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Methods to produce andstudy clusters
http://fielicke.lmsu.tu-berlin.de/
André FielickeInstitut für Optik und Atomare Physik
Technische Universität Berlin, Germany
Program
1. What are clusters and why to study them?2. Making and characterizing free clusters3. Probing the structures
1. What are clusters and why to study them?
Clusters
Oxford English Dictionary:
1. A collection of things of the same kind, as fruits or flowers, growing closely together; a bunch.
a. Originally of grapes (in which sense bunch is now the usual term).
b. Of other fruits, or of flowers; also of other natural growths, as the eggs of reptiles, the air-cells of the lungs, etc.
Cluster compounds• Thermodynamically and kinetically stable
• Chemical synthesis in bulk quantities
• Characterization with “classical” spectroscopic methods (IR, NMR, XRD etc.)
Isolated clusters• Generation through aggregation of the
(atomic or molecular) constituents
• (Nearly) free choice of size (n), composition (n/m) and charge (z): MnLm
z
• Most clusters are not stable towards aggregation
• Experimental investigations are usually performed in the gas phase Molecular beam techniques
Co4(CO)12
H6O13+
(“Zundel” cation)
B12H122-
V8+ B16
PRL 93 (2004) 023401 JCP 137 (2012) 014317
Clusters of atoms and molecules
• multiples of a simple subunit, e.g. Cn, Arn, or (H2O)n
• The cluster size n can vary and determines the properties
• small clusters have (nearly) all atoms on the surface
Number of atoms
Surface atoms
radius [nm] 1 10 102
1 10 10310 104 105 106 107 1082
10 102 103 105104
Clusters Nano-crystals
5 6 7 8 9 10 11 12 13
Nbn+
?
Volume and surface of a cluster with n atomsSpherical cluster approximation
R
2r
3
34 RV
24 RS
Assignment1.1 How many Krypton atoms are in a spherical cluster of a) 1 nm, b) 10
nm, c) 100 nm radius? Assume that a single Kr atom fills an effective volume with 0.2 nm diameter in this cluster.
1.2 What is the ratio for surface vs. volume atoms for these clusters? The surface atoms contribute to the cluster surface by only ¼ of their ‚atomic surface‘, ¾ point toward the inside of the cluster.
Clusters: nano and smaller
Effects at the (sub)nano-scale quantum confinementlarge surface/volume ratio
structural changes
Emergence of new properties e.g.: magneticoptical / luminescence
chemical / catalytic
Size dependence of properties: Each atom counts
Magnetism
Stability
Fen + H2
Reactivity
Ionization energy
Motivations for the study of free metal clusters
Fundamental aspectsHow are properties emerging when going from the atom to the bulk?
Reference systemsTest and further development of theoretical methods
New materialsInspiration from particularly stable clusters
Model systems in heterogeneous catalysis
model
application
2. Making and characterizing free clusters
Bonding in clusters
dispersion
induction
dipole/dipole
ion/dipole
metallic
covalent
ionic
increasingbond strength
and cluster stability
1s 1s
He HeHe2
EB≈100 neV
EB≈4-6 eV
Cn
C=C=C or C≡C–C
EB≈7-8 eV(Na+Cl-)n
EB≈1-5 eV + +
+
++
+
+-
---
-
-
-
Experimental techniques for Cluster studies
Cluster production: Top-down vs. Bottom-upSputtering or Aggregation of the constituents
cooling
condensation
clustersbulk material
supersaturated vapor
vaporization
sputtering
Cluster productionSupersonic expansion of a gas
Adiabatic and isenthalpic expansion leads to strong cooling formation of a cold supersonic beam
Cluster formation via 3-body collisions near the nozzlee.g. Ar + Ar + Ar Ar* + Ar2 (conservation of energy and momentum)Dimers are condensation nuclei for larger clusters
Seeded molecular beam: cooling of the internal degrees of freedom
Cluster ProductionGas aggregation
(thermal) evaporation into a cold gas
Smoke source for the production of C60, C70 and larger carbon clusters
Typical vapour pressures of ~10-2
mbar need to be reached
(°C)Na 289Al 1217Ag 1027Au 1397
Cluster ProductionLaser ablation
heating of a small surface part of a solid target by a focused, intense short-pulse laser (typically Nd-YAG, 532 nm)formation of a plasma that contains ions and electronscooling with rare gas induces aggregation formation of neutral and charged (anionic and cationic) clusters
Converts practically any solid into clusters, very frequently used!Can be easily combined with reaction or thermalization channels, etc.
see: M.A. Duncan, Rev. Sci. Instr. 83 (2012) 041101.
A molecular beam cluster experiment
Mean free path length (identical particles)
Experiments under collision-free conditions
Vacuum range Pressure in mbar Molecules / cm3 mean free pathAmbient pressure 1013 2.7*1019 68 nmMedium vacuum 1-10-3 1016-1013 0.1-100 mmHigh vacuum 10-3-10-7 1013-109 10 cm - 1 kmUltra high vacuum 10-7-10-12 109-104 1 km-105 km
Sourcechamber
big pump
smallpump
time-of-flightmass spectrometer
VUV laser
operators
fore vacuumpump
infr
ared
lase
r bea
m fr
om F
EL
Metal cluster lab at the FHI-FEL in Berlin
Mass spectrometric characterizationIonization techniques for neutral clusters
Electron impactEfficient ionization at 60-100 eVIonization potentials (IPs) are onthe order of 5-15 eVexcess energy leads to fragmentationand changes mass distribution
Photoionization
UV lasers (nm) E (eV)Nd-YAG, 3rd 355 3.5Nd-YAG, 4th 266 4.7
KrF 249 5.0ArF 193 6.4F2 157 7.9
Nd-YAG, 9th 118 10.5single photon resonant multi photonspecies and state selective
kinB EEh
Mass spectrometric characterizationTime-of-flight mass spectrometry
acceleration of charged particles (ions) in an electric fieldparticles having the same charge but different mass are accelerated to the same kinetic energy
Measurement of the arrival time on the detector gives mass informationtypical experimental conditions: s=1 cm, D=10-300 cm,
E=100-10000 kV/cmA single mass spectrum can be measured within 5-100 µs.Mass resolution up to 10 000 amu can be achieved
2
2mvzeEs
mzeEsv 2
sDDzeEsmt
2
Example: Cobalt cluster cations produced by Laser ablation
Mass spectrometric characterization
time-of-flight (µs) time-of-flight (µs)
inte
nsity
(arb
. uni
ts)
mass resolution: 3402 2/1
max ttR
2. Dirty Terbium clusters, what is in there?
Other types of mass spectrometersI Magnetic sector field
II Quadrupolemoderate to high (104) resolution experiments on beams of mass selected ions (MS/MS)
III Ion traps, FT-ICR (ion cyclotron resonance) very high resolution (106), long storage timessimultaneous detection of all ionsexpensive
I-II are often used as mass filters, measurement of a full mass spectrum requires scanning (of voltages) and is relatively time consuming.
Experiments are often performed on pulsed molecular beams, usage of a ToF-MS allows rapid and full mass analysis of a single ion pulse.
Mass spectrometric characterization
mzeB
Mass spectrometers:mass analyzermass filtersion traps
Approaches for size-selectivity in cluster studies:a) Mass selection, accumulation, spectroscopyb) Size-specific detection ( Action spectroscopy)
3. Probing the structures
We like to understand, and to explain, observed facts in terms of structure.
Linus Carl Pauling
(1901-1994)Nobel Prize in Chemistry 1954
“The place of Chemistry in the Integration of the Sciences”, Main Currents in Modern Thought, 1950, 7, 110
CLUSTERSTRUCTURE
Ion Mobility
Anion PESTrapped Ion
ElectronDiffraction
RamanSpectroscopy
Infrared Multiple Photon Dissociation Spectroscopy
Chemical probe method
Experimental methods for structure determination of clusters
Vibrationalspectroscopies
Theory
Ligand molecules are brought into reaction with a clusterComplexes of the cluster with one or more ligands are formed depending on PL and T via consecutive reactions
saturation numbers
The chemical probe method
X + L XL + L XL2 + L … XLsat
plot (average) saturation number as function of P,plateaus indicate stable complexes
The chemical probe method
5
8
6
E.K. Parks, et al., The structure of nickel-iron clusters probed by adsorption of molecular nitrogen.Chem. Phys. 262 (2000) 151.
Ion chromatography
• The collision cross section is a measure of size (number of atoms) and shape of a cluster
rotationally averaged collision cross sections:spherical < oblate < prolate
Mass selected ions are pulled through a collision gas (He) by a weak electric field leading to a resulting drift velocity: vd = K·E
Mobility K in the gas is related tothe collision cross section
Ion mobility measurements
12
1632
TkNq
UtLK
B
More compact structures have higher mobilities
Comparison with collision cross sections for various isomers from theory geometric structure
source: bowers.chem.ucsb.edu/theory_analysis/ion-mobility
IMS-MS: a commercial technique
Waters Synapt-G2 HDMS
Si cations and anions
R.R. Hudgins, M. Imai, M.F. Jarrold, P. Dugourd, J. Chem. Phys. 111 (1999) 7865.
prolate
oblate?
‘more spherical’
Several families of cluster structuresSimilar transition size from prolate to oblate structures
Electron diffraction of trapped cluster ions
wave - particle duality
de Broglie wavelength on electrons:
12 pm for Ekin=10 keV
Electron diffraction of trapped cluster ions
mass selection, trapping, thermalization~107 ions per cm3
40 keV e-beam, ~µA currentJ.H. Parks, X. Xing in The Chemical Physics of Solid Surfaces,
Vol. 12 Atomic Clusters. (2007) 377.
Overview: TIED of anionic gold clusters
Total scattering intensity shows little size specific features use of reduced molecular intensity
Gold clusters, some example structures from TIED
observation of 2D and 3D isomer for Au12-: size for 2D/3D transition for anionic Au clusters
Anion photoelectron spectroscopy (Photoemission)
• Anions can be mass selected• Excitation energies are within the UV-vis range
• Electron affinity: vertical EA > adiabatic EA
Ekin=h-EB
Measurement of photoelectron spectra
K.H. Meiwes-Broer, Appl. Phys. A 55 (1992) 430-441.
1. Production of cluster anions2. Mass selection3. Photo excitation with vis/UV Laser4. Measure kinetic energies of electrons
Anion Photoelectron Spectroscopy of Au20-
simulation
Au20: minimum in EA (2.75 eV)
A-X separation = energy to reach firstexcited state in the neutral
≈ HOMO-LUMO gap
J. Li, X. Li, H.-J. Zhai, L.-S. Wang, Science 299 (2003) 864.
“… Au20 possesses a tetrahedral structure, which is a fragment of the face-centered cubic lattice of bulk gold with a small structural relaxation.”
Structure and bonding of Au20-
large HOMO-LUMO gap: sign of stability
1.77 eV in Au20 vs. 1.57 eV in C6020 e: magic shell closing
5d10 are localized6s1 form 4-center-2-electron bonds (10x)
D.Y. Zubarev, A.I. Boldyrev, J. Phys. Chem. A 113 (2008) 866.
Isomerism in gold clusters
Isomer identification by:Ion chromatography (different cross section)Electron diffraction (different atomic positions)Chemical reactions (different reactivity)
Example: using O2 to remove contribution of more reactive isomers of Au10
- to anion photoelectron spectrum
W. Huang, L.-S. Wang, Phys. Chem. Chem. Phys. 11 (2009) 2663.
Origin of vibrational spectroscopy
1800 discovery of “invisible Rays of the Sun” by W. Herschel
1905 Coblentz: “Investigations of Infrared Spectra” (120 organic compounds)
1920/30’s Foundations of theoretical molecular spectroscopy
1928 Discovery of the Raman effect 1940’s structure of penicillin from group
frequencies
R.N.Jones Can. J. Spectr. 26 (1981) 1
Vibrational spectroscopy
Infrared absorption
Raman scattering
=0
=1
h h h h(
h h(
virtual state
=0
=1
h=E=1-E=0
Rayleigh-S. Raman-S.(Stokes)
Raman-S.(anti-Stokes)
Selection rules for vibrational transitions
Infrared absorption
0
eqq
Selection rules for vibrational transitions
Infrared absorption
Raman scattering
0
eqq
0
eqq
s as
IR spectroscopy of clusters in molecular beams
Not sensitive enough (low particle density)Not species specific (cluster distribution)
Absorption spectroscopy
Action Spectroscopy:More sensitive and selective: Mass spectrometric detection of absorption
Changes of the charge state (ionization)Changes of particle mass (dissociation)
An intense and tunable IR source is needed for the excitation
σnleII 0
σFeNN 0
“fingerprint” region(M-M)
IR photo dissociation of most systems requires absorption of multiple IR photons
Chemisorption energies: 1-3 eVBinding energies in transition metal clusters 3-6 eVPhysisorption energies <0.1 eV
(X-H)(C=O)
DFM / OPO CO2
Dissociation of rare gas complexes:Messenger technique
Free Electron Lasers as source of IR radiation
Wavelength depends on:kinetic energy of the electronsUndulator period u
magnetic field (~K)
2
2
2 1)2
1(2 cm
EK
e
U
The Free Electron Laser for Infrared eXperiments (FELIX)FOM Institute for Plasma Physics “Rijnhuizen”, Nieuwegein, The Netherlands
15-45 MeV electron beamtunable between 40-2400 cm-1
(up to ~3700 cm-1 on 3rd harmonic)
up to 100 mJ per macropulse (1010 W/cm2 in a micropulse)
bandwidth typically 0.5-2 %of the central wavelength
Magnetism in small rhodium clusters
Y.-C. Bae et al. Phys. Rev. B 72 (2005) 125427.
108 9
► Cubic growth can explain magnetic properties
► Eight-center bonding through d orbitals
1312A.J. Cox et al. Phys. Rev. B 49 (1994) 12295.
Far-IR multiple photon dissociation spectroscopy of metal cluster rare-gas complexes
IR multiple photon excitation spectroscopy
Internal vibrational redistribution thermal heating
IR excitation
action
Far-IR multiple photon dissociation spectroscopy of metal cluster rare-gas complexes
IR: 205 cm-1
depletion spectrum
frequency
inte
nsity
frequency
IR absorption spectrum
cros
s se
ctio
n resonant absorption Fragmentation of the Ar complexes
The cubic structures of rhodium clusters
Rh8 cube, Oh symmetry 1 IR active mode (t1u)
108 9 1312
e
b2
bicapped octahedral structure as identified also for other transition metals
0
+0.18 eV
+0.56 eV
+0.92 eV
Assignment of the structure of Rh8+
J. Chem. Phys. 132 (2010) 011101.J. Chem. Phys. 133 (2010) 214304.
Infrared spectroscopy of metal cluster complexes
ligand modes500-3500 cm-1 (0.06-0.43 eV)
internal cluster modes< 500 cm-1 (0.06 eV)
Structure of “bare” metal clusters
Exploring the cluster’s surface chemistry
CO at Rhn+: Size dependence of the binding site
Binding to each additional M atom leads to a shift of the C-O stetch of about 100-150 cm-1 tolower frequencies.
Observation of CO bound in 3-fold facecapping (µ3), 2-fold bridging (µ2), and linear (µ1)geometries
JACS 125 (2003) 15716J. Phys. Chem. B 108 (2004) 14591
M() CO(5)donation
M() CO(2)back donation
Assignment3.1 Rh4(CO)12 has the structure shown to the right. For the cation we
measured the infrared spectrum plotted below. What can you say about the structure of Rh4(CO)12
+?
3.2 Make suggestions for the structures of Rh3(CO)9+ and Ru3(CO)12
+ based on their given IR spectra. Both are actually very similar.Hint: Rh2(CO)8 shown below follows the 18 e valence electron rule for each metal atom, where CO is a 2e donor ligand and metal-metal single bonds are counted to contribute with one extra electron to each metal center. This gives in total 2x9 (from Rh) + 8x2 (from CO) + 2 (1 Rh-Rh bond) = 36 valence electrons, or 18 per Rh atom.For the trimers only Ru3(CO)12
+ obeys this rule for each metal atom, so, first figure out how many metal-metal single bonds are in this cluster.
Physical and chemical properties of small clusters (<100 atoms) are often strongly size-dependent
Model and reference systems
Investigation under (close to) collision free conditions
Mass spectrometry: Cluster size separation vs. sizeselective detection (action spectroscopy)
Variation of size (n), composition (n/m) and charge (z): MnLm
z
Cluster-size specific methods for characterizationAdsorption probes Ion mobility spectrometryTrapped ion electron diffractionAnion photo electron spectroscopyInfrared spectroscopy
Summary