecular orbitals close to theFermi level. These orbitals show the shape of the angular momenta of a hydrogen atom (s, p). Someof these excitations have a collective character.

Zincblende- and wurtzite-derived particles show very similar spectra, whereas the spectra of rocksalt-derived particles are rather featureless. Particle shapes that confine the orbital wavefunctionsstrongly (tetrahedron) give rise to less pronounced spectra with lower oscillator strengths. Theagreement of our data with experimentally available spectra and excitation energies is very good.

137


Chemical Reactivity of Small Palladium Clusters with H2 andO2 in a Cluster Beam Experiment

Mats Andersson and Arne Rosén

Department of Physics, Göteborg University, SE-41296 Göteborg, Sweden

The reactivity of neutral palladium clusters towards diatomic molecules such as O2 and H2 wasinvestigated in a cluster beam experiment. A beam of clusters seeded in He gas is generated in apulsed laser-vaporisation source. After expansion the beam passes a skimmer and then two collisioncells, in which a low pressure of a reactive gas can be maintained. The clusters, with or withoutadsorbed molecules, are detected with laser ionisation and time-of-flight mass spectrometry. Bymeasuring the number of molecules adsorbed on the clusters as a function of pressure in the collisioncells, the reaction probability in a cluster-molecule collision can be determined using a pseudo-first-order kinetic model [1].

The reaction probability of the Pdn with O2 molecules was investigated for cluster sizes up to n=28.The formation of PdnO2 products was observed for sizes from n=5, and PdnO4 products from aboutn=15. The reaction probability with the first O2 molecule is around 0.5-0.7 for sizes n=10-28, andthe size-to-size variations are limited. For the smallest sizes the measured reaction probability islower. The observation of threshold sizes and a lower reactivity of the smaller sizes is thought tobe an effect of insufficient stability of the reaction products for clusters having only few vibrationaldegrees of freedom to accommodate the chemisorption energy [1]. The reaction probability of O2

on palladium clusters has a similar value as on nickel clusters [2], but it is slightly lower than forcobalt [1] and rhodium [3] clusters, and higher than for platinum clusters [4].

When the Pd clusters react with O2 in the first collision cell and H2 (D2) in the second cell, thereis evidence of reactions between oxygen and hydrogen on the clusters. If the O2 pressure is keptconstant and the H2 pressure is successively increased, it is observed that the number of adsorbedoxygen atoms is decreasing. This behavior is similar to what was observed for O2 and D2 adsorptionon platinum clusters [4]. The loss of oxygen atoms can be explained by formation of water moleculesthat subsequently desorb from the clusters.

[1]M. Andersson, J.L. Persson and A. Rosén, J. Phys. Chem., 100, 12222 (1996).[2]M. Andersson, L. Holmgren, J.L. Persson, T. Åklint, and A. Rosén, Mat. Res. Soc. Symp. Proc., 351, 299 (1994).[3]M. Andersson, L. Holmgren, and A. Rosén, Surf. Rev. Lett., 3, 683 (1996).[4]M. Andersson and A. Rosén, J. Chem. Phys., 117, 7051 (2002).

138


Hydrogen Sorption Characteristics of Boron, Carbon and/orNitrogen-Based Materials Decorated by Transition Metal

Clusters

Nobuyuki Nishimiya

Department of Materials Science, Toyohashi University of Technology, Tempaku-cho 1-1, Toyohashi441-8580, Japan

Nano-structured carbonaceous materials and B-C-N compounds (BN, CN and BCN) were compar-atively prepared and characterized in order to discover novel materials with high hydrogen capacity.Nano-structured carbonaceous materials decorated by Pd or 3d transition metals such as Ti, V, Cr,Fe, Co etc. possessed limited hydrogen capacity of 0.1-0.4 wt.% at 298 K under 100 kPa of hydrogen,which was lower than the maximum hydrogen concentration, 0.86 wt.% [1], in single walled carbonnanotubes containing both Y and Ni. The hydrogen capacity positively correlates with surface areaas illustrated in Figure 1, where data are plotted for 3d transition metals. The straight line isdrawn so as to have a slope of 0.303 wt.%/200 m2 g−1, which is essentially the same as reportedfor nanocarbons at 77 K [2]. The capacity was also governed by content of carbon and strength ofgraphitic G band of Raman spectrum. The effect of decoration with metals is qualitatively inter-preted by stability of hydrogen in assumed C2M2 tetrahedron evaluated by quantum mechanicalcalculation, DV-Xα.Decoration of BN, CN and BCN with Pd again enhanced hydrogen capacity, but maximum hydro-gen concentrations were as low as 0.1 wt.% at 298 K and 100 kPa. Decorated BCN had the largesthydrogen capacity, but lost the decoration effect on repeated sorption and desorption of hydrogen.Decorated BN showed reversed behavior. While specific surface area of BN (before decoration)increased through heat treatments, hydrogen capacity did not monotonously increase with specificsurface area. It is to be resolved how starting BN powders are reproducibly prepared with surfacearea and porosity regulated.

Figure 1: Variation of hydrogen capacity at 298 K and 100 kPa with surface area.

[1]N. Nishimiya, K. Ishigaki, H. Takikawa, M. Ikeda, Y. Hibi, T. Sakakibara, A. Matsumoto and K. Tsutsumi, J.Alloys and Comp. 339, 275-282 (2002).

[2]A. Züttel, P. Sudan, Ph. Mauron, T. Kiyobayashi, Ch. Emmenegger and L. Schlapbach, Int. J. Hydrogen Energy27, 203-212 (2002).

139


Hydrogen peroxide and ammonia on protonated ice clusters

Martin Schmidt1, Albert Masson1 and Catherine Bréchignac1, Hai-Ping Cheng2

1. Laboratoire Aimé Cotton, CNRS, Bât 505, Université Paris sud, 91405 Orsay Cedex, [email protected]

2. Department of Physics & QTP, University of Florida Gainesville, FL 32611, USA

A temperature-controlled source for protonated water clusters has been combined with high-resolutionmass spectroscopy to study the stability pattern of ice-clusters and compounds with ammonia andhydrogen peroxide depending on temperature. The stability pattern of pure protonated ice showsthe two well-known peaks at 21 and 28 molecules and also less pronounced structure up to n =55. Ammonia and hydrogen peroxide do not destroy this pattern but shift it by a number of watermolecules. The additives are therefore integrated in the persisting crystalline structure of the pureprotonated ice. Based on this structural information, DFT calculations reveal that hydrogen perox-ide and ammonia occupy surface positions on a dodecahedral 21-molecule cluster and are not cagedin the center.

140


Hydrogen Induced Transition From Dissociative to MolecularChemisorption of CO on Vanadium Clusters

Ingmar Swart1, AndrÈ Fielicke2, Britta Redlich3, Gerard Meijer2, Bert Weckhuysen1 andFrank de Groot1

1. Department of Chemistry, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands2. Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195, Berlin, Germany3. FOM Institute for Plasmaphysics, Edisonbaan 14, 3439 MN, Nieuwegein, The Netherlands

Transition metal nanoparticles play an important role in catalysis and hydrogen storage materials[1]. The presence of adsorbates on nanoparticles can strongly influence their properties with respectto these applications. Transition metal clusters, on which molecules are adsorbed, are useful modelsystems to study the fundamental aspects of the metal-adsorbate interaction.

Here we present results on the determination of the structure of hydrogen covered transition metalcomplexes and on the effect of co-adsorption of H2 and CO. Vibrational spectroscopy is applied toobtain structural information on the binding geometries of H2 and CO on cationic vanadium clustersin the gas-phase. Experimental IR spectra of cluster complexes have been obtained via InfraRedMultiple Photon Dissociation (IR-MPD) spectroscopy [2] using the Free Electron Laser for Infraredexperiments (FELIX) as an intense and tunable light source in the 300-2200 cm−1 spectral range.The experimental spectra are augmented with density functional calculations to obtain the groundstate geometries of the cluster complexes as well as the corresponding IR spectra.

We report on the size-dependent interaction of carbon monoxide molecules with hydrogen coveredvanadium clusters containing between 5 and 20 metal atoms. It is shown that hydrogen is pre-dominantly adsorbed in three fold hollow sites with a minor number of hydrogen atoms adsorbedin bridging sites. The CO molecule dissociates on bare vanadium clusters, in agreement with COadsorption on bulk vanadium [3]. Co-adsorption of H2 leads to molecular binding of CO on themetal clusters as evidenced by the presence of a ν(CO) stretching band in the infrared spectra. Weshow that dissociative chemisorption is prevented when the potential binding sites of atomic C andO atoms are blocked by H atoms [4].

[1]A.T. Bell, Science, 299, 1688 (2003).[2]A. Fielicke, G. Meijer, G. von Helden, J. Am. Chem. Soc. 125, 3659 (2003).[3]G. Broden, T.N. Rhodin, C. Brucker, Surf. Sci. 59, 593 (1976).[4]I. Swart, A. Fielicke, B. Redlich, G. Meijer, B.M. Weckhuysen, F.M.F. de Groot, J. Am. Chem. Soc. in press

(2007).

141


Controlling the Activation of CO Adsorbed on CobaltClusters by Co-adsorption of H2

Ingmar Swart1, André Fielicke2, David M. Rayner3, Bert Weckhuysen1, Gerard Meijer2 andFrank de Groot1

1. Department of Chemistry, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands2. Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195, Berlin, Germany

3. Steacie Institute for Molecular Sciences, National Research Council, 100 Sussex Drive Ottawa, Ontario,K1A 0R6, Canada

Reactions of small molecules with transition metal nanoparticles have attracted considerable inter-est over the past decades since they can provide a conceptual framework for applications such asheterogeneous catalysis and hydrogen storage. Especially the reaction of H2 and CO with Fe andCo has been widely studied due to the relevance to the Fischer-Tropsch process in which a mixtureof H2 and CO is converted into long chain hydrocarbons.

Here we report on the co-adsorption of H2 and CO on small cationic Co clusters (Co+n , n=4-20) in

the gas-phase, specifically addressing the effect on the C-O bond. Vibrational spectroscopy is usedto monitor the C-O bond strength, relying on InfraRed Multiple Photon Dissociation (IR-MPD)spectroscopy [1] to measure the IR spectra of (hydrogenated) mono-carbonyl complexes in the rangeof the C-O stretch vibration (1600 - 2200 cm−1). The presence of a C-O stretch band in the 1950 -2100 cm−1 spectral range indicates that CO is exclusively linearly coordinated to bare and hydrogencovered cationic cobalt clusters. Upon increasing H2 coverage, the C-O stretch frequency shifts tohigher energy. To explain and quantify the observed effect, we extended a recently developed model[2] that describes the size and charge dependence of the C-O stretch vibration of a CO moleculebound to late transition metal clusters to incorporate the co-adsorption of hydrogen. We show thatby forming Co-H bonds, hydrogen controls the electron density available for back-donation to theCO. On average each adsorbed H atom formally withdraws 0.09-0.25 electrons, depending on thesize of the cluster [3].

[1]A. Fielicke, G. Meijer, G. von Helden, J. Am. Chem. Soc. 125, 3659 (2003).[2]A. Fielicke, G. von Helden, G. Meijer, D. B. Pedersen, B. Simard, D. M. Rayner, J. Chem. Phys. 124, 194305

(2006).[3]I. Swart, A. Fielicke, D.M. Rayner, G. Meijer, B.M. Weckhuysen, F.M.F. de Groot, submitted for publication

142

Deposited Clusters


Production and Nanolithography on Silicon Cluster Films

B. Rasul, S. Jaksch, F. Zappa, P. Scheier

Institut für Ionenphysik und Angewandte Physik, Leopold-Franzens-Universität Innsbruck,Technikerstrasse 25, A-6020 Innsbruck, Österreich

Films of silicon clusters are formed by magnetron sputtering and deposited onto freshly cleavedhighly oriented pyrolitic graphite (HOPG) surfaces. The topography of the films and the growthbehavior as a function of the sputtering parameter are analyzed with a variable temperature scan-ning tunneling microscope (STM). This instrument enables to investigate the silicon clusters in atemperature range from 25K to approximately 1300K. The electronic properties of the films areprobed by local scanning tunneling spectroscopy. The sample preparation chamber is connected viaan ultra high vacuum gate directly to the STM chamber which reduces oxidation and passivationof the silicon clusters to a minimum.

In the present study, we analyze the dependence on the distance between the HOPG surface andthe magnetron, the argon gas pressure, the discharge current, and also the deposition time. Thedistance has the strongest influence on the sputtering behavior. The Ar pressure affects both thegrowth rate of the films and the sticking coefficient of the silicon clusters on the HOPG. For highpressure we observe an increased mobility of the clusters. Furthermore, the overall roughness ofthe film increases with higher pressure. Very low pressure (0.4 Pa) results in fractal structures anddecoration of defects in the HOPG substrate.

Well defined manipulation in the nanometer regime of the films with the STM tip is demonstratedwith different techniques. At increased bias-voltage, applied between the tip and the silicon clusterfilm, we are able to fuse silicon nanoparticles into larger objects. Thereby the electronic densityof states is strongly increased and probed with complementary scanning tunneling spectroscopymeasurements. At increased tunneling current silicon clusters can be gently vaporized which resultsin regions with reduced roughness. Finally at low bias voltages the silicon clusters can be wiped awaywith the tip. Surprisingly this extreme procedure generally does not affect the quality of the tip andhigh resolution images of the manipulated sample can be taken with the same tip. It is planned todetermine the conditions for a film that has optimum properties for subsequent nanolithography asdescribed above. A high resolution grey scale image will be imprinted on such a film. The stabilityof freshly formed films and also the imprinted nanolithography is studied at elevated temperaturesin the STM.

145


Electronic excitations induced by the impact of coinage metalions and clusters on a rare gas matrix: neutralization and

luminescence

Christoph Sieber1, Wolfgang Harbich1, Karl-Heinz Meiwes-Broer2 and Christian Félix1

1. Institut de Physique des Nanostructures, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015Lausanne, Switzerland

2. Fachbereich Physik, Universität Rostock, D-18051Rostock, Germany

Rare gas matrices are model systems in solid state physics. They are used to provide softlandingconditions to size-selected clusters impinging on a surface because they are inert and relatively soft.Classical molecular dynamics calculations have been used to describe and understand the nuclearmechanics of the deposition process, however much less studies have focused on electronic processes.

Here we show that collisions of atomic ions and cluster ions with rare gas covered metal substrateslead to light emission which is element and size specific (fig.1). In these low energy ion substratecollisions, it is not the direct energy transfer into the electronic system of the cluster, but thecreation of an exciton, which is trapped in the rare gas layer, that carries the excitation energy.We propose a model for the excitation mechanism that explains the observed luminescence as wellas the neutralization of the clusters in the rare matrix. Strong links with thermoluminescence andother thermostimulated processes are found.

Figure 1: (a) Spectrally resolved luminescence of Ag+1 , Ag+

2 and Ag+3 and (b) of Au+

1 , Au+2 during

the deposition in an argon matrix.

[1]C. Sieber, W. Harbich, K.-H. Meiwes-Broer, & C. Félix, Chem. Phys. Lett.,433, 32-36 (2006)

146


Absorption spectroscopy of size-selected neutral gold clustersin argon matrices

V. Rodrigues1,2, F. Conus1, A. Rydlo1, S. Lecoultre1 and C. Félix1

1. Institut de Physique des Nanostructures, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015Lausanne, Switzerland

2. Instituto de Física "Gleb Wataghin," UNICAMP, C.P.6165, 13083-970 Campinas SP, Brazil.

The interest for gold clusters has been renewed both for fundamental reasons as well as for potentialapplications. Different calculations of low energy structure show that for small gold clusters, therelativistic effects result in an increased stability of the planar structures that are the most stablestructures for small clusters in the gas phase. From the point of view of applications, recent studieshave shown their potential for catalysis as well as their interesting optical properties. Only veryfew reports on theoretical and experimental investigations of the optical properties of gold clustersexist, this being probably related to their absorption cross section that is an order of magnitudesmaller than for silver.Here we report new absorption spectra of gold Au1−7 nanoclusters. Because of the lower crosssection, these measurements required the use of a device recently developed in our group [1] thathas an increased optical path and therefore sensitivity. The spectra are compared to the availableexperimental and theoretical data. A first analysis of the spectra is performed using the Mie formal-ism by introducing explicitly the lowest energy structures resulting from the reported calculations.Despite the simplicity of this model, this analysis yields surprisingly good results for the clustersthat have a structure with symmetry compatible with an ellipsoid.

[1]Conus, F.; Lau, J.T.; Rodrigues, V. & Félix, C. Rev. Sci. Instrum., 77, 113103 (2006).

147


Oxidation/Reduction of Aun (n=2-13) clusters on SiO2/Si andHOPG

Rainer Dietsche1, Dong Chan Lim1, Moritz Bubek1, Thorsten Ketterer1, Karsten Vetter1,Young Dok Kim2, and Gerd Ganteför1

1. Department of Physics, University of Konstanz, Universitätsstrasse 10, 78457 Konstanz, Germany2. Division of Nano Sciences and Department of Chemistry, Ewha Womans University, Seodaemoon-gu,

Daehyun-dong 11-1, 120-750 Seoul, Korea

Au nanoparticles show enhanced catalytic activity in contrast to bulk Au, which is chemically inert[1]. One of the central questions for these Au nanoparticles is the origin of this activity. Possiblereasons are the electronic structure due to the confined electrons or geometric effects like under-coordinated atoms. The metal-oxygen interaction may also be important. Results for small Auclusters on metal-oxide surfaces show that chemical properties can change drastically with eachadditional atom [2]. In our experiment small mass-selected Au cluster anions were softlanded onvarious substrates. Oxidation and Reduction behavior was studied by X-Ray Photoelectron Spec-troscopy (XPS).On Si wafers covered with native oxide layers mass-selected Aun (n=2-13) cluster anions were de-posited. In order to increase defect density the wafers were etched before introducing to the UHVsystem. Regarding the appearance of Au-oxide peaks the results indicate an interesting even-oddpattern in oxidation behavior for n=4-8 as it has previously been found for Au anions in gas phaseexperiments [3]. Because of their low reactivity towards atomic oxygen Au5 and Au7 can thereforebe suggested to be highly stable. The oxidized clusters can be reduced by CO. Exposure of theseclusters to NaOH solution leads to an inversion of the observed even-odd alternation.

Figure 1: Au4f XPS spectra of Aun clusters on SiO2/Si after exposure to oxygen atmosphere.

For Aun (n=2-9) clusters on sputtered HOPG this even-odd pattern in chemical properties is notobserved. However, Au8 seems to exhibit a more readily uptake of atomic oxygen compared to theother clusters. Again the oxidized Au8 cluster can be reduced by CO. STM measurements of Au7

clusters show that this cluster is stable on defects and exhibits two layer, 3D structures.

[1]R. Meyer et al., Gold Bulletin 37, 72 (2004).[2]S. Lee et al., J. Am. Chem. Soc. 126, 5682 (2004); B. Yoon et al., Science 307, 403 (2005).[3]B.E. Salisbury et al., Chem. Phys. 262, 131 (2000); D. Stolcic et al., J. Am. Chem. Soc. 125, 2848 (2003)

148


Ag islands on C60 functionalized Au(111) compared todeposited Ag clusters

Stefanie Duffe1, Thomas Irawan1, Torsten Richter1, Benedikt Sieben1, Chunrong Yin2,Bernd von Issendorff2, Heinz Hövel1

1. Universität Dortmund, Experimentelle Physik I, 44221 Dortmund, Germany2. Universität Freiburg, Fakultät für Physik, 79104 Freiburg, Germany

We are interested in the electronic structure of clusters in contact with a surface. In addition toexperiments with mass selected Ag clusters we grew Ag islands by evaporation of Ag atoms on aC60 functionalized Au(111) surface at a temperature below 50 K.

The investigation of the silver islands with a low temperature scanning tunnelling microscope showsquite broad height distributions. Tempering the cluster/surface system up to different temperaturesleads to a higher mobility of the silver islands and more and more coalescence. We observed a strongcoalescence at temperatures above 150 K.

These experiments can be compared with experiments of mass selected geometrically magic clustersdeposited on C60/Au(111) at room temperature and at T ≈ 165 K. Here we observed very narrowheight distributions.

We also studied the position of the clusters relative to the C60 molecules. We found out that thesize selected clusters are on top of a C60 molecule and the grown islands at a position between twoC60 molecules.

(a) (b)

Figure 1: (a) Ag islands grown by silver atom deposition at T < 50 K and subsequent annealing to265 K show a broad height distribution. Image size: 50 x 50nm2. Height scale: 0 − 2 nm. (b) Ag309

clusters deposited at T ≈ 165 K. All clusters show the same height of h ≈ 2.65 nm. Image size: 65x 65 nm2. Height scale: 0 − 2.75 nm.

149


Size-selected clusters on surfaces: Chemical and spectroscopicproperties

Stefan Gilb

Chair Physical Chemistry I, Technical University Munich, Germany

On the basis of specific examples taken from our laboratory, the most important steps in preparingsize-selected metal clusters supported on thin oxide films and investigating their chemical and spec-troscopic properties are presented. The clusters are generated by a laser vaporization source. Aftermass selection in ultrahigh vacuum the clusters are softlanded on various substrates. Temperatureprogrammed reaction spectroscopy, Fourier-transformation infrared spectroscopy as well as absorp-tion spectroscopy are available for probing the properties of the clusters. With these techniques, itwill be shown that the reactivity of nanoscale systems is mainly dominated by quantum-size effectsthat govern the electronic spectra of clusters, by the structural dynamical fluxionality of clusters,as well as by impurity-doping effects.Specific systems of various sizes will be presented. First, the CO oxidation catalyzed by mass selectedgold cluster on MgO will be discussed. Secondly, cavity ringdown spectroscopy, a highly sensitivephotoabsorption technique, will be discussed in detail. First results on monodispersed samples ofAun=1,2,4,8,20 are presented and discussed in conjunction with TD-DFT calculations.

150


MD Simulations of Porous Single and Multiple Layers of Ge

Ari Harjunmaa1, Jura Tarus2, Kai Nordlund1 and Juhani Keinonen1

1. Accelerator Laboratory, University of Helsinki, P. O. Box 43, Helsinki FIN-00014, Finland2. CSC - Scientific Computing Ltd., P. O. Box 405, Espoo FIN-02101, Finland

Porous semiconductor surfaces have been an object of interest ever since the discovery of the strongvisible photoluminescence of porous Si at room temperature [1]. Porous films are usually madeusing anodic etching or a similar top-down method. However, some attention has also been givento bottom-up methods such as low-energy cluster beam deposition (LECBD)[2],[3]. Using clusterbeams, it is easier to grow multiple superimposed layers having different porosities, which can beused for waveguide applications. By switching between sputtering targets, it is also possible tocreate multielemental multilayers, which allows greater variety in the properties of the layers.

The purpose of this study is to expand the results of our previous work, where we used classicalmolecular dynamics (MD) to simulate the deposition of Ge clusters on a Si surface in an effort toproduce porous Ge films [4]. More clusters are deposited at high energy (1 eV per atom) on theporous samples obtained in that study, with the goal of seeing how a porous surface would be able tosustain the high-energy cluster bombardment necessary for growing porous multilayers. We presentthe results for the high-energy depositions of forty 1018-atom Ge clusters on porous layers grownusing clusters with energies of 10 and 100 meV per atom.

(a) (b)

Figure 1: Atomic cross-sections of the high-energy layers deposited using an energy of 1 eV ontoporous surfaces grown with a deposition energy of 10 meV (a) and 100 meV (b) per atom.

[1]L.T. Canham, Appl. Phys. Lett. 59, 1046 (1990).[2]P. Mélinon, P. Kéghélian, B. Prével, A. Perez, G. Guiraud, J. LeBrusq, J. Lermé, M. Pellarin and M. Broyer, J.

Chem. Phys. 107, 10278 (1997).[3]D. Amans, S. Callard, A. Gagnaire, J. Joseph, G. Ledoux and F. Huisken, J. Appl Phys. 93, 4173 (2003).[4]A. Harjunmaa, J. Tarus, K. Nordlund and J. Keinonen, Eur. Phys. J. D, accepted for publication.

151


Electrocatalysis on Size-Selected Cluster

Mayrhofer, K.J.J.; Wiberg, G.; Heiz, U.; Arenz, M.

TU München, Lehrstuhl für Physikalische Chemie; D-85748 Garching, Germany

The aim of our newly started research project is to understand the size-dependent electrocatalyticproperties of metal particles supported on planar substrates. In contrast to previous studies, the in-vestigated metal particles will be prepared in UHV utilizing a laser evaporation source, size selectedin a quadrupole mass spectrometer, and softlanded onto different substrates. This procedure willallow for the first time in electrochemistry the investigation of particle size effects in the non-scalablesize regime, i.e., where the electronic, and thus electrocatalytic, properties of the particles change ina non-monotonic manner and are not scalable from the bulk properties. On these size-selected par-ticles, electrocatalytic measurements will be performed focusing on processes occurring in polymerelectrolyte membrane fuel cells (PEMFC). That is, the oxidation of hydrogen, the influence of COin the hydrogen fuel, methanol oxidation as well as the oxygen reduction reaction. By comparingclusters of less than 1nm in diameter, with nanoparticles of up to 5 nm in diameter (prepared by wetchemical means), important insights about the active reaction centers on nanoparticles and clustersare expected.

Figure 1: Comparison of the results from cyclovoltametry, CO bulk oxidation and Oxygen reductionreaction, showcasing the adsorption of OH and its impact on electrochemistry on the Pt electrodesurface depending on the particle size.

152


Liquid Drop Model for Supported Clusters

Veronika Semenikhina, Andrey Lyalin, Andrey Solov’yov, and Walter Greiner


We have performed a theoretical exploration of structure formation and stability of atomic clustersdeposited on a surface. In order to describe energetics, stability and shape of an atomic cluster thatinteracts with a surface we adapt the simple Liquid Drop Model (LDM). In particular, we investigatethe role of cluster - surface interaction in the stability and deformation of Ar clusters with numberof particles N (ranging from 4 to 150 particles) deposited on a graphite surface (001). The proposedmodel takes into account the cluster-surface interaction as well as the cluster deformation due to theinteraction with the surface. We proceed from the fact that the total energy of free clusters of noblegases can be predicted within LDM with high accuracy [1],[2]. In the present work we elucidatethe applicability of LDM to the description of properties of deposited clusters of noble gases. Weobtain a good agreement between the results of LDM and the results of numerical simulations basedon the dynamical search of the most stable isomers forming in the cluster growth process. Thenumerical simulations were performed with the use of cluster fusion algorithm (CFA) [1],[2]. Wedemonstrate that LDM can be used for simple estimation of the shape, stability and energetics ofdeposited atomic clusters.

(a) (b) (c)

Figure 1: (a): binding energy per atom for Ar clusters deposited on a graphite surface (001) calcu-lated within LDM (line) and CFA (dots); (b): free Ar55 cluster; (c): Ar55 deposited on a graphite(001) surface.


[1]I.A.Solov’yov, A.V.Solov’yov, W.Greiner et al., Phys. Rev. Lett. 90, 053401 (2003).[2]I.A.Solov’yov, A.V.Solov’yov, W.Greiner, Int. J. Mod. Phys. 13, 697 (2004).[3]V.V. Semenikhina, A. Lyalin, A. Solov’yov, and W. Greiner in preparation for JETP (2007).

153


Inner-shell photoelectron spectroscopy of size-selectedCu-clusters on Si

N. Ferretti, B. Balkaya, M. Neeb, and W. Eberhardt

BESSY, Albert-Einstein Str. 15, D-12489 Berlin-Adlershof

XPS and XANES spectra of supported Cun-clusters up to n=70 atoms have been measured with softX-ray synchrotron radiation and are compared with the respective bulk spectra. The clusters wereproduced by a magnetron sputter cluster source and mass-selected prior to deposition by a magneticsector field. After passing the magnetic mass filter an individual cluster mass was softly landed ontoa biased Si-wafer substrate. A cluster coverage of ∼5x1012 cluster (∼100 pA) was reached within∼30 minutes for each sample.

XANES spectra (Cu-L3-edge) of individual cluster sizes are exemplarily shown in the adjoiningfigure. With respect to the bulk, the L3-edge for all clusters is shifted to a higher absorption energyby 1-2 eV for small clusters [1] and converging smoothly towards the bulk value with increasingcluster size. Similarly, a blue shift of the core electron binding energy is observed which amountsto ∼1 eV for small clusters and converge towards the bulk value with increasing cluster size. Theabsorption energy is located within the ionization continuum, i.e. above the 2p-XPS threshold,indicating a closed d-shell of the supported Cu-clusters. Upon oxidation, as experimentally shownfor an oxidized Cu10-sample, an excitonic peak below the 2p-XPS threshold reveals a charge transferfrom the filled d-band towards oxygen. Another distinct difference between the XANES spectra ofthe bulk and cluster is the missing fine structure [2]-[3] beyond the L3 edge at energies >936 eVfor sizes smaller than 10 atoms. At a cluster size ≥13 atoms a second absorption feature arises at∼938 eV (see figure) indicating a morphology change from flat to 3-dimensional compact structures.The band gap of the supported clusters has been estimated from a rigid band model. A stronginteraction between the Si-wafer and the Cu cluster is indicated by a clear deviation from the liquiddrop model.

[1]N. Ferretti, B. Balkaya, A. Vollmer, M. Neeb, W. Eberhardt, J. Electron Spect. Relat. Phenom., in press.[2]H. Ebert, J. Stöhr, S. Parkin, M. Samant, A. Nilsson, Phys. Rev. B 53, 16067 (1996).[3]T. Tiedje, J.R. Dahn, Y. Gao, K.M. Colbow, E.D. Crozier, D.T. Jiang, W. Eberhardt, Solid State Comm. 85, 161

(1993).

154


DFT-Investigations of the Coalescence Behaviour of SmallMagic Si Clusters on Surfaces

W. Quester, M. Schach, and P. Nielaba

Department of Physics, University of Konstanz, 78457 Konstanz, Germany

Experimental results indicate that small magic Si clusters do not form islands of bulk Si on weaklyinteracting surfaces (HOPG). Therefore these clusters might be building blocks for new cluster ma-terials.Earlier simulations [1] for Si4 were extended to Si7 and Si10. Potential energy curves of two ap-proaching Si clusters were calculated for different reaction channels using Density Functional Theoryimplemented in the CPMD code available at [2]. For Si7 it was shown that the cluster-cluster in-teraction is either repulsive or there are fusion barriers that block spontaneous fusion of clusters atsufficient low temperatures. So far the potential energies found for different reaction channels ofSi10 were repulsive or had minima which were too weak to be interpreted as covalent bonds.In the calculations neglecting the cluster-surface interaction none of the above mentioned clusterstends to form bulk Si. In the next step a graphite surface is included in the simulations to get abetter match with experiments. The influence of surface defects is studied as well.

[1]M. Grass, D. Fischer, M. Mathes, G. Ganteför and P. Nielaba, Appl. Phys. Lett., 81, 3810 (2002).[2]www.cpmd.org.

155


WnSm-Clusters: Search for new nano-materials

W. Westhäuser1, T. Mangler1, T. Fischer1, N. Bertram1, S. Gemming2, G. Seifert2, and G.Ganteför1

1. Department of Physics, University of Konstanz, Germany2. Department of Physical Chemistry and Electrochemistry, University of Dresden, Germany

The bulk semiconductors WS2 and MoS2 form layered structures similar to bulk graphite. Therefore,small clusters of these materials might also build up cage-structures analogous to the famous C60.Small WnSm- and MonSm-clusters are studied theoretically using density functional theory andexperimentally with photoelectron spectroscopy of gas phase clusters [1], [2]. These studies revealmagic species with a high stability, which might be suitable as a building block for new clustermaterials. In contrast to graphite, the larger species form nanoplatelets with triangular shape [3].

To investigate the suitability as building blocks for new cluster materials, size-selected WnSm-clusters are deposited in soft-landing mode on Ag(111) and SiO2/Si surfaces at ultrahigh vacuumconditions. These samples are analyzed using electron energy loss spectroscopy (HREELS), andX-ray photoelectron spectroscopy (XPS). The data are compared to bulk WS2.

[1]N. Bertram, Y.D. Kim, G. Ganteför, Q. Sun, P. Jena, J. Tamuliene and G. Seifert, Chem. Phys. Lett. 396 (2004),341

[2]S. Gemming, J. Tamuliene, G. Seifert, N. Bertram, Y.D. Kim and G. Ganteför, Appl. Phys. 82 (2006), 161[3]N. Bertram, J. Cordes, Y.D. Kim, G. Ganteför, S. Gemming and G. Seifert, Chem. Phys. Lett. 418 (2006), 36

156


Nanocluster Metrology: Mass-spectrometry and shapedetermination using size-selected clusters

N.P. Young, Z.Y. Li, S.Palomba, Y. Chen, R.E. Palmer

Nanoscale Physics Research Laboratory, School of Physics & Astronomy, University of Birmingham,Edgbaston, Birmingham B15 2TT, United Kingdom

[email protected]

Clusters are often referred to as nanoscale ’building blocks’, with physical and chemical propertiesthat vary significantly with size. Mass-selected cluster beam deposition has allowed detailed stud-ies of such size-dependent phenomena. However, mass-selected beams are not always accessible,or indeed appropriate in certain cluster-based applications. For example, the catalytic activity ofnanoparticles is well known to be associated with cluster size and shape. However the exact atomicmass of catalytic nanoparticles is often a poorly defined quantity; a difficulty exacerbated by thesintering of supported particles during processing. Additional methods of accurate nanoscale char-acterisation are required to further comment upon structural and size-dependent phenomena.Characterisation of supported clusters has traditionally advanced via spectroscopic analysis or mi-croscopical imaging techniques. The former often only provides average values of a collection ofclusters, while imaging methods are generally limited to measurements of spatial dimensions, oftenproviding only a two-dimensional view of the three-dimensional cluster. In the present study, wereport on the successful application of high-angle annular dark field scanning transmission electronmicroscopy (HAADF-STEM) to quantify a range of supported Au nanoclusters in terms of theiratomic mass. When only electrons scattered through large angles are collected, HAADF imagecontrast is strongly related to the type and number of atoms within a crystalline atomic column [1].Deposited mass-selected clusters are a model system to study using HAADF-STEM, since their welldefined size allows for the prospect of calibrating HAADF image intensity as a function of clustermass [2]. For clusters whose depth perpendicular to the substrate is sufficient to maintain approxi-mately incoherent imaging, a reliable calibration curve of scattered electron intensity as a functionof cluster mass was obtained. The resulting empirical knowledge allowed the atomic mass of otherunknown Au nanoparticles to be deduced. Colloidal Au nanoparticles and Au islands depositedvia vacuum sublimation are presented as two contrasting examples. Comparison of the estimatedcluster mass with simple geometric models allows the overall three-dimensional cluster morphologyto be deduced. Prediction of a quasi-spherical morphology for the colloids confirmed the capabilityof the method.The mass-spectrometry aspects of the technique are particularly suited to the sub 2-3 nm size range,and thus particularly appropriate to application in the characterisation of catalytic nanoparticles.The technique has advantages over traditional tomographic methods in terms of speed of acquisitionof results. Metallic clusters from two to several thousand atoms in size may be characterised. Largeclusters, in excess of 104 atoms are inaccessible due to the large projected column depth and crystaltwinning effects at this size.

[1]A. Singhal, J.C. Yang, J.M. Gibson, Ultramicroscopy 67, 191 (1997).[2]Z.Y. Li, N.P.Young et al. Submitted

157


Three-dimensional atomic-scale structure of size-selected goldnanoclusters

Z.Y. Li1, N.P. Young1, M. Di Vece1, S. Palomba1, R.E. Palmer1, A. Bleloch2, B.C. Curley3,R. L. Johnston3, J. Jiang4 and J. Yuan4

1. Nanoscale Physics Research Laboratory, School of Physics & Astronomy, University of Birmingham,Edgbaston, Birmingham B15 2TT, United Kingdom

2. UK SuperSTEM Laboratory, Daresbury Laboratory, Daresbury, WA4 4AD, United Kingdom3. School of Chemistry, University of Birmingaham, Edgbaston, Birmingham B15 2TT, United Kingdom

4. Department of Materials Science & Engineering, Tsinghua University, Beijing 100084, [email protected]

The aberration-corrected scanning transmission electron microscope (STEM) is used to study size-selected gold nanoclusters, preformed in the gas phase [1] and soft-landed onto an amorphous carbonsubstrate. Atomic resolution high-angle annular dark field (HAADF) imaging, coupled with imagesimulations using relaxed atomic coordinates provided by a genetic algorithm search, allows theidentification of not only the size and shape, but also the atomic structure and orientation of theultra-small Au309 ’magic number’ clusters. We show that structures can be identified with eitherdecahedral or cuboctahedral geometries. This phenomenon is consistent with energetic considera-tions, specifically, the existence of many local minima with relatively small energy barriers betweenparticularly structures [2]. The work illustrates a new and very promising way to study structureand stability of supported ultra-small metal clusters in the nanometre size range, e.g. catalystparticles, with single atom sensitivity.

[1]S. Prantontep, S. J. Carroll, C. Xirouchaki, M. Streun, R. E. Palmer, Rev. Sci. Instrum. 76, 045103 (2005)[2]B.C. Curley et al, Submitted to JPC

158


Thin-film Effects on MgO/Mo-supported Au clusters

Pentti Frondelius1, Hannu Häkkinen1,2, Karoliina Honkala2

1. NanoScience Center / Department of Physics, University of Jyväskylä, P.O. Box 35, Jyväskylä,FIN-40014, Finland

2. NanoScience Center / Department of Chemistry, University of Jyväskylä, P.O. Box 35, Jyväskylä,FIN-40014, Finland

The adsorption of small gold clusters on Mo(100) supported MgO(100) ultra thin films was studiedby the means of the density functional theory [1]. The optimal geometries, adsorption energies (fig.1) as well as the charge states of the clusters of up to six atoms were investigated systematicallyboth on regular and defected surfaces. It was found that the Mo(100) support induces clear changesto all the cluster properties compared to the adsorbates on a single crystal MgO(100).On a regular MgO/Mo flat cluster geometries wetting the surface are energetically the lowest oneswhile on a single crystal MgO the best geometries are upright or slightly tilted ones. Also the oxygenvacancy of a MgO/Mo stabilizes the flat clusters compared to the vacancy on a single crystal MgO.On a MgO/Mo the adsorption energies of the clusters are increased compared to the adsorptionenergies on a single crystal MgO independent of the investigated site. However, on a terrace site theadsorption energy oscillates as a function of a cluster size which is not seen in the case of an oxygenvacancy. Furthermore the clusters gain more charge in the presence of the Mo metal. On a regularMgO/Mo the clusters become anionic while on a regular single crystal MgO they are essentiallyneutral. On an oxygen vacancy the clusters are charged both on a MgO/Mo and a single crystalMgO.The anionic nature of the adsorbates in the presence of a Mo film explains the even odd oscillationsin the cluster adsorption energy on a regular MgO/Mo. The difference in the adsorption energiesof the clusters between a single crystal MgO and a MgO/Mo substrates correlates with the electronaffinities of the clusters. The electron affinity oscillates as a function of a cluster size leading to theoscillations in the adsorption energy.

Figure 1: The cluster adsorption energies on a regular MgO(100) (circles), on a regularMgO(100)/Mo(100) (filled circles), on an O vacancy of a MgO(100) (squares) and on an O va-cancy of a MgO(100)/Mo(100) (filled squares).

[1]P. Frondelius, H. Häkkinen and K. Honkala, to be published.

159


Softlanding, STM imaging and thermally activated clusterdecay for Ag309 and Ag561 clusters on a C60 monolayer

Heinz Hövel1, Stefanie Duffe1, Thomas Irawan1, Markus Bieletzki1, Torsten Richter1,Benedikt Sieben1, Chungrong Yin2, Bernd von Issendorff2, Michael Moseler3,4

1. Universität Dortmund, Experimentelle Physik I, 44221 Dortmund, Germany2. Universität Freiburg, Fakultät für Physik, Hermann-Herder Straße 3, 79104 Freiburg, Germany3. Fraunhofer-Institut für Werkstoffmechanik IWM, Wöhlerstraße 11, 79108 Freiburg, Germany

4. Freiburg Materials Research Center, Stefan-Meier-Straße 21, 79104 Freiburg, Germany

Mass selected Ag+561±5 and Ag+

309±3 clusters were deposited with low kinetic energy on an Au(111)surface functionalized with an ordered monolayer of C60 molecules. This substrate system provedto be a new and promising choice for the investigation of mass selected clusters attached to asurface. Stable cluster samples could be obtained for deposition at a temperature of 165 K, whichin STM images measured at 77 K gave an extremely narrow height distribution with (3.1±0.2) nmand (2.6±0.2) nm cluster height, respectively. Molecular dynamics simulations of the depositionsuggest that the experimental conditions indeed are close to softlanding with only minor distortionsof the icosahedral cluster shape at the cluster-fullerene interface. After annealing the samples up toroom temperature we observed thermally activated cluster decay and penetration of the Ag materialthough the C60 film. Very interesting is the appearance of a sharp maximum at about 1.7 nm clusterheight during the decay of the deposited clusters, which was also observed for room temperaturedeposition for a broad range of cluster sizes (55, 147, 309, 561, 923 atoms). This seems to be anindication for some ’magic’ cluster size in the cluster-surface system.

Figure 1: STM images measured at 77 K and height distributions for Ag309 clusters deposited on anAu(111) surface functionalized with 1 monolayer of C60 molecules. The left image shows the sampleas deposited, the middle and right image after annealing to room temperature for 45 and 180 min,respectively. We observe cluster decay and the emergence of a ’magic’ metastable cluster height of1.7 nm. The Ag material penetrates the C60 film and forms monolayer islands below (cf. the lineprofile in the right image).

160

Dynamics


Statistical Decay of C60

B. Climen, B. Concina, M.A. Lebeault, F. Lépine, B. Baguenard, C. Bordas

Université Lyon 1 ; CNRS ; LASIM UMR 5579, 43 boulevard du 11 novembre 1918,F-69622 Villeurbanne Cedex, France

Two experimental techniques have been combined to get a deeper insight into the decay of C60 excitedby a nanosecond laser (355 nm). Our experimental set-up consists in a time-resolved velocity-mapimaging spectrometer coupled with a time-of-flight mass spectrometer [1]. Time-resolved electronkinetic energy spectra were measured for the first time on a large delay range (from 0.1 to 10 µs). Inaddition to time-of-flight mass spectra, the ion formation was characterized by mass-resolved kineticenergy distributions, which is an original aspect of this contribution [2].These three sets of experimental data are compared with a statistical model based on detailedbalance: (1) ion time-of-flight spectra (2) time-resolved electron kinetic energy distribution givingdirect access to delayed (thermionic) emission, and (3) mass-resolved ion kinetic energy distributionfor C+

2n (44 ≤ 2n ≤ 60). Ion time-of-flight mass spectra are compared to a Monte-Carlo simula-tion. Parameters (activation energy and pre-exponential factor in the Arrhenius decay formula)characterizing C2 emission and delayed ionization agree with a similar study from the literature [3].The temperatures extracted from the time-resolved kinetic energy distributions of the electrons giveinformation on the activation energy and the pre-exponential factor for C2 emission from C60 [4].The measured distributions in ion kinetic energy characterize the chains of sequential C2 emissionsleading to the ion fragments. They are compared to a simulation of these decay chains using asinputs fragment temperatures from the Monte-Carlo simulation [2]. Agreement with the experimentis then achieved, confirming the validity of the statistical approach which describes three series ofexperimental data in the same frame.

[1]F. Pagliarulo, B. Climen, B. Baguenard, F. Lépine, M.A. Lebeault, A. Ollagnier, J. Wills, C. Bordas, Int. J. MassSpectrometry 252, 100 (2006).

[2]B. Climen, B. Concina, M.A. Lebeault, F. Lépine, B. Baguenard, C. Bordas, submitted to Chem. Phys. Lett.[3]B. Concina, S. Tomita, J. U. Andersen, P. Hvelplund, Eur. Phys. J. D 34, 191 (2005)[4]F. Lépine, C. Bordas, Phys. Rev. A 69, 53201 (2004)

163


Delayed Electron Emission as a Probe of Finite-SizeProperties of Small Carbon Clusters

F. Pagliarulo, B. Baguenard, B. Concina, F. Lépine, C. Bordas

Université Lyon 1 ; CNRS ; LASIM UMR 5579, 43 boulevard du 11 novembre 1918,F-69622 Villeurbanne Cedex, France

We have experimentally studied the electron emission from small carbon cluster anions excited bynanosecond laser (Xe:Cl, 308 nm, or Xe:Cl pumped dye laser). Time-resolved velocity-map imagingallows us to separate direct emission and delayed emission which is regarded as a statistical process.Kinetic energy spectra of emitted electrons from size-selected C−n (10 ≤ n ≤ 22) have been recordedfor a delay of 90 ns at λ = 308 nm showing only the contribution of delayed emission [1], [2]. Inthe framework of detailed-balance theory, the kinetic energy spectrum is shown to depend on thedaughter temperature Td:

P (ε) ∝ εγexp(−εkBTd

)with γ = 1

2 deriving from the assumption of a spherical daughter cluster interacting with the emittedelectron via a Langevin potential. Measured daughter temperatures Td are in good agreement withthe detailed balance predictions. Wavelength dependent measurements at a given delay do not showany significant variation of Td in accordance with the statistical approach.More interestingly, the shape of the kinetic energy distribution should be a promising way to studygeometrical properties of small clusters or finite-size effects in general. Indeed, it is related to thecross-section of the reverse process within the detailed-balance formalism. A first approach is todetermine the exponent γ from the experimental spectra. A size-dependence is found: this parameterincreases with the size from about 0.2 for n = 10 to about 0.5 for n > 17. A value of 0.5 correspondsto a (large) spherical cluster (with the assumption of a Langevin potential) but the interpretationfor values significantly different is not yet clear. Wavelength dependent measurements do not showany variation of γ. Thus, this parameter does not depend on the excitation process and seems tocorrespond to an intrinsic property of the cluster. PST calculations are underway to find a relationbetween cluster properties (possibly geometry) and shape of the kinetic energy distribution.

[1]J.B. Wills, F. Pagliarulo, B. Baguenard, F. Lépine, C. Bordas, Chem. Phys. Lett. 390, 145 (2004).[2]F. Pagliarulo, B. Climen, B. Baguenard, F. Lépine, M.A. Lebeault, A. Ollagnier, J. Wills, C. ++[3]Bordas, Int. J. Mass Spectrometry 252, 100 (2006).

164


Sequential decay in clusters over long time scales: insightsfrom phase space theory

F. Calvo1, P. Parneix2

1. Laboratoire de Chimie et Physique Quantiques, IRSAMC, Université Paul Sabatier, 118 Route deNarbonne, F31062 Toulouse, France

2. Laboratoire de Photophysique Moléculaire, Bât. 210, Fédération Lumière-Matière, Université Paris-Sud,F91405 Orsay, France

A general theoretical framework for describing the thermally induced sequential decay in atomicclusters is presented. Our scheme relies on a full treatment of individual dissociation steps based onphase space theory (PST), built into a kinetic Monte Carlo (kMC) procedure [1]. This combinedPST/kMC approach allows us to follow the evolution of several statistical properties such as the size,the angular momentum, or the temperature of the cluster over arbitrarily long time scales. Quan-titative accuracy is achieved by incorporating anharmonicities of the vibrational densities of states,the rigorous conservation of angular momentum via the effective dissociation potential, and a propercalibration of the rate constants. Our approach is tested and validated on selected Lennard-Jones(LJ) clusters in various situations, using molecular dynamics (MD) simulations as a benchmark.Several approximations, including a mean-field rate equation treatment, are critically discussed. Fi-nally, by coupling the present method with explicit non-adiabatic dynamical trajectories, the longtime relaxation in electronically excited Ar+n can also be investigated [2].

Figure 1: Average cluster size for an initial LJ56 cluster thermally excited at several temperatures,obtained using our kMC method and compared with explicit MD simulations.

[1]F. Calvo and P. Parneix, J. Chem. Phys. 126, 034309 (2006).[2]work in progress.

165


Directed Electron Emission from Metal Clusters Induced byPlasmon-Enhanced Excitation with Intense Femtosecond

Laser Pulses

Thomas Fennel, Johannes Passig, Tilo Döppner, Christian Schaal, Josef Tiggesbäumker,Karl-Heinz Meiwes-Broer

Institute of Physics, University of Rostock, Universitätsplatz 3, 18051 Rostock, Germany

Silver clusters AgN (N=500..2000) generated with a magnetron sputtering source are irradiatedwith intense dual femtosecond laser pulses (Ilaser=1013...1014 W/cm2). In previous experiments wehave demonstrated that the charging of the clusters can be enhanced by the resonant excitation ofthe dipolar mode of the delocalized electrons. When tuning the optical delay of the dual pulses toachieve such plasmon enhanced ionization both the yield of highly charged ions and the maximumkinetic energy of the emitted electrons could be maximized [1],[2]. As an extension of these studies,here we present the results of energy and angular resolved measurements of the electron emission.The most energetic electrons are emitted along the laser polarization axis, induced by resonantplasmon excitation at optimum pulse delay (cf. Fig. 1). A microscopic analysis of correspondingsemiclassical simulations indicates strong temporal beating in the electron emission at resonanceand identifies phase-matched electron-cluster recollisions leading to multi-plasmon deexcitation, toproduce the observed directional preference of most energetic electrons [3].

Figure 1: Electron time of flight spectra of silver clusters irradiated with intense (8 x 1013 W/cm2)dual femtosecond laser pulses. At the optimum optical delay of ∆t=1.3ps a clear difference is ob-served for electrons emitted parallel (squares) and perpendicular (triangles) to the laser polarizationaxis. The straight lines are guides to the eye only.

[1]T. Döppner, Th. Fennel, Th. Diederich, J. Tiggesbäumker, and K.-H. Meiwes-Broer, Phys. Rev. Lett. 94, 013401(2005)

[2]T. Döppner, Th. Fennel, P. Radcliffe, J. Tiggesbäumker, and K.-H. Meiwes-Broer, Phys. Rev. A 73, 031202(R)(2006)

[3]Th. Fennel, T. Döppner, J. Passig, Ch. Schaal, J. Tiggesbäumker, and K.-H. Meiwes-Broer, submitted.

166


Molecular wavepacket oscillations of ultracold Rb2 andoptimal control of multi-photon ionization of Rb2 molecules in

a MOT

A. Merli, F. Weise, S. Birkner, S. M. Weber, F. Sauer, L. Wöste, A. Lindinger1,W. Salzmann, T. G. Mullins, J. Eng, M. Albert, R. Wester, and M. Weidemüller2

1. Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, D-14195, Berlin, Germany2. Physikalisches Institut, Universität Freiburg , Hermann Herder Str. 3, D-79104, Freiburg, Germany

Our long-term aim is the efficient formation and vibrational cooling of ultracold Rb2 moleculesto their vibrational ground state by pump-dump like processes via an intermediate excited state,theoretical predicted by [1]. First pump-probe experiments with femtosecond light pulses in adark SPOT magneto-optical trap were successfully performed in order to gain information aboutthe molecular dynamic in the excited state of the Rb-Dimer. The observed wavepacket oscillationperiods are depending from the cut-off of the spectral frequencies (made in the Fourier plane ofa zero-dispersion compressor) in the pump pulse below the Rb atomic D1 and D2 resonances,respectively. Linear chirps of the excitation pulse influence the pump-probe spectra. Measurementsat different bright state fractions [2] of the trapped molecules provide advice about the origin ofthe molecules which are oscillating. By applying a closed feedback loop in order to optimize theionization process of the rubidium dimers, two dominant frequency bands in the optimal spectrumwere observed. The obtained data reveal the ionization process and the involved excited states.

Figure 1: Experimental Rb+2 count rates as a function of pump-probe delay between the red excitation

pulse and the green ionization pulse. The oscillations reflect the wavepacket dynamics in the excitedstate. The oscillation periods decrease as spectral cut-off in pump pulse is detuned from the Rb - D1atomic resonance.

[1]C. P. Koch, R. Kosloff, and F. Masnou-Seeuws, Phys. Rev. A 73, 043409 (2006).[2]C.G. Townsend, N.H. Edwards, K.P. Zetie, C.J.Cooper, J. Rink, and C.J Foot, Phys. Rev. A 53, 1702, (1996)

167


Electronic Relaxation in Ag Nanoclusters

Marco Niemietz1 Markus Engelke1, Young Dok Kim2, and Gerd Ganteför1

1. Department of Physics, University of Konstanz, D-78457 Konstanz, Germany2. Division of Nano Sciences and Department of Chemistry, Ewha Womans University, 120-750 Seoul,

Korea

In small metal clusters, the density of electronic states is very much reduced compared to the bulk.Accordingly, many properties related to the electronic structure of matter can change dramaticallyfor clusters. The low density of states in small clusters having large gaps between electronic states,inhibits Auger-like decay channels that are responsible for ultrafast relaxation of optically excitedstates in bulk metals. Phonon-assisted relaxation is also inhibited by the fact that the electroniclevel spacing is much larger than the typical energy of a phonon. Only multi-phonon processescan lead to the relaxation of an optically excited state, which is rather unlikely and referred to asthe "phonon bottleneck". Thus, much longer relaxation times are expected for these clusters. Inthe presented work, the decay mechanism of excited electronic states in small Ag nanoclusters isstudied using time-resolved photoelectron spectroscopy. In contrast to the expectation mentionedabove, lifetimes below 1 ps were observed for most of the Ag−n (n < 22) studied here. The onlyexception is the magic Ag−7 with a relaxation time of ∼4 ps (Fig. 1). The observed fast relaxationsare discussed in view of their ability to undergo fast shape deformations [1]. This approach can alsoexplain the slow relaxation of the rigid Aun- nanoclusters (> 1 ns, Fig.1) [2]-[4] and the extremelyfast relaxation of the flexible magic Al13 − (∼200 fs) [3]-[5].

Figure 1: Examples of time-resolved photoelectron spectra of Ag−7 [1] and Au−6 [2],[3]. The photonenergies of the pump and probe pulses are 1.55 eV and 3.1 eV, respectively.

[1]M. Niemietz, M. Engelke, Y. D. Kim, and G. Ganteför, Phys. Rev. B, in press (2007).[2]M. Niemietz, P. Gerhardt, G. Ganteför and Y. D. Kim, Chem. Phys. Lett. 380, 99 (2003).[3]Y. D. Kim, M. Niemietz, P. Gerhardt, F. v. Gynz-Rekowski, and G. Ganteför, Phys. Rev. B 70, 035421 (2004).[4]P. Gerhardt, M. Niemietz, Y. D. Kim and G. Ganteför, Chem. Phys. Lett. 382, 454 (2003).[5]V. V. Kresin and Yu. N. Ovchinnikov, Phys. Rev. B 73, 115412 (2006).

168


Time-resolved Pump/probe-Photoelectron Spectroscopy ofIsolated Fullerene and Phthalocyanine Negative Ions

Oli T. Ehrler1, Ji Ping Yang1, Christian Rensing1, Albert Sugiharto1, Christof Hättig2,Andreas-N. Unterreiner3, Horst Hippler3, and Manfred M. Kappes1

1. Lehrstuhl für Physikalische Chemie Mikroskopischer Systeme, Universität Karlsruhe (TH),Kaiserstrasse 12, 76128 Karlsruhe, Germany

2. Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, Universitätsstrasse 150, 44801 Bochum,Germany

3. Lehrstuhl für Molekulare Physikalische Chemie, Universität Karlsruhe (TH), Kaiserstrasse 12, 76128Karlsruhe, Germany

Pump-probe spectroscopy in the gas phase opens up the possibility to study time constants andbranching ratios of excited state dynamics in the ultimate intramolecular limit without any per-turbing influence of a matrix. We present experimental results on the spectroscopy of mass selectedfullerene and phthalocyanine negative ions with 270 fs time resolution.

We have measured pump/probe photoelectron spectra of mass-selected, near room temperaturefullerenes C−60 in the gas phase [1]. Excitation of a higher vibronic transition in the characteristicnear infrared absorption band was performed at a wavelength of 780 nm. Lifetime of the vibra-tionally excited B̃−(2Eg) state at a calculated energy of 1.26 eV was found to be τ = 2.2 ± 0.2 ps.The main decay process corresponds to intramolecular radiationless transitions into ground stateC−60. This is in contrast to neutral C60 for which pumping at the absorption onset (1.95 eV) leadsto predominantly intersystem crossing.

Fourfold negatively charged metastable phthalocyanine tetrasulfonate ions with different centermetal ions [MPc(SO3)4]4 (with M = Cu, Ni, H2) were excited in the low energy wing of the in-tense Q-band band [2]. The temporal evolution of the transient photoelectron spectra shows multi-exponential decay mechanisms with lifetimes ranging from approximately hundreds of femtosecondsto several tens of picoseconds. While in the closed-shell nickel compound, Q-band absorption pop-ulates the lowest excited singulet state which rapidly relaxes (τ ≤250 fs) directly to the electronicground state, CuPc∗ shows a sequential decay due to non-adiabatic coupling involving three exciteddublet configurations and lifetimes of τ1 ≤ 250 fs, τ2 = 2.1 ± 0.6 ps, and τ3 = 83 ± 11 ps. Contrarily,de-excitation in the sulfonated free base phthalocyanine occurs also through a competing tunnelingelectron loss channel in the excited state leading to a characteristic autodetachment feature in thephotoelectron spectra. From extracted lifetimes the tunneling rate in the excited state can be es-timated between ktunnel = 1.4 · 1010 s−1 and 2.1 · 1012 s−1, more than ten orders of magnitudehigher than measured in the ground state [3]!

[1]O.T. Ehrler, J.P. Yang, C. Hättig, A.N. Unterreiner, H. Hippler, and M.M. Kappes, J. Chem. Phys., 125, 074312(2006).

[2]O.T. Ehrler, Thesis, Universität Karlsruhe (TH), 2006.[3]K. Arnold, T.S. Balaban, M.N. Blom, O.T. Ehrler, S. Gilb, O. Hampe, J.E. van Lier, J.M. Weber, and M.M.

Kappes, J. Phys. Chem. A, 107, 794 (2003).

169


Temperature dependent femtosecond two-photonphotoemission from ultra-thin MgO films

Mihai E. Vaida, Tobias Gleitsmann and Thorsten M. Bernhardt

Universität Ulm, Institut für Oberflächenchemie und Katalyse

Ultra-thin insulating oxide films were originally invented as support for nano-scale model catalysts.Investigations indicate that these thin films have almost the same chemical and physical propertiesof their bulk analogs. In particular MgO(100) film deposited onto Mo(100) single crystal exhibits theelectronic properties of a wide band gap (∼6eV) insulator. On the other hand it allows applying theelectronic spectroscopy, because charging effects are avoided due to the electron tunneling throughthe few monolayers film.Epitaxially magnesia thin films supported on a Mo(100) single crystal surface are produced in situby evaporating magnesium in oxygen atmosphere. The films composition and quality are analyzedby temperature programmed desorption spectrometry, Auger electron spectroscopy, and low energyelectron diffraction, respectively. Changes in the electronic structure of these films induced by postdeposition annealing were investigated via femtosecond two photon photoemission spectroscopy andelectron energy loss spectroscopy.

170


Dynamics in Atomic Clusters: Analysis and Control

Roland Mitrić, Ute Werner, Melanie Nößler, Vlasta Bonačić-Koutecký

Humboldt Universität zu Berlin, Institut für Chemie, Brook-Taylor-Strasse 2, 12489 Berlin, Germany

We present study of dynamics and ultrafast observables in the frame of pump-probe negative-to-neutral-to-positive ion (NeNePo) and time-resolved photoelectron spectroscopy (TRPES) in contextof cluster reactivity [1],[2]. This will be illustrated on examples of pure and mixed silver-gold trimersand their complexes with molecular and dissociated oxygen. First principle multistate adiabaticdynamics allows us to determine timescales of different ultrafast processes and conditions underwhich these processes can be experimentally observed. Furthermore, we present a strategy foroptimal control of ultrafast processes by shaped infrared laser fields based on the quantumchemicalMD ”on the fly” and our Wigner distribution approach [1], in context of isomerization in clusters,biomolecules and their complexes. The shapes of pulses can be assigned to underlying processes andtherefore optimal control can be used as a tool for analysis.

[1]V. Bonačić-Koutecký, R. Mitric, Chem. Rev. 105, 11 (2005).[2]V. Bonačić-Koutecký, R. Mitric, U. Werner, L. Wöste, R. S. Berry, Proc. Natl. Acad. Sci. 103, 10594 (2006).

171


New reactivity-promoting criterion based on internalvibrational energy redistribution

Roland Mitrić, Christian Bürgel, and Vlasta Bonačić-Koutecký

Humboldt Universität zu Berlin, Institut für Chemie, Brook-Taylor-Strasse 2, 12489 Berlin, Germany

We propose to introduce intrinsic dynamical properties as new criterion for promoting reactivity ofsmall size noble metal reactive centers relevant for heterogeneous catalysis. In order to illustratethe concept, collisions between Ag−6 or Au−6 clusters and molecular oxygen have been investigatedwith direct ab-initio molecular dynamics using DFT. We show that different nature and efficiencyof internal vibrational energy redistribution (IVR) during reaction dynamics is responsible for sig-nificantly different sticking probabilities of O2 to gold and to silver clusters. In the case of Au6 andO2 collisions, resonant IVR occurs between two subunits activating O2 and promoting subsequentreaction. In contrast, a dissipative IVR in Ag−6 and O2 molecule prevents O2 to react with otheradsorbates. These findings allow us to introduce the nature of IVR as new criterion for promotingreactivity of noble metal clusters: Resonant IVR between reactants promotes reactivity towardsadsorbates.

172


Analysis and Control of Metal Cluster and AdsorbatComplexes within the Framework of NeNePo-Spectroscopy

Shaohui Li, Juri Demuth, Bruno Schmidt, Waldemar Unrau, Xin Zhang, Aldo Mirabal,Torsten Siebert and Ludger Wöste

Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany

The presented work aims to further develop femtosecond time-resolved NeNePo (negative ion-to-neutral-to-positive ion) spectroscopy towards studying the catalytic properties of noble metal clus-ters and the respective adsorbate complexes [1]. We seek to expand this spectroscopic techniqueto allow for a comprehensive characterization of the electronic structure and nuclear dynamics ofthe respective cluster system. This goal is pursued in order to ultimately achieve the capability ofsteering the cluster dynamics along a specific reaction coordinate and herby reach a desired reactivegeometry via photo-excitation (see Fig. 1) [2]. Towards this goal, a tandem mass-spectrometer cou-pled with a hexadecapol ion trap allows for mass-selection of cluster anions, trapping and synthesisof adsorbate complexes, as well as a further mass-selection in the detection of the products generatedby means of photo-excitation with ultrashort laser pulses. For the photo-excitation, femtosecondwhite-light pulses are generated by means of self-phase modulation through filamentation in noblegas atmosphere. This laser source provides pulses that span a spectral range form the visible toNIR and can be modulated by a liquid crystal modulator. Laser pulses with a specifically tailoredelectric field can be generated that drive the electron detachment and subsequent transitions in thecluster system so that the nuclear dynamics lead to the desired reactive geometry as shown in Fig.1. [1], [2]

Figure 1: Potential energy scheme illustrating the controlled photo-excitation of a cluster system toa desired reactive geometry within the framework of femtosecond time-resolved NeNePo spectroscopy.The temporal distribution of the frequencies within the bandwidth of the laser pulse can be adaptedoptimally to the dynamics of the system for maximum efficiency in reaching the desired final state.

[1]"Analysis and Control of small isolated molecular systems," A. Lindinger, V. Bonacic-Koutecký, R. Mitric, D.Tannor, C. P. Koch, V. Engel, T. M. Bernhardt, J. Jortner, A. Mirabal, and L. Wöste, In Analysis and Controlof Ultrafast Photoinduced Reactions, O. Kühn, L. Wöste (eds.), Springer Series in Chemical Physics Vol. 87,Springer, Heidelberg, p25 (2007).

[2]V. Bonacic-Koutecký, R. Mitric, U. Werner, L. Wöste, and R. S. Berry, First Proc. Natl. Acad. Sci 103, 10594(2006).

173


Time-resolved photoelectron spectroscopy of gold-oxideclusters

Kiichirou Koyasu, Marco Niemietz, Matthias Götz, and Gerd Ganteför

Department of Physics, University of Konstanz, D-78464 Konstanz, Germany

Gold (Au) nanoparticles supported on metal oxides act as very active catalysts [1] although Au isquite inert in bulk form [2]. The catalytic activity has been shown for example for oxidation of COat low temperature [3], and has stimulated many further investigations. Moreover, the importanceof electron transfer from a metal oxide substrate to Au nanoparticles has been found [4], and hencepure and oxidized Au cluster anions [5],[6] have been studied.We have investigated the time evolution of the electronic structure in AunO−

m clusters in the gasphase by femtosecond (fs) time-resolved photoelectron spectroscopy: The energy of the excitingphoton is stored in the cluster and distributed over a limited number of degrees of freedom [7].Using a photon energy of 3.1 eV for both, the pump and the probe pulse, Au2O− (Fig. 1) showeda long-lived excited state (peak A, τ > 10 ps) and two subsequently appearing peaks (B and C).The peaks B and C can be assigned to the fragments Au− (Fig. 1b) and AuO− (Fig. 1c). In thiscase, the cluster might remain in the excited state and directly dissociates into different fragmentchannels, which show different temporal behavior.

Figure 1: Time-resolved PES spectra of (a) Au2O− and PES spectra of (b) Au− and (c) AuO−. Along-lived (>10 ps) excited state (peak A) was observed, followowed by dissociation into Au− (peakB) and AuO− (peak C).

[1]A. Stephen K. Hashmi and Graham J. Hutchings, Angew. Chem. Int. Ed., 45, 7896 (2006).[2]B. Hammer and J. K. Nørskov, Nature, 376, 238 (1995).[3]M. Haruta, Catal. Today, 36, 153 (1997).[4]A. Sanchez, S. Abbet, U. Heiz, W.-D. Schneider, H. Häkkinen, R. N. Barnett, and U. Landman, J. Phys. Chem.

A, 103, 9573 (1999).[5]L. D. Socaciu, J. Hagen, T. M. Bernhardt, L. Wöste, U. Heiz, H. Häkkinen, and U. Landman, J. Am. Chem. Soc.,

125, 10437 (2003).[6]M. L. Kimble, A. W. Castleman, Jr., R. Mitrić, C. Bürgel, and V. Bonačić?-Koutecký, J. Am. Chem. Soc., 126,

2526 (2004).[7]G. Lüttgens, N. Pontius, P. S. Bechthold, M. Neeb, and W. Eberhardt, Phys. Rev. Lett., 88, 076102 (2002).

174


Dynamics of O2 Photodesorption from Metal Clusters: ASignificant Difference from Bulk Behaviour

Matthias Götz1, Marco Niemietz1, Kiichirou Koyasu1, Young Dok Kim2, and GerdGanteför1

1. Department of Physics, University of Konstanz, D-78457 Konstanz, Germany2. Division of Nano Sciences and Department of Chemistry, Ewha Womans University, 120-750 Seoul,

Korea

The surprising catalytic properties of small noble metal clusters and nanoparticles [1]-[5] motivatethe research on the reaction dynamics and photochemistry of such particles [6]. The interaction ofoxygen with metal surfaces has been suggested to be one of the most important elementary stepsin heterogenous catalysis [7]. Photodesorption of O2 from size-selected AgnO−

2 cluster anions withn = 2, 3, 4 and 8 corresponding to the process AgnO−

2 + AgnO−∗2 → Ag−n + O2 was studied in

the gas phase using time-resolved photoelectron spectroscopy. The spectra indicate that relaxationsof photo-excited AgnO−

2 clusters with n = even numbers accompany ultrafast direct O2 photodes-orption. For the odd-numbered cluster Ag3O−

2 , in contrast, a long-lived excited state is observed,since O2 might be dissociatively chemisorbed, suppressing direct photodesorption of oxygen. Both,direct desorption and long-lived excited states, have not been observed from adsorbate covered metalsurfaces, suggesting unique photochemical properties of such small clusters.

Figure 1: Series of photoelectron spectra recorded using a probe pulse of 1.55 eV after photoexci-tation of Ag2O−

2 with 3.1 eV [8]. The experimental data display each state of the process: Theexcitation into a well defined electronic state (light background), its decay via desorption of O2 andthe appearance of the bare metal cluster (dark background).

[1]A. Sanchez, S. Abbet, U. Heiz, W.-D. Schneider, H. Häkkinen, R. N. Barnett, and Uzi Landman, J. Phys. Chem.A 103, 9573 (1999).

[2]W. T. Wallace and R. L. Whetten., J. Am. Chem. Soc. 124, 7499 (2002).[3]M. Haruta, Catal. Today, 36, 153 (1997).[4]M. Valden, X. Lai, and D. W. Goodman, Science 281, 1647 (1998).[5]D. C. Lim, I. Lopez-Salido and Y. D. Kim, Surf. Sci. 598, 96 (2005).[6]L. D. Socaciu-Siebert, J. Hagen, J. Le Roux, D. Popolan, M. Vaida, S. Vajda, T. M. Bernhardt and L. Wöste,

Phys. Chem. Chem. Phys. 7, 2706 (2005).[7]T. S. Kim, J. D. Stiehl, C. T. Reeves, R. J. Meyer, and C. B. Mullins, J. Am. Chem. Soc. 125, 2018 (2003).[8]M. Niemietz, M. Engelke, Y. D: Kim, and G. Ganteför, accepted in Appl. Phys. A (2007).

175

Reacted Clusters and Catalysis


Transition Metal Cluster-CO Complexes

A. Fielicke1; P. Gruene1; G. von Helden1; G. Meijer1;I. Swart2; B. Weckhuysen2; F. de Groot2; D. B. Pedersen3; B. Simard3; D. M. Rayner3

1. Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany2. Inorganic Chemistry and Catalysis, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The

Netherlands3. Steacie Institute for Molecular Sciences, NRC, 100 Sussex Drive, Ottawa, Ontario, Canada

The adsorption of carbon monoxide is a common probe to characterize the surfaces of transitionmetals or of deposited transition metal nanoparticles. Furthermore, the interaction of CO withtransition metals directly plays a central rule in model studies of catalytic oxidations or, e.g., in theFischer-Tropsch reaction. The characterization of the bonding situation of the CO can be achievedusing vibrational spectroscopy of the ν(CO) stretch, since the C-O bond strength is highly sensitiveto the coordination of the CO and the electron density on the metal. Here, Infrared Multiple PhotonDissociation (IR-MPD) spectroscopy is used to study the interaction of CO with transition metal(Co, Rh, Ni, Au) clusters containing 2 to more than 30 metal atoms. IR spectra are measured in therange of the ν(CO) vibration for complexes of neutral, cationic and anionic clusters. The detectionof a ν(CO) band and its position directly gives information on the adsorption state of the COmolecule (dissociative or molecular, atop or bridging configuration). In the case of rhodium we findthat for most clusters molecular adsorption in an atop position (µ1) is preferred; however for someclusters CO in bridging (µ2) or hollow (µ3) sites can be identified as well.[1] For atop bound COligands on charged clusters, the ν(CO) frequency shifts with cluster size and a quantitative modelis developed to explain this size dependence taking into account the effect of M → C back bondingand electrostatic interactions.[2] The gas phase experiments provide reference data on particles withdefined charge states to gauge the charging of deposited clusters and can thereby give new insightsinto mechanisms of catalytic reactions involving electron transfer. A second focus will be given tosaturated cluster carbonyls. For these systems, the vibrational spectra have been extended into therange of the M-C stretches as well as M-CO bending modes that are located in the far-infrared.For gold clusters, the structure of the clusters saturated with CO can be compared to the structureof the bare clusters. In certain cases we find evidence suggesting that successive adsorption of COcan distort the metal cluster framework.[3] Finally, we present vibrational spectra of the cationsof "classical" cluster carbonyls like Co2CO+

8 Co4CO+12 or Rh6CO+

16 that can be compared to theneutral counterparts. Our initial results suggest that the structure of the cations in the gas phasecan be different to the neutral species.

[1]A. Fielicke, G. von Helden, G. Meijer, D. B. Pedersen, B. Simard, and D. M. Rayner, J. Phys. Chem. B, 108,14591 (2004).

[2]A. Fielicke, G. von Helden, G. Meijer, D. B. Pedersen, B. Simard, and D. M. Rayner, J. Chem. Phys., 124, 194305(2006).

[3]A. Fielicke, G. von Helden, G. Meijer, D. B. Pedersen, B. Simard, and D. M. Rayner, J. Am. Chem. Soc., 127,8416 (2005).

179


Size selective reactivity of Cobalt clusters for the reactionwith benzene and ethane

Britta Pfeffer, Stephanie Jaberg, Gereon Niedner-Schatteburg

Fachbereich Chemie, Physikalische Chemie,TU Kaiserslautern,Erwin-Schrödinger-Str. 52, 67663Kaiserslautern, Germany

We investigate the size dependent reactivity of Co±n . Cationic and anionic cobalt cluster are pro-duced in a laser vaporisation source and the ion reactions are investigated by Fourier TransformIon Cyclotron Resonance Mass Spectrometry (FT-ICR-MS). Benzene reacts with metal clusters viatwo different reaction pathways [1],[2]. It either adsorbs intact to the cluster surface or disintegratesunder complete dehydrogenation. The cationic cobalt cluster only adsorb intact benzene. Co−n showthe intact adsorption for large clusters (n=9-13). Smaller cluster react under complete dehydrogena-tion.As already known from former studies [3] the reaction of metal cluster with hydrocarbons maybe strongly size dependent. Co−n does no react with ethane, cationic clusters do react under H2

elimination and with size dependent efficiencies: this reaction takes place only for Co+n (10 ≤ n ≤

16). The reactivity is high for odd and low for even cluster sizes. In further studies we want toinvestigate the influence of attached argon molecules on the cluster on the reactivity. It would beinteresting to see if the "argon cooling" leads to a higher reactivity for the cobalt cluster as has beenseen for platinum [4].

[1]C. Berg, M. Beyer, U. Achatz, S. Joos, G. Niedner-Schatteburg and V. Bondybey, J. Chem. Phys. 108, 5398(1998).

[2]G. Niedner-Schatteburg et al., Nb-heteroaromates, in preparation.[3]I. Baltenau, O. Balaj, M. Beyer, and V. Bondybey, Int. J. Mass Spectrom. 255, 71 (2006).[4]U. Achatz, M. Beyer, S. Joos, B. Fox, G. Niedner-Schatteburg and V. Bondybey, J.Phys. Chem. A. 103, 8200

(1999).

180


Reactions of V4O+10 with small alkanes

Jens Döbler, Joachim Sauer

Theoretical Chemistry, Humboldt University Berlin, Unter den Linden 6, Berlin, 10099, Germany

We study the reaction of V4O+10 with methane and propane using density functional theory and

compare the results to mass spectroscopic experiments. All calculations are performed with TUR-BOMOLE 5.8, using the B3LYP functional and the TZVP basis set. V4O+

10 is a radical cation witha V-O · group and thus highly reactive. For the reaction with methane we find a barrierless reactionforming V4O9OH+ · · ·CH3 with a reaction energy of 122 kJ/mol. In the experiment under singlecollision conditions V4O10H+ is found as product [1], corresponding to methyl loss. The fact that areaction occurs under single collision conditions is consistent with a barrierless reaction.The reaction of V4O+

10 with propane is more complex. The ionization potential of propane is lowerthan that of methane and it is also lower than the IP of V4O+

10. Thus the encounter complex of thereaction is not the electronic ground state and even an elastic collision concurrent with an electrontransfer corresponds to an energy gain. This result is in agreement with the occurrence of C3H+

8 inexperiments. The study of the initial reaction step is not straightforward with theoretical methodsdue to the excited nature of the electronic state. From the methane results it can be assumed thata hydrogen transfer to the V-O · will be the first step, as propane is more reactive than methane.Two different possibilities exist, leading to a primary or secondary propyl radical. The formationof the secondary radical is energetically favored, while the primary H abstraction is more likely dueto statistics. In both cases structural optimizations did not yield a stable structure for a single hy-drogen transfer. For the secondary H abstraction a second hydrogen atom of the propane interactswith the VOH group, preforming a propene molecule. The second hydrogen transfer, forming aVOH2 group and propene is subject to a shallow barrier. The primary hydrogen abstraction yieldsa propanol molecule attached to the vanadium via a rebound mechanism of the propyl radical tothe VOH group and the propanol formation also barrierless.These barrierless mechanisms are in agreement with the experimental MS results, as V4O10H+

2

(propene loss) and V4O+9 (propanol loss) are found under single collision conditions [2]. However,

the V4O+9 signal is small, indicating that propanol is only a side product. To look into this con-

tradiction, an MD simulation with a structure corresponding to the primary H abstraction wasperformed. The simulation reveals that a hydrogen shift from the secondary to the primary posi-tion of the propyl radical can occur. This path is driven by the higher stability of the secondaryradical and will yield propene as product. The result that initial H abstraction from primary andsecondary position can lead to the same reaction channel explains the occurrence of propene asmain product and creation of propanol as a minor reaction channel. Additional experiments undermultiple collision conditions yield V4O9OH2C3H+

8 as main product, due to attachment of excesspropane to V4O9OH+

2 . The calculated IR spectrum of V4O9OH2C3H+8 is in good agreement with

an experimental IRMPD spectrum [3].

[1]S. Feyel, J. Döbler, D. Schröder, J. Sauer and H. Schwarz, Angew. Chem. Int. Ed. 45, 4681 (2006).[2]S. Feyel, D. Schröder and H. Schwarz, J. Phys. Chem. A 110, 2647 (2006).[3]G. Santamrogio, E. Janssens, J. Döbler, in preparation.

181


Interaction of acetonitrile with Cobalt and Vanadium clusters

S. Jaberg, B. Pfeffer, G. Niedner-Schatteburg

Fachbereich Chemie, Physikalische Chemie, TU Kaiserslautern, Erwin-Schrödinger-Str. 52, 67663Kaiserslautern, Germany

We continue to investigate the reactivity of transition metal [1],[2] yield with acetonitrile. Undersingle collision conditions the gas phase reactivity of Co+/−

n and V+/−n clusters with acetonitrile

(CH3CN) is studied in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer. Weobserved two different main reaction products: complete dehydrogenation in competition with intactadsorption of acetonitrile. Propensities depend on the cluster size. Small Co+

n clusters favourintact adsorption whereas larger clusters prefer complete dehydrogenation, such as all anionic cobaltclusters do (n=7-21). Stepwise addition of CH3CN to Co+/−

n or V+/−n leads to various products:

either stepwise elimination of H2 is observed or the acetonitrile adsorb intact on the cluster surface.To compare the reactivity of different cluster sizes, kinetic studies were performed at backgroundpressures of 8 · 10−10 mbar and 1 · 10−9 mbar. The results of these studies are discussed in terms ofsimple reaction models.

[1]G. Niedner-Schatteburg et al., J. Chem. Phys. 102(12), 4870 (1995).[2]G. Niedner-Schatteburg et al., Nb-Heteroaromates, in preparation.

182


A hollow tetrahedral cage of hexadecagold dianion provides arobust backbone for a tuneable sub-nanometer oxidation and

reduction agent via endohedral doping

Michael Walter and Hannu Häkkinen

Department of Physics, Nanoscience Center (NSC), University of Jyväskylä, Finnland

We show, via density functional theory calculations, that dianionic Au2−16 cluster has a stable, hollow,

Td symmetric cage structure, stabilized by 18 delocalized valence electrons. The cage maintains itsrobust geometry, with a minor Jahn-Teller deformation, over several charge states (q = -1,0,+1),forming spin doublet, triplet and quadruplet states according to the Hund’s rules. Endohedraldoping of the Au16 cage by Al or Si yields a geometrically robust, tuneable oxidation and reductionagent. Si@Au16 is a magic species with 20 delocalized electrons. We calculate a significant bindingenergy for the anionic Si@Au16/O−

2 complex and show that the adsorbed O2 is activated to asuperoxo-species, a result which is at variance with the experimentally well-documented inertnessof Au−16 anion towards oxygen uptake [1].

Figure 1: a) Structure, b) electron localisation function and c) the electronic density of states ofSi@Au16. The jellium-derived delocalized states are indicated and visulaized in c).

[1]Michael Walter and Hannu Häkkinen, Phys. Chem. Chem. Phys. 8, 5407-5411(2006).

183


The effect of gold doping on the reactivity of free silverclusters: Comparison of Ag+

3 and Ag2Au+

Denisia M. Popolan, Mihai E. Vaida, Thorsten M. Bernhardt

Institut für Oberflächenchemie und Katalyse, Universität Ulm, Albert-Einstein-Allee 47, Ulm, 89069,Germany

The chemistry of noble metal clusters was intensively studied in recent years. This is due to theirfascinating properties acting as promising low temperature catalytic materials. However, the sur-prising reactive behavior is still far from being completely understood. Free gas phase clustersinvestigated by mass spectrometric methods might help to shed light on the reaction mechanismsand size effects in reactivity. Besides the pure free gold and silver clusters also mixed silver-goldclusters have been investigated. Insight into the active role of gold as a catalyst and into the possi-bilities of doping effects using gold atoms will help in the comprehension of the reactive propertiesof bimetallic clusters. Our investigations aim to elucidate the influence on the reactivity when silveratoms in Agn clusters are replaced atom by atom. In this contribution results on the reactivityof Ag+

3 with carbon monoxide at temperatures between 100 and 300 K are presented. The effectof introducing one gold atom on the chemical reaction kinetics is studied by comparison with thecluster Ag2Au+. The investigations are performed in a low energy ion beam apparatus consisting ofa sputter cluster ion source and of a temperature variable octopole ion trap inserted into a multiplequadrupole mass spectrometer arrangement.

184


Oxidation Reactions on Size-Selected Cluster Based Catalysts

Stefan Vajda1,2, Gregory E. Ballentine1, Stephanie Mucherie1, Christopher L. Marshall3,Jeffrey W. Elam4, Michael J. Pellin5, Byeongdu Lee6, Chieh-Tsung Lo6, Sönke Seifert6,

Randall. E. Winans6, Armin Kleibert7, Kristian Sell7, Viola von Oyenhausen7 andKarl-Heinz Meiwes-Broer7

1Chemistry Division, 2Center for Nanoscale Materials, 3Chemical Engineering Division, 4Energy SystemsDivision, 5Materials Science Division, 6X-ray Science Division, Argonne National Laboratory, 9700 South

Cass Avenue, IL 60439, USA 7Institut für Physik, Universitätsplatz 3, D-18051 Rostock, Germany

IntroductionThe objective of this work is to achieve high catalytic activity and selectivity in oxidative reactionsby using highly monodisperse sub-nm size atomic metal clusters, as well as few nm size particles.The applied combination of techniques allows for ultimate control of both: surface composition, aswell as catalytic particle size and composition - prerequisites in producing highly uniform activesites on technologically relevant supports [1]-[4].

Materials and MethodsSupport preparation. Our earlier studies on various oxide films showed, that small Pt and Au clus-ters are exceptionally stable on thin alumina films fabricated by atomic layer deposition (ALD)[5]-[6]. These films were selected to pre-coat the anodized aluminum oxide (AAO) membranes andflat silicon wafers prior to cluster and nanoparticle deposition. Using ALD, the diameter of themembrane pores was reduced to 10 nm.Catalyst deposition. Size-selected metal clusters and nanoparticles were produced for deposition ina continuous laser vaporization [6] and an arc cluster ion source (ACIS) [7]. Pt8−10/Al2O3/AAO andPt8−10/SnO/Al2O3/AAO Catalyst Tests. The tests were performed under atmospheric pressure ina freestanding AAO flow reactor.Ag and Au Catalyst Tests. The performance of the catalysts was tested in a unique setup at theAdvanced Photon Source which, at atmospheric pressure, allows for simultaneous mass-analysis andin situ acquisition of grazing incidence small angle X-ray scattering (GISAXS) images for a directcorrelation of catalytic performance to particle size and shape.

Results and DiscussionODH of Propane on Pt8−10/Al2O3/AAO and Pt8−10/SnO/Al2O3/AAO Catalysts. Selectivity to-wards propylene up to 85% were observed, depending temperature, presence of SnO promoter andlocation of the clusters in the membranes. Two to three orders of magnitude higher turn-overfrequencies than those reported in the literature for the best Pt and VOx based catalysts wereobserved. The superb selectivity (only one or maximal two side-products) can be explained by veryshort contact times on highly uniform dispersed catalytic particles. Catalytic performance did notchange even after lengthy heat treatment, thus indicating highly stable nanocatalysts.Oxidation of Ethylene and Propylene on Au and Ag Nanocatalysts. Onset of product formationwas observed between 160-200 °C. At higher reaction temperatures, silver particles tended to sinter,while gold clusters stabilized with a protective alumina overcoat retained their size.

[1]Bell, A. T., Science 299, 1688 (2003).[2]Heiz, U., Abbet, S., Häkkinen, H., and Landman, U., Materials Research Society Symposium Proceedings, 648,

P9.1.1-P9.1.10. (2001)[3]Lee, S., Fan, C., Wu, T., and Anderson, S. L., J. Am. Chem. Soc. 126, 5683 (2004)[4]Valden, M., Lai, X., and Goodman, D.W., Science 281, 1647 (1998)[5]Vajda, S., Winans, R.E., Elam, J., Pellin, M.J., Seifert, S., Tikhonov, G.Y., and Tomczyk, N.A. Top. Catal. 39,

161 (2006)[6]Winans, R.E., Vajda, S., Elam, J., Lee, B., Pellin, M.J., Seifert, S., Tikhonov, G.Y., and Tomczyk, N.A. Top.

Catal. 39, 145 (2006)[7]Methling, R.-P., Senz, V., Klinkenberg, E.-D., Diederich, Th., Tiggesbäumker, J., Holzhüter, Bansmann, G J., and

Meiwes-Broer, K.H., Europ.Phys. J D 16, 173 (2001)

185


Interaction of acetonitrile with Cobalt and Vanadium clusters

S. Jaberg, B. Pfeffer, G. Niedner-Schatteburg

Fachbereich Chemie, Physikalische Chemie, TU Kaiserslautern, Erwin-Schrödinger-Str. 52, 67663Kaiserslautern, Germany

We continue to investigate the reactivity of transition metal [1],[2] yield with acetonitrile. Undersingle collision conditions the gas phase reactivity of Co+/−

n and V+/−n clusters with acetonitrile

(CH3CN) is studied in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer. Weobserved two different main reaction products: complete dehydrogenation in competition with intactadsorption of acetonitrile. Propensities depend on the cluster size. Small Co+

n clusters favourintact adsorption whereas larger clusters prefer complete dehydrogenation, such as all anionic cobaltclusters do (n=7-21). Stepwise addition of CH3CN to Co+/−

n or V+/−n leads to various products:

either stepwise elimination of H2 is observed or the acetonitrile adsorb intact on the cluster surface.To compare the reactivity of different cluster sizes, kinetic studies were performed at backgroundpressures of 8 · 10−10 mbar and 1 · 10−9 mbar. The results of these studies are discussed in terms ofsimple reaction models.

[1]G. Niedner-Schatteburg et al., J. Chem. Phys. 102(12), 4870 (1995).[2]G. Niedner-Schatteburg et al., Nb-Heteroaromates, in preparation.

186


Gas-phase Kinetic Studies on Reaction of Methane with[V3O7]+

Xin Zhang, Shaohui Li, Ewald Janssens, Torsten Siebert, Knut R. Asmis, Ludger Woeste

Xin Zhang, Shaohui Li, Ewald Janssens, Torsten Siebert, Knut R. Asmis, Ludger Woeste

Activation and functionalization of light hydrocarbon (C1-C4) has attracted much attention becauseof its great importance in fundamental research and industrial application. Vanadium-oxide basedcatalysts have been widely used in oxidative conversion of light hydrocarbon, but detail reactionmechanism of light hydrocarbon on these catalysts is still unknown. Studies on the reaction of lighthydrocarbon with gas-phase vanadium-oxide cluster ions by mass spectroscopy provide an effectiverout to reveal the reaction mechanism [1]. Up to date, little attention has been paid to the reactionof methane with [V3O7]+, which is the smallest polynuclear vanadium-oxide cluster cation in theV5+ oxidation state and is considered to be more reactive than large vanadium-oxide cluster ions [2]-[3]. In this present work, reaction kinetics of methane with gas-phase [V3O7]+ was investigated byradio-frequency ion-trap tandem mass spectrometer coupled with laser vaporization source. Possiblereaction pathways and kinetic model on the reaction are proposed and the rate-limiting reactionsteps as well as the effects of reaction conditions on rate constants are discussed here.During the reaction of methane with [V3O7]+ at an ion-trap temperature of 100 ∼ 120 K and CH4

partial pressure of 0.015 ∼ 0.075 Pa, the activation of methane and the subsequent formation of[V3O7(CH4)]+, [V3O7(C2H6)]+, [V3O7(C2H4)]+, [V3O7(C2H2)]+ were observed. [V3O7(CH4)]+,[V3O7(C2H6)]+ and [V3O7(C2H4)]+ are intermediates and contentiously converted to the domina-tive product [V3O7(C2H2)]+. The reaction of [V3O7]+ with methane proceeds to equilibrium withthe increase of the reaction time (∼ 6000 ms). The reaction goes through a pathway of ion-molecularassociation (1-1), coupling dehydrogenation (1-2) and contentious reversible dehydrogenation (1-3and 1-4) described as,

[V3O7]++CH4 →[V3O7(CH4)]+ k(1)1 (1-1)

[V3O7(CH4)]++CH4 →[V3O7(C2H6)]++H2 k(1)2 (1-2)

[V3O7(C2H6)]++CH4 ↔[V3O7(C2H4)]++H2 k(1)2 , k

(1)−3 (1-3)

[V3O7(C2H4)]++CH4 ↔[V3O7(C2H2)]++H2 k(1)4 , k

(1)−4 (1-4)

The ion-molecule association and coupling dehydrogenation reactions are involved in methane acti-vation. The pseudo-first-order rate constant k(1)

n and absolute rate constant k(abs)n of each reaction

step are respectively estimated according to the obtained kinetic data. The ion-molecule associationis the rate-limiting step in the reaction. The reaction steps involving in methane activation areslow and the subsequent dehydrogenation steps are much faster. CH4 partial pressure and ion-traptemperature affected k(1)

n and k(abs)n of each reaction step. Moreover, k(1)

n and k(abs)n have a negative

temperature and pressure dependency, due to the low-pressure limit of Lindemann mechanism andthe nature of ion-molecule reactions. The negative temperature dependence of k(1)

n and k(abs)n im-

plies that there is no noticeable activation energy barrier for each reaction step under the reactionconditions.

[1]D.K. Böhme, H. Schwarz, Angew. Chem. Int. Ed. Engl. 44, 2236 (2005).[2]K.A. Zamsik, D.R. Justes, A.W. Castleman Jr., J. Phys. Chem. A 105, 10237 (2001).[3]S. Feyel, D. Schröder, X. Rozanska, J Sauer, H. Schwarz, Angew. Chem. Int. Ed. 45, 4677, (2006).

187


Gas-phase chemistry of [SiAun]±, n = 1-4

Yali Cao, Martin Beyer∗

Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135, D-10623, Germany

It has been demonstrated that nanoscale gold particles can play an important role in catalysis,which has sparked interest in gold clusters in the gas phase [1],[2]. The Au-Si interfaces have beenstudied extensively owing to their importance in microelectronics. The group of Lai-Sheng Wang[3] has reported the electronic structure and photoelectron spectroscopy of anions [SiAun]-(n=2-4),which revealed that the structures of [SiAun] clusters were nearly identical to silicon hydride [SiHn]molecules. We investigate the structure and reactivity of small gold-silicon cluster ions. Specialfocus lies on experiments which reflect the gold-hydrogen analogy in [SiAun]±.In our experiments, [SiAun]± are produced by laser vaporization followed by supersonic expansion,stored in an FT-ICR mass spectrometer, and their reactions with a selection of small inorganicand organic molecules are investigated. Detailed insight and quantitative reaction rate constantsare obtained by fitting the time-intensity profile of reactant and product intensities to pseudo-first-order kinetics. Quantum chemical calculations on selected species are employed to interpret theexperimental findings and to get quantitative information on the reaction potential energy surface.

[1]A. Sanchez, S. Abbet, A. U. Heiz, W.-D. Schneider, H. Hakkinen, R. N. Barnett, and U. Landman. J. Phys. Chem.A 103, 9573 (1999).

[2]M. L. Kimble, A. W. Castleman, Jr., R. Mitri?, C. Bürgel, V. Bona?i?-Koutecký. J. Am. Chem. Soc., 126, 2526(2004).

[3]B. Kiran, X. Li, H. J. Zhai, L. F. Cui, and L. S. Wang. Angew. Chem. Int. Ed. 43, 2125-2129 (2004).

188


Catalytic Properties of Size-Selected Pt Clusters on TiO2(110)Surface

Yoshihide Watanabe1,2, Noritake Isomura1,2, Hiroyuki Matsubara1

1. Toyota Motor Corporation 1200 Misyku Susono, Shizuoka, Japan2. Toyota Central R & D Lab.Incs, Nagakute, Aichi 480-1192, Japan

Heterogeneous catalysts such as automotive exhaust catalysts consist of precious metal particlesupported on oxide surfaces. The reduction of precious metal usage will be strongly required. Metalcluster has been speculated to have a strong size-dependence in catalytic activity. The cluster on thesurface would give further specificity because of the interaction between the clusters and the surface.Our main purpose is the improvement of catalytic activity by cluster size control the interactionbetween the clusters and the surface.Catalytic oxidation of CO on size-selected Pt clusters on TiO2(110) surface was investigated byhigh-pressure reaction cell (Fig.1). This high-pressure reaction cell was designed for high-pressurestudies using small-area samples and a retractable internal isolation cell with quartz lining whichconstitutes a micro batch reactor in the ∼20kPa pressure range. The reaction cell and the externalrecirculation loop were connected to a stainless steel bellows pump for circulation.

(a) (b) (c)

Figure 1: (a) High-pressure reaction cell. (b) Overview of the experimental setup. (c) STM imageof Pt10/TiO2(110).

The size-selected Pt clusters were deposited on the TiO2(110) surfaces prepared by the typical proce-dure using a new ultrahigh vacuum (UHV) cluster deposition apparatus supplied by the magnetron-sputter ion source. Both crystal faces were cleaned by repeated Ar ion sputtering, and annealing,until a well-defined (1x1) LEED pattern was observed, and no impurities were detected by AES.The deposited Pt cluster and TiO2 surface was observed by a scanning tunneling microscope (STM).All of the chambers were connected with each other and the sample was transferred under UHVcondition. This system is also equipped with LEED/AES and XPS/UPS (Fig. 2).The STM image (Fig. 3) show Ptn clusters deposited firmly on TiO2 surface without aggregation.The result of catalytic activity using high-pressure cell will be given.

189

Index

INDEX

List of Authors

Abd El Rahim, M., 56Adilah, H., 127Albert, M., 167Alonso, J. A., 35, 88Andersson, M., 138Antoine, R., 56Antonietti, J.-M., 22Arenz, M., 100, 152Armentrout, P., 44Arndt, M., 38, 58Asmis, K. R., 34, 187

Böttcher, A., 17, 128Bürgel, C., 172Baguenard, B., 163, 164Balkaya, B., 154Ballentine, G. E., 185Bansmann, J., 67Bartels, C., 9Belosludov, R. V., 7Benoit, D., 106Bernhardt, T. M., 105, 170, 184Bertram, N., 156Beyer, M. K., 33, 76, 188Bieletzki, M., 160Bierweiler, T., 91Birkner, S., 167Bleloch, A., 158Blom, M. N., 61Boese, A. D., 133Boldyrev, A. I., 45Bonačić-Koutecký, V., 26, 171, 172Bopp, J. C., 57Bordas, C., 163, 164Bostedt, C., 82Bowen, K. H., 21Bowers, M. T., 28Bréchignac, C., 116, 140Broyer, M., 26, 46, 56Bubek, M., 148Bulut, F., 67Burgert, R., 21

Cabria, I., 35Cahuzac, P., 116Calvo F., 165Cao, Y., 188Castleman Jr., A. W., 13Chen, M., 43Chen, Y., 157Cheng, H.-P., 140

Cheshnovsky, O., 37Chirot, F., 19, 96Choi, C. H., 89Christen, W., 115Climen, B., 163Compagnon, I., 26, 61Compton, R. N., 18Concina, B., 133, 163, 164Conus, F., 147Cordes, J., 21, 99Curley, B. C., 158Cwiklik, L., 55

Döbler, J., 81, 181Döppner, T., 101, 166Dörner, R., 82de Groot, F., 141, 142, 179Del Vitto, A. , 22Demuth, J., 102, 173Denifl, S., 25, 62, 119Di Domenicantonio, G., 10Di Vece, M., 158Dietsche, R., 148Duffe, S., 149, 160Dugourd, P., 26, 56

Eberhardt, W., 49, 82, 154Echt, O., 119Egashira, K., 95Ehrler, O. T., 131, 169Elam, J. W., 185Elliott, B. M., 57Eng, J., 167Engelke, M., 168

Félix, C., 10, 146, 147Farajian, A. A., 7Feiden, P., 116Fennel, T., 166Ferretti, N., 154Fielicke, A., 75, 86, 141, 142, 179Fischer, T., 82, 99, 156Fournier, R., 83, 84Frenzel, J., 137Frondelius, P., 159Furche, F., 90

Göde, S., 20, 87Götz, M., 99, 174, 175Ganteför, G., 21, 82, 99, 148, 156, 168, 174,

175

193

INDEX

Garand, E., 34Gebhardt, R. K., 67Gemming, S., 156Getzlaff, M., 67Gilb, S., 22, 150Glaser, L., 70Gleitsmann, T., 170Gloess, A. N., 74Goebbert, D. J., 34Goodman, D. W., 43Greiner, W., 27, 60, 68, 69, 79, 104, 111, 127,

130, 132, 153Grubisic, A., 21Gruene, P., 86, 179

Häkkinen, H., 77, 159, 183Hätting, C., 169Hövel, H., 149, 160Ha, D. G., 59Hackermüller, L., 58Hammer, N., 57Hampe, O., 85, 133Harbich, W., 146Harding, C.J., 100Harjunmaa, A., 151Hartl, K., 22Headrick, J. M., 34, 57Heiz, U., 22, 100, 152Henriques, E., 104Hervieux, P.-A., 120Hillenkamp, M., 10Hippler, H., 169Hock, C., 9Honkala, K., 159Hrivnák, D., 121Huber, B., 77, 103Huwer, J., 9Hwang, K. W., 59

Irawan, T., 149, 160Isomura, N., 189

Jaberg, S., 48, 180, 182, 186Jagoda-Cwiklik, B., 110Jaksch, S., 145Janssens, E., 102, 187Jena, P., 21, 32Jester, S.-S., 17, 128Jiang, J., 158Johnson, M. A., 34, 36, 57Johnston, R. L., 158Jones H. , 22Joswig, J.-O., 137Jungwirth, P., 55

Kalus, R., 121Kappes, M. M., 17, 74, 85, 90, 91, 97, 128,

131, 133, 169Karlický, F., 121Kartouzian, A., 22Kawazoe, Y., 7Keinonen,J., 151Ketterer, T., 148Khanna, S. N., 13Kiesewetter, G., 58Kim, H. S., 89Kim, N. D., 59Kim, S. K., 59Kim, Y. D., 148, 168, 175Kiran, B., 21Kleibert, A., 67, 185Knickelbein, M. B., 76Kondow, T., 95Kordel, M., 97Korol, A. V., 134Koskinen, P., 77Kostko, O., 90, 103Koyasu, K., 174, 175Krause, T., 115Kubala, D., 119Kulesza, A., 26

L’Hermite, J.-M., 19, 96Löffler, D., 17, 128Lépine, F., 163, 164Labastie, P., 19, 96Lagutschenkov, A. , 109Lang, S. M., 105Lau, J. T., 78Lebeault, M.A., 163Lebedkin , S., 17Lechtken, A., 90Lecoultre, S., 147Lee, B., 185Lee, S. H., 59Lee, Y.-T., 109Lemaire, J., 133Leygnier, J., 116Li, S., 102, 173, 187Li, X., 21Li, Z. Y., 157, 158Lim, K. H. , 22Lim, C. T., 89Lim, D. C., 148Lindinger, A., 98, 167Lineberger, W. C., 5Liu, F., 44Liu, S., 44Lo, C.-T., 185

194

INDEX

Lo, S., 134Lopez, M. J., 35Lyalin, A., 68, 69, 79, 104, 153

Mähr, I., 25, 62, 119Märk, T. D., 25, 62, 119Möller, T., 78, 82Maître, P., 133Majima, T., 95Mangler, T., 156Marksteiner, M., 58Marshall, C. L., 185Martínez, J. I., 88Martins, M., 70, 82Masson, A., 140Matheis, K., 131Mathew, M., 130Matsubara, H., 189Mayrhofer, K. J. J., 152McCunn, L., 57Meijer, G., 24, 61, 75, 86, 141, 142, 179Meiwes-Broer, K.-H., 20, 67, 82, 87, 101, 146,

166, 185Merli, A., 167Michalski, M., 22Mirabal, A., 173Mitrić, R., 26, 171, 172Mizuseki, H., 7Moon, W. H., 89Morgner, N., 90Moseler, M., 8, 77, 103, 129, 160Mucherie, S., 185Mullins, T. G., 167

Nößler, M., 171Nakajima, A., 11Nakamura, M., 120Neeb, M., 82, 154Neumaier, M., 85, 133Neumark, D. M., 34Neville, J., 82Niedner-Schatteburg, G., 48, 109, 133, 180,

182, 186Nielaba, P., 155Niemietz, M., 168, 174, 175Nishimiya, N., 139Nordlund, K., 151Novara, F., 63

Obolensky, O. I., 60, 104, 111, 132Oelßner, P., 82Oger, E., 91Oleksy, K., 121Oomens, J., 61

Pacchioni, G. , 22Pagliarulo, F., 164Paizs, B., 61Palmer, R. E., 157, 158Palomba, S., 157, 158Pankewitz, T., 109Parneix, P., 165Passig, J., 67, 101, 166Pastewka, L., 129Pedersen, D.B., 179Pellin, M. J., 185Peter, J., 22Petersen, J., 26Pfeffer, B., 48, 180, 182, 186Plewicki, M., 98Polfer, N., 61Popolan, D. M., 184Przystawik, A., 20, 87

Quester, W., 155

Rösch, N. , 22Röttgen, M., 100Rühl, E., 82Rabeus, S., 115Rabin, I., 75Radcliffe, P., 101Rademann, K., 39, 115Rasul, B., 145Rayane, D., 56Rayner, D. M., 47, 142, 179Redlich, B., 141Rensing, C., 169Richter, T., 149, 160Rittmann, J., 78Rizzo, T. R., 23Rodrigues, V., 147Rosén, A., 138Roscioli, J. R., 57Rydlo, A., 147

Salzmann, W., 167Santambrogio, Gabriele, 34Sauer, F., 167Sauer, J., 31, 81, 181Schöffler, M., 82Schaal, C., 166Schach, M., 155Schadow, T., 78Scheier, P., 25, 62, 119, 145Schmidt, B., 173Schmidt, M., 140Schmidt-Böcking, H., 82Schnöckel, H., 21

195

INDEX

Schneider, H., 74Schooss, D., 90, 97Schröder, D., 63Schwarz, H., 63Scribano, Y., 106Seifert, G., 137, 156Seifert, S., 185Sell, K., 185Semenikhina, V., 153Senz, V., 82, 101Sieben, B., 149, 160Sieber, C., 146Siebert, T., 102, 173, 187Sierka, M., 81Simard, B., 179Skruszewicz, S., 101Solov’yov, A. V., 27, 60, 68, 69, 79, 104, 111,

127, 130, 132, 134, 153Solov’yov, I. A., 27, 60, 79, 104, 127, 130, 132Stairs, J. R., 90Stanzel, J., 82Stokes, S. T., 21Sugiharto, A., 169Suhai, S., 61Sun, Y., 83, 84Swart, I., 141, 142, 179

Tabarin, T., 26Tan, L., 44Tarus, J., 151Terasaki, A., 95Teslenko, V., 100Thomas, H., 82Tiago, M. L., 14Tiggesbäumker, J., 20, 82, 87, 101, 166

Uehara, T., 7Ulbricht, H., 58Unrau, W., 173Unterreiner, A.-N., 169

v. Issendorff, B., 9, 90, 103Vaida, M. E., 170, 184Vajda, S., 185Vetter, K., 99, 148Vitek, A., 73Vogel, M., 78von Helden, G., 61, 179von Issendorff, B., 37, 77, 78, 149, 160von Oyenhausen, V., 185

Wöste, L., 102, 167, 173Wabnitz, H., 101Walter, L., 97

Walter, M., 183Walter, T., 48Wang, L.-S., 6Watanabe, Y., 189Weber, J. M., 74Weber, S. M., 98, 167Weckhuysen, B., 141, 142, 179Weidemüller, M., 167Weigend, F., 85Weis, P., 17, 91, 128Weise, F., 98, 167Wellhöfer, M., 70Werner, U., 171Wester, R., 167Westhäuser, W., 156Whetten, R. L., 12Wiberg, G., 152Willis, M., 21Winans, R. E., 185Woeste, L., 187Wurth, W., 70, 82

Xantheas, S. S., 109

Yakubovich, A. V., 27, 104Yang, P. J., 169Yin, C., 149, 160Young, N. P., 157, 158Yuan, J., 158

Zamith, S., 19, 96Zamudio-Bayer, V., 78Zappa, F., 25, 62, 119, 145Zhang, M., 80Zhang, X., 173, 187Zhou, J., 34

196

Documents

Symposium on Size Selected Clusters 2007