Ions colliding with molecules andmolecular clusters: fragmentationand growth processes

Tao Chen

Abstract

In this work we will discuss fragmentation and molecular growth processes incollisions of Polycyclic Aromatic Hydrocarbon (PAH) molecules, fullerenes,or their clusters with atoms or atomic ions. Simple collision models as wellas molecular structure calculations are used to aid the interpretations of thepresent and other experimental results. Fragmentation features at center-of-mass collision energies around 10 keV are dominated by interactions betweenthe fast ion/atom and the electron cloud in the molecules/clusters (electronicstopping processes). This electronic excitation energy is rapidly distributed onthe vibrational degrees of freedom of the molecule or of the molecules in acluster and may result in fragmentation. Here, the fragmentation is statisticaland favors the lowest-energy dissociation channels which are losses of intactmolecules from clusters, H- and C2H2-losses from isolated PAHs, and C2-lossfrom fullerene monomers. We will also discuss the possibility of formation ofmolecular H2 direct from native PAHs which reach high enough energies wheninteracting with ions, electrons, or photons.

For the experiments at lower center of mass collision energies (∼ 100 eV)a single atom may be knocked out in close atom-atom interaction. Such non-statistical fragmentation are due to nuclear stopping processes and gives highlyreactive fragments which may form covalent bonds with other molecules in acluster on very short time scales (picoseconds). This process may be importantwhen considering the formation of new species. For collision between 12 keVAr2+ and clusters of pyrene (C16H10) molecules, new molecules, e.g. C17H+

10,C30H+

18, C31H+19, etc are detected. We also observe molecular fusion processes

for He and Ar ions colliding with clusters of C60 molecules. These and relatedmolecular fusion processes may play a key role for understanding moleculargrowth processes under certain astrophysical conditions.

c©Tao Chen, Stockholm University 2015

ISBN 978-91-7649-063-1

Printer: Holmbergs, Malmö 2015

Distributor: Department of Physics, Stockholm University

to Xiaoyu

List of Papers

The following papers, referred to in the text by their Roman numerals, areincluded in this thesis.

PAPER I: Formations of Dumbbell C118 and C119 inside Clusters of C60Molecules by Collision with alpha ParticlesH. Zettergren, P. Rousseau, Y. Wang, F. Seitz, T. Chen, M.Gatchell, J. D. Alexander, M. H. Stockett, J. Rangama, J. Y.Chesnel, M. Capron, J. C. Poully, A. Domaracka, A. Méry, S.Maclot, H. T. Schmidt, L. Adoui, M. Alcamí, A. G. G. M. Tie-lens, F. Martín, B. A. Huber, and H. Cederquist PHYSICAL RE-VIEW LETTERS, 110, 185501 (2013).DOI: 10.1103/PhysRevLett.110.185501

PAPER II: Ions colliding with clusters of fullerenes-Decay pathways andcovalent bond formationsF. Seitz, H. Zettergren, P. Rousseau, Y. Wang, T. Chen, M.Gatchell, J. D. Alexander, M. H. Stockett, J. Rangama, J. Y.Chesnel, M. Capron, J. C. Poully, A. Domaracka, A. Méry, S.Maclot, H. T. Schmidt, L. Adoui, M. Alcamí, A. G. G. M. Tie-lens, F. Martín, B. A. Huber, and H. Cederquist THE JOURNALOF CHEMICAL PHYSICS, 139, 034309 (2013).DOI: 10.1063/1.4812790

PAPER III: Non-statistical fragmentation of PAHs and fullerenes in col-lisions with atomsM. Gatchell, M. H. Stockett, Patrick Rousseau, T. Chen, K Ku-lyk, H. T. Schmidt, J. Y. Chesnel, A. Domaracka, A. Méry, S.Maclot, L. Adoui, K. Støchkel, P. Hvelplund, Y. Wang, M. Al-camí, B. A. Huber, F. Martín, H. Zettergren, H. Cederquist IN-TERNATIONAL JOURNAL OF MASS SPECTROMETRY, 365-366, 260, (2014).DOI: 10.1016/j.ijms.2013.12.013

PAPER IV: Non-statistical fragmentation of large moleculesM. H. Stockett, H. Zettergren, L. Adoui, J. D. Alexander, U.

Berzinš, T. Chen, M. Gatchell, N. Haag, B. A. Huber, P. Hvelplund,A. Johansson, H. A. B. Johansson, K. Kulyk, S. Rosén, P. Rousseau,K. Støchkel, H. T. Schmidt and H. Cederquist PHYSICAL RE-VIEW A, 89, 032701 (2014).DOI: 10.1103/PhysRevA.89.032701

PAPER V: Absolute fragmentation cross sections in atom-molecule col-lisions: Scaling law for non-statistical fragmentation of poly-cyclic aromatic hydrocarbon moleculesT. Chen, M. Gatchell, M. H. Stockett, J. D. Alexander, Y. Zhang,P. Rousseau, A. Domaracka, S.Maclot, R. Delaunay, L. Adoui,B. A. Huber, T. Schlathölter, H. T. Schmidt, H. Cederquist andH. Zettergren JOURNAL OF CHEMICAL PHYSICS, 140, 224306(2014).DOI: 10.1063/1.4881603

PAPER VI: Formation of H2 from internally heated Polycyclic AromaticHydrocarbons: Excitation energy dependenceT. Chen, M. Gatchell, M. H. Stockett, R. Delaunay, A. Do-maracka, P. Rousseau, L. Adoui, B. A. Huber, H. T. Schmidt,H. Cederquist and H. Zettergren JOURNAL OF CHEMICALPHYSICS, 142, 144305 (2015).DOI: 10.1063/1.4917021

PAPER VII: Molecular growth inside polycyclic aromatic hydrocarbonclusters induced by ion collisionsR. Delaunay, M. Gatchell, P. Rousseau, A. Domaracka, S.Maclot,Y. Wang, M. H. Stockett, T. Chen, L. Adoui, M. Alcamí, F.Martín, H. Zettergren, H. Cederquist, and B. A. Huber JOUR-NAL OF PHYSICAL CHEMISTRY LETTERS, 6, 1536 (2015).DOI: 10.1021/acs.jpclett.5b00405

Reprints were made with permission from the publishers.Articles not included in this thesis are listed in Appendix.

Author’s contribution

For Paper I-IV, I have modeled the electronic and nuclear stopping processesfollowing collisions between atoms/ions and PAH/fullerene molecules/clusters.I have also calculated the molecular structures and electron density distribu-tions by using quantum chemical methods.

For Paper V, I have calculated the electronic and nuclear stopping energiesfor various collision systems. I have derived a simple scaling law for esti-mating non-statistical fragmentation cross sections. I have also analyzed theexperimental data and produced the model data. I drafted the publication andmade all figures.

For Paper VI, I have calculated the dissociation energies for single H-emission, sequential H-emissions (H+H) and H2-emission from several PAHs,and transition barriers for H2-emission from PAHs. I drafted the publicationand made all the figures.

For Paper VII, I have participated in the experimental study of the molecu-lar growth process and contributed with calculations of energy transfers throughelectronic stopping processes. I have participated in discussions of the resultsand their interpretations.

This thesis is partly based on my licentiate thesis "Statistical and Non-statistical fragmentation of large molecules in collisions with atoms".

Sammanfattning

I denna avhandling presenteras resultat rörande sönderfall- och tillväxtpro-cesser efter kollisioner mellan fulleren-molekyler, PAH-molekyler (polycyk-liska aromatiska kolväten) eller kluster av dessa, och atomer eller atomärajoner. Två olika experimentella uppställningar har använts, en för höga kol-lisionsenergier (> 10 keV i masscentrumsystemet) och en för låga kollision-senergier (< 1 keV i masscentrumsystemet). De experimentella resultatendiskuteras och tolkas med hjälp av en kollisionsmodell, molekylära struk-turberäkningar och molekyldynamik-simuleringar.

Vi visar att vid höga kollisionsenergier (flera keV i masscentrumsystemet)är den dominerande processen för energiöverföring växelverkan med mole-kylernas elektronmoln (elektronisk excitationer). Den energi som överförsfördelas sedan statistiskt över alla inre frihetsgrader innan molekylen ellerklustret börjar kylas genom att skicka ut fotoner, elektroner, atomer eller mole-kyler. De energier som överförs ligger generellt långt över tröskelenergin förmolekylär dissociation vilket leder ett innehållsrikt masspektra med mångatoppar. Med strukturberäkningar visar vi att dissociationsenergier för PAH-molekyler har ett svagt beroende på molekylernas storlek, åtminstone för devanligt förkommande fragmenteringsprocesserna. Dissociationsenergierna föratt en PAH-molekyl ska avge H eller H2 ligger på cirka 5 eV respektive 4eV, men reaktionsbarriärer gör att den senare processen i praktiken är mycketlångsammare än den förra. Detta leder till att H2-emission blir en viktig pro-cess först vid höga PAH-temperaturer (> 2200 K). Temperaturer i detta områdeär vanliga i våra kollisioner med keV-joner/atomer, men nås inte exempelvisnär PAH-molekyler absorberar enskilda UV fotoner med energier lägre än 13.6eV (jonisationsenergin för atomärt väte).

Icke-statistiska sönderfallsprocesser är processer i vilka enskilda kolatomerslås ut från PAH-molekyler eller fullerener i Rutherford-liknande spridningspro-cesser. Vi har observerat icke-statistiskt sönderfall i experiment där vi kollid-erat PAH-/fulleren-molekyler med atomer vid låga (< 1 keV) kollisionsen-ergier. Vi har utvecklat enkla samband för att beräkna absoluta tvärsnitt föricke-statistiskt sönderfall av molekyler.

I kollisioner mellan keV-joner och molekylära kluster spelar de snabbaicke-statistiska sönderfallsprocesserna en nyckelroll i bildandet av nya molekyler.De fragment som skapas genom icke-statistiskt sönderfall är oftast mer reak-

tiva än de som bildas i statiska sönderfallsprocesser. Detta leder till att dereaktiva fragment som skapas i kollisionerna kan bilda nya kovalenta bind-ningar med andra molekyler i klustren. Vi har utfört experiment där vi kollid-erat olika atomära joner med kluster av C60 eller pyren (C16H10). I fallet medfulleren-kluster såg vi att den hantelformade C119-molekylen bildades. MedPAH-kluster skapades en rad nya molekyler med formen C16+mHx, där m =1-21.

Contents

Abstract ii

List of Papers iii

Author’s contribution v

Sammanfattning vii

1 Introduction 1

2 Experimental techniques 72.1 Collision induced dissociation experiments with PAH and fullerene

cations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.2 Ion-impact induced fragmentation and molecular growth in molec-

ular clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 Theoretical tools and models 133.1 Collision model . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.1.1 Nuclear stopping . . . . . . . . . . . . . . . . . . . . 133.1.2 Electronic stopping . . . . . . . . . . . . . . . . . . . 17

3.2 Molecular structure calculations . . . . . . . . . . . . . . . . 18

4 Results & Discussion 214.1 Collisions with isolated PAH or fullerene molecules . . . . . . 21

4.1.1 Statistical fragmentation . . . . . . . . . . . . . . . . 214.1.2 Competition between statistical and non-statistical frag-

mentation . . . . . . . . . . . . . . . . . . . . . . . . 264.2 Collisions with clusters of PAH or fullerene molecules . . . . 30

4.2.1 Molecular growth . . . . . . . . . . . . . . . . . . . . 31

5 Concluding remarks 37

Appendix: Additional publications xxxix

Acknowledgements xliii

References xlv

1. Introduction

Polycyclic Aromatic Hydrocarbons (PAHs) contain only carbon and hydrogenatoms (CmHn). They typically have three or more fused benzene-like rings. Anexample of a PAH molecule, pyrene is shown in Figure 1.1a. Pyrene (C16H10)has a planar structure with four benzene-like rings fused together. PAHs areubiquitous and abundant on Earth and in Space. On Earth, it has been com-monly found in combustion products of organic materials [59]. In space, manyobservations suggest that PAHs are present in meteorites [44], in comets [38],and in the interstellar medium (ISM) as the carriers of the strong infrared emis-sion features from galactic and extragalactic objects [6, 65, 68, 69]. Anotherrelated class of molecules are the fullerenes, which contain only carbon atoms,and have hollow spherical structures. An example of a fullerene with 60 carbonatoms (C60) is shown in Figure 1.1b. This molecule has been unambiguouslyidentified in space [10, 62, 79]. A recent study suggests that fullerenes could beformed from PAHs in space by multistep dehydrogenation and fragmentationprocesses [5].

Photons

Electrons ~7eV

Atoms H ~ 5eV

Molecules H2, C2H2 ~ 5eV

Molecules C2 ~ 10eV

Photons

Electrons~ 7eV

(a) (b)

Figure 1.1: Internally heated molecules (a. pyrene, b. C60) may cool by emittingphotons, electrons, and/or molecules or atoms.

When internally heated, PAHs predominantly fragment by emitting H-atoms or C2H2-molecues which correspond to the lowest-energy dissociationchannels. It is much more difficult to remove a single C-atom which is re-

1

flected in the fact that the corresponding dissociation energy is much higherthan for H- or C2H2-loss. Similarly, the fullerenes have C2-loss as their low-est dissociation channel while the removal of a single C-atom is much moreenergetically unfavourable. The fullerenes have been extensively investigatedboth experimentally and theoretically - see for examples [13, 31, 35, 72, 78].Larsen et al performed a pioneering experiment on the fragmentation of C60anions in 50 keV collisions with rare gas targets. There, they observed two dif-ferent types of fragmentation processes: weak direct "knock-out" processes ofsingle C-atoms and strong delayed (statistical) fragmentation process, whichare dominated by C2 emission [35]. However, it is in general hard to dis-tinguish the knock-out and statistical fragmentation processes for large andcomplex molecules. Due to the three-dimensional hollow sphere structure, thesignature of a direct knock-out process namely the loss of a single C atomis often disguised by secondary (statistical) fragmentation process and alsoby additional knock-out processes. The latter are more likely for three thanfor two-dimensional molecular structures. Attention has therefore be givento PAH molecules, which have planar, simple and regular structures. A vastnumber number of studies [15, 27, 29, 36, 47, 53, 58] have been performed onexcitation of PAHs by absorption of photons [24, 40], by interaction with en-ergetic electrons [25], or atoms [9]. In a pioneering work, Postma et al studiedthe fragmentation of the PAH molecule anthracene (C14H10) in collisions withprotons and helium ions at keV energies. They introduced a method to calcu-late the molecular excitation due to electronic stopping, in which the electrondensities are obtained by means of Density Functional Theory (DFT). In boththeir experiments and their model simulations it was found that the fragmen-tation increased with increasing projectile velocity [53]. Reitsma et al studiedthe fragmentation of anthracene due to double ionization by 5 keV protons andfound clear experimental evidence for dominance of C3H+

3 over C2H+2 in this

particular case [58]. However, Johansson et al found that C2H2-loss was thelowest energy dissociation channel for singly charged anthracene molecules[29]. Mishra et al were interested in the electron emission and electron trans-fer processes in proton-naphthalene collisions at velocities between 1.41 and2.68 au. They found that electron capture cross sections decrease rapidly overthe corresponding energy range [47].

The study of PAHs and fullerenes is also a hot topic in astronomy. Re-cent investigations suggest that molecular hydrogen (H2) could be formed fromPAHs, in which the PAHs in some potential schemes may act as catalytic cen-ters [2, 8, 12, 26, 45, 52, 64]. Figure 1.2 shows three possible routes lead-ing to molecular hydrogen formation from a coronene molecule. In the firstcase (Fig 1.2a.), a coronene molecule captures a hydrogen atom to form analiphatic carbon. This step is exothermic and the barrier for emitting a H2

2

+

+

+

PAH [PAH+H] PAH+H2

[PAH-H2]+H2

[PAH-H]+H2

(a)

(c)

(b)

[PAH+H]PAH

PAH

+H

+Energy

+Energy

+H

+H

Figure 1.2: Three possible routes for the formation of H2-molecules from PAHmolecules. The examples shown are for coronene C24H12. In processes a, andb, the coronene molecule acts as a catalytic center absorbing one or two hydro-gen atoms, respectively. In c, the H2 molecule is formed directly from pristinecoronene molecules (see the text for more details).

3

molecule from the aliphatic carbon is very low [34, 57]. In the second case(Fig 1.2b.), the coronene with an aliphatic carbon can lead to the formation ofH2 without adding a second H-atom. This however, requires that a substantialamount of energy is transferred to the system as the corresponding activationbarrier is high. There is another possible route, in which the H2 moleculecould be formed directly from a fully aromatic PAH as shown in Fig 1.2c. Ithas been shown experimentally [74], that the last route is an inefficient processfor singly charged PAHs with low internal energy (i.e. close to the dissociationthreshold). In this work, we focus on the latter route and calculate dissocia-tion and transition state energies for H- and H2-emission from the naphthalene(C10H8), and the PAHs anthracene (C14H10), pyrene (C16H10) and coronene(C24H12). This work demonstrate that H- and H2-formation from PAHs aretwo competing process. For high energy collisions (center of mass energyhigher than 10 keV), the internal PAH temperatures typically exceed 2200 Kand H2 molecules may then be efficiently formed (see Paper VII for details).

We have also studied reaction within clusters of molecules. In all casesthese molecules are weakly (< 1 eV) bound to each other through weak (e.g.van der Waals) forces. Two examples of clusters - pyrene and fullerene clustersare shown in Figure 1.3. PAH and fullerene clusters have been suggested to beomnipresent in nature [5, 7, 14, 18, 56, 68]. Many experiments and theoreticalworks have dealt with clusters of PAHs and fullerenes [28, 30, 72, 78]. Holmet al reported the first experimental study of keV ions interacting with clustersof PAH molecules. They found that anthracene clusters easily fragment in thecollisions just like many other weakly bound clusters [28]. Johansson et al theninvestigated the ionization and fragmentation of anthracene and coronene clus-ters with similar experimental techniques as Holm et al. They found that singlycharged and internally rather cold monomers are dominant products when PAHclusters fragment due to collisions with keV ions [30]. In this work, we presentexperimental evidence for the formation of so called dumbbell fullerene sys-tems (see Paper I), and molecular growth of PAHs inside clusters (see PaperVII), which are ignited by non-statistical knockout processes. The cluster en-vironment is essential here as it helps to cool the fragments such that they havetime to react with neighbouring molecules in the cluster before they fragmentfurther.

This work focuses on fragmentation processes of PAHs or fullerenes fol-lowing collision with energetic ions or atoms. In general there are two differ-ent fragmentation processes (see Paper IV for details): statistical fragmenta-tion refer to processes in which there is time for a given excitation energy tobe distributed over all the internal degrees of freedom in the molecule beforefragmentation. Such processes favour the lowest-energy dissociation chan-nels. The emissions of C2 molecules from fullerenes [35] are typical examples

4

(a) (b)

Figure 1.3: Cluster of pyrene (a) and fullerene (b). The pyrene cluster containsfour pyrene molecule, and the fullerene cluster includes five C60 molecules. Thestructures were optimized using the semi-empirical PM6 method as implementedin Gaussian09 [23].

of statistical fragmentation processes. Examples of non-statistical fragmen-tation processes are processes in which atoms are knocked-out directly fromthe molecule in head-on collision between atomic ions or atoms and individ-ual atoms in the molecules. Such knockout processes are fast (femtosecondtimescales) and there is no time for the excitation energy to redistribute itselfover all the degrees of freedom of the molecule before the initial fragmenta-tion step. Micelotta et al simulated the interaction between interstellar PAHsand plasma shock waves from supernova explosions and concluded that non-statistical fragmentation is an important destruction mechanism: interstellarPAHs (number of carbon atom NC = 50) do not survive in shocks with veloc-ities greater than 100 km s−1 and larger PAHs (NC = 200) are destroyed forshocks with velocities ≥ 125 km s−1 [46]. Similar fragmentation processeshave been unambiguously observed in experiments at center of mass energiesfrom about 100 eV and up to about 1 keV for interaction between PAHs orfullerenes and atoms, in which single atoms are lost from the molecules. Thisleads to the formation of highly reactive fragments.

Statistical and non-statistical fragmentation processes often compete. Af-ter knockout, the molecule may still have high enough internal energy to decayfurther in secondary statistical fragmentation steps. In general, the balance be-tween processes in which energy is transferred to the electronic degrees of free-dom (electronic stopping processes) and processes in which energy is trans-ferred directly to the vibrational modes (nuclear stopping processes) affects thebalance between statistiscal and non-statistical fragmentation processes. Fig-ure 1.4 shows the ratio between electronic and nuclear stopping as a function

5

of projectile energy for helium atoms colliding with anthracenes in solid phase[80]. One can see from Figure 1.4 that nuclear stopping is more important forion energies lower than ∼ 2 keV, while the electronic stopping becomes moreimportant at higher energies (> 2 keV). In order to study both fragmentationprocesses the present work covers the energy range from roughly 100 eV to100 keV (gray area in Fig 1.4).

100 101 102 103 104 105 106 107

Projectile ion energy, eV

10-1

100

101

102

103

Ene

rgy

loss

(eV

/nm

)

Electronic

Nuclear

Figure 1.4: The nuclear and electronic stopping energies as functions of pro-jectile ion energy in the case of helium atoms colliding with anthracene in thesolid phase [80]. The gray area indicates the energy range of main interest forthe present work.

The remainder of this thesis is organized as follows: In Chaper 2 the ex-perimental technique is described for studies of PAH or fullerene monomercations and different noble gases at center of mass energies from about 100eV to 1 keV. In the same chapter we describe the experimental technique usedfor collision experiments with atomic projectiles with kinetic energies in therange above 10 keV and neutral PAH or fullerene monomers or their clusters.The theoretical calculations and the Monte Carlo simulations of electronic andnuclear stopping processes are discussed in Chapter 3. The results on H2 for-mation, the simple scaling law for non-statistical fragmentation and moleculargrowth processes are discussed in Chapter 4. Finally, concluding remarks andan outlook are given in Chapter 5.

6

2. Experimental techniques

This section describes two complementary experimental setups for studies offragmentation and molecular growth processes in collisions between ion/atomsand molecules or molecular clusters. The setup at Stockholm University isutilized for low energy (< 1 keV) Collision Induced Dissociation (CID) ex-periments of molecular ions, where nuclear stopping is the dominant energytransfer process in the collisions (see Figure 1.4). The setup in Caen, France,is utilized for higher energy (> 10 keV) collisions. There, electronic stoppingis more important than nuclear stopping (see Figure 1.4).

2.1 Collision induced dissociation experiments with PAHand fullerene cations

The experimental setup at Stockholm University is a part of the DESIREE fa-cility [61, 67] and is designed for low energy collisions between molecularions/anions and neutral gases (atoms and molecules) and for studies of inter-actions with laser photons. A schematic of the experimental setup is shown inFigure 2.1. The molecules under study are dissolved in a solution and broughtinto gas phase by means of electrospray ionization [19, 76]. In this technique,the needle of a syringe carrying the solution is kept at a high voltage (2-3 keV)to create a strong electric field between the needle and a capillary serving asa counter electrode. The syringe plunger is then pushed at a controllable ratesuch that charged droplets are ejected from the top of the needle [50]. The tem-perature of the capillary may be adjusted for evaporation of unwanted solventmolecules [77].

The so formed isolated molecules leave the capillary and enter a radio-frequency ion funnel [32, 33, 63]. The ion funnel focuses the ions into twosets of octopoles [66] used to accumulate the ions in bunches (not used in thepresent work), and to guide the ions to a quadropole mass filter [51].

The quadruploe mass filter is used to mass-to-charge select ions for theprimary projectile beams for the experiments. The mass-selected PAH cationsenter a quadrupole deflector before they leave the high-voltage platform, areaccelerated and enter the experimental beam line which is at ground poten-tial. The accelerated ions are guided to the collision chamber by means of

7

AcceleratioGas Cell

Acceleration Lens DeflectorsCapillary 8-pole Trap Mass Filter

MCP

Slits

Ion Funnel 8-pole Guide

Electron Multiplier

Quadrupole DeflectorNeedle

Figure 2.1: Schematic of the experimental setup used for collision induced dis-sociation (CID) type experiments at Stockholm University. The molecular ionsare produced by means of electrospray ionization, collected by an ion funnel,guided by two octopoles, mass-selected by a quadrople mass filter, and acceler-ated before entering the collision cell. There they collide with a target gas andthe so formed fragments are analyzed by means of electrostatic deflectors and amicro-channel plate detector with a resistive anode (see text).

Pressure (mTorr)

0.1

1

Norm

aliz

ed

Rate

1 2 3 4 5 6

C14H10++He

C24H12++He

Figure 2.2: Normalized count rates of the primary PAH+-beams as functions ofgas cell pressure for different PAHs colliding with He at 110 eV center-of-massenergy. The slope of the fitted lines yield the total fragmentation cross sections.The pressure is measured by means of a capacitance manometer. Statistical errorsare smaller than the data points.

8

an einzel lens and deflector elements to enter a 4 cm long collision cell. Theabsolute pressure of the target gas in the collision cell is measured with a ca-pacitance manometer and the pressure may be adjusted in a controlled man-ner by a needle valve. The beam intensity for keV-ions entering the collisioncell is typically a few thousands of ions per second. The intact PAH cationsand fragment ions leaving the collision cell are mass-to-charge analyzed bymeans of two sets of electrostatic deflectors and focused by means of an einzellens. The ions are then detected by a micro-channel plate (MCP) equippedwith a resistive anode for position information. The position information isstored for each collision event together with the corresponding deflector andlens settings. This may be used to achieve the necessary resolution in the massspectrum without using slits in front of the detector.

The beam attenuation method [71] was used to determine absolute frag-mentation cross sections. The ion beam count rate is then measured as a func-tion of the gas pressure in the cell. Figure 2.2 shows such trends for differentPAH cations colliding with He at 110 eV center of mass energies. The slopesof the fitted lines in the lin-log plots give the corresponding total fragmentationcross sections.

He gas at ~ 1 mbar

Liquid N2 < 100 K

PAH

Figure 2.3: Schematic of the the cluster aggregation source. An oven is mountedinside a container filled with He buffer gas at a pressure of about 1 mbar. Liquidnitrogen is used to cool the He gas to temperatures below 100 K such that weaklybound clusters may be formed.

2.2 Ion-impact induced fragmentation and molecular growthin molecular clusters

The experimental setup for high center of mass energy (> 10 keV) collisions islocated at the ARIBE facility in Caen, France [4, 16]. There, keV ion beams

9

are produced in an Electron Cyclotron Resonance (ECR) ion source [42, 75]and guided to interact with a beam of neutral molecules or molecular clustersinside the extraction region of a linear time-of-flight mass spectrometer.

The neutral targets are produced in electrically heated ovens loaded withcommercially available molecular powders. There are separate ovens for pro-ducing neutral monomer and cluster beams. When the monomer oven is heated,the molecules are evaporated and effuse into the interaction region through theoven exit hole. The cluster source oven has a similar design as the monomeroven but it is mounted inside a container filled with helium buffer gas wherethe pressure is of the order of millibars (see Figure 2.3). The molecules ef-fuse from the oven and enter the condensation region where the helium gasis cooled down by liquid nitrogen to temperatures below 100 K. This leadsto formation of weakly bound neutral clusters with a broad (lognormal) sizedistribution [28, 30]. The clusters are guided to the interaction region. Thecluster size distribution depends on the oven temperature, increasing the tem-perature (number density) gives larger clusters. The temperature depends onthe molecular species, e.g. 60 ◦C and 250 ◦C is typically set for producinganthracene and coronene monomers, respectively. Production of clusters oftenrequire higher temperatures.

-4 kV

-23

kV

-15.4 kV-13 kV0V

MCP

2.14

5kV

-4kV

0V

Weak magnetic field

Interaction region

e-

e- e-

e-

e-e-

Target collisionproducts

~10-6 sec

Figure 2.4: Schematic of the time-of-flight mass spectrometer. A pulsed ionbeam enters the interaction region of the spectrometer. The charged collisionproducts are extracted and accelerated along the spectrometer axis shortly afterthe beam pulse has left the interaction region. The ions hit a metal plate at theend of the spectrometer. This produces secondary electrons which are guided bya weak magnetic field towards a MCP detector.

A schematic of the mass spectrometry setup is shown in Figure 2.4. Theion beam is chopped into ∼ 1µs long pulses before entering the interactionregion with a monomer or a cluster target. After the beam pulse has left the

10

interaction region, the charged collision products (intact and fragment ions) areextracted by a pulsed homogeneous electric field. The ions are then acceleratedand enter a field free drift region. The acceleration system and the length of thedrift region are designed to let ions with the same mass to charge ratio arriveat the same time at the end of the drift region regardless where they wereproduced in the interaction region. After the drift free region, the ions arefurther accelerated to increase the yield of secondary electrons emitted whenthey hit the metal plate at the end of the spectrometer. A weak magnetic field isthen used to guide these electrons towards a MCP detector. The time differencebetween the start of the extraction pulse and a hit on the MCP detector givesthe ion flight time. The metal plate combined with the weak magnetic field andthe MCP system gives a high detection efficiency, which allows for detectionof several charged fragments per collision event (coinicidence measurements).

11

12

3. Theoretical tools and models

This section describes the theoretical tools and models which have been usedto guide the interpretations of the experimental results. In Section 3.1 wepresent a model for energy transfer processes when ions and atoms pene-trate the molecular electron clouds and interact with the individual nuclei inmolecules or molecular clusters. In this model, we use a Monte Carlo tech-nique where we calculate such electronic and nuclear stopping processes forrandom ion/atom trajectories through or close to the molecules or clusters.By performing a large set of trajectories we get information about the energytransfer distributions. The collisional energy transfer is typically statisticallyredistributed across all degrees of freedom before the system cool down byemission of photons, electrons, atoms, or molecules. However, prompt (non-statistical) atom knockout processes may also be important under certain cir-cumstances (in low energy collisions with isolated molecules or high energycollisions with clusters).

In Section 3.2 we describe the molecular structure calculations which wasused to investigate how the molecules and clusters respond to energy transferprocesses. These calculations give information about e.g. the lowest dissocia-tion energy pathways and reaction barriers for the intact molecules and the re-activity of fragments from non-statistical fragmentation. They were also usedto produce collision model input parameters (nuclear coordinates and electrondensities).

3.1 Collision model

3.1.1 Nuclear stopping

Nuclear stopping, i.e. when the projectile lose some of its kinetic energy byscattering on individual nuclei in the molecule, is the dominant energy lossmechanism for low collision energies (see Figure 1.4). In a classical picture,the energy lost by projectile atom with mass M1, atomic number Z1 and atarget atom (M2, Z2), is

Tnuc = Tmaxsin2 φ

2(3.1)

13

where

Tmax =4M1M2

(M1 +M2)2 E (3.2)

is the maximum energy transfer in head-on collisions and E is kinetic energyof projectile in laboratory system. The scattering angle in the center of masssystem φ (see Figure 3.1) is related to the impact parameter p by [80]:

sinφ

2= cos

∫

∞

rmin

pdr

r2√

1− V (r)ECM− ( p

r )2

(3.3)

where ECM is the center of mass collision energy, rmin is the closest distancebetween the projectile and target atom during the collision. This distance ofclosest approach can be found by solving the following equation

1− V (rmin)

ECM−(

prmin

)2

= 0 (3.4)

Figure 3.1: Schematic showing the relation between the impact parameter (p)and the scattering angle (φ ) in the center of mass (CoM) energy system for thecollision between two atoms (M1 and M2).

14

where V(r) is the interaction potential. In the present work, we use aCoulomb potential multiplied by different types of screening functions, f(x),

V (r) =Z1Z2

rf (x) (3.5)

Lindhard [39] introduced screening function of the type

fLindhard(x) =ks

sx1−s (3.6)

where s is an integer and ks a constant determined by s. The variable x is theratio between the radial distance and the screening length (aLindhard)

x =r

aLindhard=

r√

Z2/31 +Z2/3

2

0.8853a0(3.7)

where a0 is the Bohr radius. Lindhard applied an approximate method to eval-uate φ (Eq. 3.3) and arrived at the following expression for s=2 and k2=0.831[35, 39]

sinφ

2=

π

8 p20

p2 + π

8 p20

(3.8)

The corresponding analytical solution is (see Paper V)

sinφ

2= cos

π p

2√

p2 + p20

(3.9)

wherep2

0 =0.831Z1Z2aLindhard

2ECM(3.10)

The present analytical solution (Eq. 3.9) and the approximate solution(Eq. 3.8) give results in close agreement with each other. This is illustrated inthe left panel of Figure 3.2, which shows a comparison between the two ex-pressions for 100 eV collisions (ECM=100 eV) between hydrogen (Z1=1) andcarbon (Z2=6). In the right panel of Fig. 3.2, we show a comparison betweenthe analytical expression for the Lindhard potential (Eq. 3.9) and the corre-sponding numerical results for the so called ZBL (Ziegler-Biersack-Littmark)potential [80]. The latter is commonly used for ions or atoms interacting withsolids. The ZBL screening function is

fZBL(x) = 0.1818e−3.2x +0.5099e−0.9423x+

0.2802e−0.4029x +0.02817e−0.2016x (3.11)

15

0 1 2 3 4 5Impact parameter, p(a0 )

ZBL

Analytical

0 1 2 3 4 50.20.40.60.81.0

ZB

L/A

naly

tica

l

0 1 2 3 4 5Impact Parameter, p (a0 )

0.0

0.2

0.4

0.6

0.8

1.0Lindhard

Analytical

sin(φ

/2)

0 1 2 3 4 5

0.992

0.996

1.000

Lindhard

/Analy

tica

l

Figure 3.2: Comparison between Lindhard approximate expression and thepresent analytical solution for the scattering angle φ as a function of impactparameter (left), and comparison between scattering angles obtained with theLindhard (analytical solution) and the ZBL (numerical evaluation) potentials asa function of impact parameter for collision between hydrogen and carbon (ECM= 100 eV) (right). The insets show the ratios between the two curves.

where

x =r

aZBL=

r(Z0.231 +Z0.23

2 )

0.8853a0(3.12)

and aZBL is the ZBL screening length. We find that the ZBL and Lindhardpotentials give rather similar results for light atoms (e.g. H or He) interactingwith hydrogen or carbon atoms in the 100 eV collisions energy range (see theright panel of Figure 3.2). This allows us to use the analytical solution (Eq.3.9) for the relation beween the impact parameter (p) and the scattering angle(φ ) to get an analytical formula for single atom knockout out cross section peratom in the molecule

σ = π p2 =4π p2

0

π2 arccos−2(√

Eth/Tmax)−4(3.13)

where Eth is the threshold energy for knockout. Here, we use the results frommolecular dynamics simulations which show that the threshold energies forknocking out a carbon or a hydrogen atom from a PAH molecule are close to27 eV and 9 eV, respectively (see Paper V for details). In Figure 3.3 we showthe calculated cross sections for exceeding the thresholds for carbon knockout(EC

th=27 eV) in H+C, He+C collisions and hydrogen knockout (EHth=9eV) in

H+H and He+H collisions.

16

2000 4000 6000 8000 10000Kinetic energy, eV

0.0

0.5

1.0

1.5

2.0

2.5

3.0C

ross

sect

ion, a2 0

EthC=27 eV

H + C

He + C

2000 4000 6000 8000 10000Kinetic energy, eV

H + H

He + H

200 600 10000.00.40.81.2

EthH =9 eV

Figure 3.3: Calculated absolute cross sections for H + C, He + C, H + H and He+ H scattering above the threshold energy for knocking out a carbon (left panel)or a hydrogen atom (right panel) from a PAH molecule.

3.1.2 Electronic stopping

The electronic excitation energies due to interactions with the PAH molecularelectron clouds are calculated by treating the valence electrons (all electronsexcept the C 1s electrons) as a free electron gas. The energy lost by ion/atomstraversing an electron gas depends on the electron density distribution alongthe ion trajectories [21]. Here we assume that the electronic energy loss Te

is proportional to projectile velocity [70] and the electronic stopping power isthen given by

S =dTe

dR= γ(rs)v (3.14)

where γ(rs) is the so called friction coefficient, which is a function of the one-electron radius rs

rs =

(3

4πn0

) 13

(3.15)

where n0 is the valence electron density. The electron density is inhomoge-neously distributed within the molecules and is calculated by means of molec-ular structure calculations (see Section 3.2). The friction coefficient is relatedto the scattering phase shifts (δl) through

γ(rs) =3

kFr3s

∞

∑l=0

(l +1)sin2(δl−δl+1) (3.16)

17

where kF is the magnitude of the Fermi wave vector. The friction coefficientsfor various atoms and a few rs-values have been calculated by Puska and Niem-inen [55]. We fit exponential functions to their results (see the examples inFigure 3.4) to calculate the friction coefficients for any rs-value.

In the simulations, we calculate the electronic stopping power (Eq. 3.15)for each point along random atom trajectories through the PAH molecular elec-tron clouds, The stopping energy is then given by the stopping power multi-plied by the step size. Simultaneously, we calculate the nuclear stopping en-ergy as described in the preceding section. For each individual atom trajectory,the sum of these two contributions gives the model total stopping energy.

0 1 2 3 4 5 6rs , a0

10-2

10-1

100

γ(r s)

Hydrogen: 0.64e−( rs )

Puska & Nieminen

0 1 2 3 4 5 6rs , a0

10-3

10-2

10-1

100

101

Helium: 4.15e−( rs )

Puska & Nieminen

0.46 1.14

Figure 3.4: The friction coefficient for hydrogen and helium as a function ofdensity parameter rs. The solid curves are exponential fitting functions, and thered squares are data from Puska and Nieminen [55].

3.2 Molecular structure calculations

Molecular structures calculations are carried out using the Gaussian09 package[23]. These give information about the dissociation energies and reaction bar-riers for the most important decay pathways, the coordinates of the optimizedmolecules (input for the nuclear stopping model), and the valence electrondensities (input for the electronic stopping model).

In a typical calculation, the structure is optimized and the vibrational fre-quencies are then calculated. The latter is done to include the zero-point vi-brational energy and to verify that the structure corresponds to a minimum

18

on the potential energy surface (all frequencies are real) or a transition state(one imaginary frequencies). In these calculations, we have mainly used Den-sity Functional Theory (B3LYP functional) and the 6-311++G(2d,p) basis set[3, 37]. We have also used the computationally more demanding CompleteBasis Set (CBS-QB3) method [48, 49] for computing more accurate energies(paper VI).

19

20

4. Results & Discussion

In Section 4.1, we will discuss the results on PAH- and fullerene-monomerfragmentation in collisions with ions or atoms at low (about 100 eV) and high(several keV) center of mass collision energies. In Section 4.2, we will discussresults on molecular growth in clusters of PAHs and fullerenes.

4.1 Collisions with isolated PAH or fullerene molecules

4.1.1 Statistical fragmentation

Figure 4.1 shows the time-of-flight mass spectrum for collisions between 11.25keV He+ ions and coronene. The two highest peaks in the spectrum corre-spond to intact singly and doubly charged coronene. These are associated withrather low collisional energy transfers such that the intact, singly or doubly ion-ized molecule does not fragment on the experimental timescale of microsec-onds. The highest peak in the spectrum is attributed to single electron cap-ture processes in large impact parameter collisions well outside the molecule(see the picture to the upper left in Fig. 4.1). The so formed singly chargedcoronene molecules are then produced cold enough to survive further fragmen-tation. The second highest peak is most likely due to somewhat closer non-penetrating ion-coronene collisions leading to capture of two electrons or todelayed thermionic emission processes [1] following single-electron capture.To the far left in the spectrum there are peaks representing small hydrocarbonfragments due to statistical fragmentation of coronene molecules which havebeen more strongly heated in collisions where the ion trajectories pass directlythrough the molecules (penetrating collisions). The insets in Fig. 4.1 show theregions for losses of one or several (n) hydrogen atoms from intact singly anddoubly charged coronene. It can be seen clearly in Fig. 4.1 that even numbersof H-losses are highly preferred, which is similar to what has been observedearlier for PAH-fragmentation in collision with photons [43], electrons [17]and keV ions [15, 36].

The nH-loss peaks must be associated with comparatively low internal en-ergies such that the remaing large PAH fragments, [PAH+ - nH] remain suffi-ciently cold to survive the extraction phase in the time-of-flight spectrometer(microsecond timescale). By analysis of simple mass spectra alone it is im-

21

0 50 100 150 200 250 300Atomic mass units per charge (u/e)

Inte

nsi

ty(a

rbit

rary

unit

s)

C24H12+

C24H122+

He+ + C24H12

296 298 300

n= 4 3 2 1

148.0 149.0 150.0

n= 4 3 2 1

5 a.u.

Ionization+FragmentationIonization

Figure 4.1: Mass-to-charge spectra for collisions between 11.25 keV He+ andcoronene (C24H12). The two highest peaks correspond to singly and doubly ion-ized coronene without fragmentation on the experimental timescale of about tenmicroseconds. The right and the left insets show the intensity distributions forlosses of different numbers, n, of H-atoms from the coronene cat- and dications.Other peaks are due to fragmentation of the carbon-skeleton. The inset to the up-per left shows the outer bound for one electron transfer from coronene to 11.25keV He+ (blue surface) according to the classical over-the-barrier model by Fors-berg et al [22]. The outer bound for ion trajectories transferring at least 5 eV tothe electron cloud of coronene is shown as a red surface.

22

possible to unambiguously determine whether the loss of two hydrogen atomsare due to H2 emission or sequential H-emissions (H+H). To investigate thisfurther, the dissociation energies, barriers and energy transfers to the PAHs inthe collisions are required (see below).

Dissoc. energies 1+ 2+H 4.79 4.72H2 4.19 4.17H+H 8.68 8.66

Reaction barriers 1+ 2+H2 5.04 4.95 Units: eV

Figure 4.2: The lowest adiabatic dissociation energies and reaction barriers foremission of H, H+H and H2 molecules from singly and doubly charged coronene.The red circle represents the lowest dissociation energy channel, while the bluecircle indicates the initial positions of two hydrogen atoms that connect to thelowest dissociation energy channels for (H+H)- and H2. It is thus energeticallyfavourable to emit H2 from the same ring. It is also necessary to consider transi-tion states and barriers when discussing the competition between H2-loss and theloss of single H-atoms (see text).

The DFT (B3LYP/6-311++G(2d,p)) calculation results for the lowest adi-abatic dissociation energies and reaction barriers for H, H+H and H2-lossesfrom singly and doubly charged coronene are shown in Figure 4.2. The low-est dissociation energies correspond to emission of H2 (∼4 eV), followed byemission of H (∼5 eV) and H+H (∼9 eV). For H2 or H+H emission it is ener-getically favourable when both hydrogen atoms are from the same ring. Thisseems to suggest that H2-emission could be a dominant decay channel evenfor modest internal excitation energies. However, the H2-emission pathwayinvolves several transition states (see paper VI). The H- and (H+H)-emissions,on the other hand, do not involve any barriers. As shown in Fig. 4.2, the low-est barrier for H2-emission is comparable with the energy required for singlehydrogen dissociation. Therefore, these two processes could be competing indifferent ways for different internal energies. Figure 4.3 shows that the disso-ciation energies and reaction barriers are rather independent of PAH size and

23

PAH size

0

1

2

3

4

5

Energ

y (

eV

)

H-loss (diss. energy)

H2 -loss (diss. energy)

H2 -loss (reaction barrier)

PAH size

0

1

2

3

4

5

H-loss (diss. energy)

H2 -loss (diss. energy)

H2 -loss (reaction barrier)

C14H10

+

C16H10

+

C24H12+

C24H122+

C16H10

2+

C14H10

2+

Figure 4.3: The dissociation energies and reaction barriers for singly (left panel)and doubly (right panel) charged PAHs.

24

charge state.

01020304050

H(1.6 keV)+C10H8He(11.25 keV)+C10H8

0

5

10

15

20H(1.6 keV)+C10H8He(11.25 keV)+C10H8

0246810

Diff.crosssectiondσ/dE(10−18cm

2/eV)

He(11.25 keV)+C14H10F(3 keV)+C14H10

0246810

Diff.crosssectiondσ/dT(10−20cm

2/K)

He(11.25 keV)+C14H10F(3 keV)+C14H10

0246810

He(11.25 keV)+C16H10

0246810

He(11.25 keV)+C16H10

0 20 40 60 80 100Stopping energy, E (eV)

020406080100

He(11.25 keV)+C24H12H(1.6 keV)+C24H12H(100 keV)+C24H12

0 2000 4000 6000 8000 10000 12000Internal temperature (K)

020406080100

He(11.25 keV)+C24H12H(1.6 keV)+C24H12H(100 keV)+C24H12

Figure 4.4: The model (see Paper V) results of the energy and temperature dis-tributions for various collision systems.

Figure 4.4 shows calculated energy and temperature differential cross sec-tions for different collision systems at different collision energies. We haveperformed calculations for four different PAH targets and in each case we havelaunched large numbers (200000) of ion trajectories with random orientationsof the molecules. In the left panels of Fig. 4.4 the energy distributions coverthe 5-100 eV range. For collisions with 11.25 keV He+ there is a distinct peakat about 40 eV in all cases, which is due to the interaction between He andcarbon-carbon bonds in the PAH molecule (see Paper V). In the right panels ofFig. 4.4, we have converted the horizontal axis to a temperature scale throughE = (3N−6)kBT +Ed/2, where N is the number of atoms in the molecule, kB

is the Boltzmann constant, and Ed=5 eV is the dissociation energy of the PAHmolecule. As it has been shown in Paper VI, the temperature at which sig-nificant H2 could be emitted is about 2200 K (see paper VI for more details).Clearly, most collisions yield internal temperatures exceeding that, suggest-ing that H2 indeed may be efficiently formed in the present keV-ion collisions.This also means that the remaining large fragments ([PAH-2H]+) have highinternal energies. Therefore it is likely that they fragment further and that the

25

2H-loss peak in the mass spectrum does not directly reflect the importance ofH2-loss even in cases where H2 is likely to be produced.

4.1.2 Competition between statistical and non-statistical fragmenta-tion

In Figure 4.5, we show the mass spectra due to C14H+10 + He and He+ + C14H10

collisions at center of mass energies of 110 eV and 11 keV, respectively. Re-sults from Monte Carlo simulations of electronic and nuclear stopping pro-cesses are shown in the insets for the particular cases when the trajectories areperpendicular to the molecular planes. Electronic stopping processes domi-nate the energy transfer to the molecule at the higher collision energy (see theupper panel in Figure 4.5). Typical total internal energies are around 40 eV,which is well above the lowest dissociation energies of about 5 eV for H- orC2H2-loss. Many collisions will lead to statistical fragmentation processes inthe 11 keV case [17, 36]. This is consistent with the distribution of fragmentsin the mass spectrum shown in the upper panel of Figure 4.5. The highest peakis due to intact C14H+

10 at a mass-to-charge ratio of 178. The smaller peaksto the left are due to moderately heated systems which lose H- atom(s) and/orC2H2 molecule(s). The lighter fragments in the spectrum are due to more vio-lent collisions where large amounts of energy are transferred (see Paper V fordetails).

The fragment mass spectrum is different in the 110 eV case. Here the nu-clear stopping dominates as can be seen in the inset. According to the molec-ular dynamic simulations by Postma et al. [54], only nuclear stopping aboveabout 27 eV will lead to prompt carbon knockout while 9 eV is sufficient forknocking out of a H-atom (see Paper V for details). Our calculations show thatthere are many collisions which lead to nuclear energy transfers above thesethresholds at 110 eV. On the average the masses of fragments are larger herethan in the cases of the higher energy collisions reflecting cooler anthtracenemolecules and fragments in the 110 eV case. The single carbon atom loss isimportant at 110 eV which is a clear signature of a non-statistical fragmenta-tion process. As shown in Paper V, the much higher dissociation energies forC- or CHx-loss in relation to those for C2H2- or H- loss makes this channelinsignificant for statistical fragmentation processes.

The calculated non-statistical, statistical and total (statistical + non-statistical)fragmentation cross sections using the ZBL potential and the total experimen-tal cross sections for collisions between PAH cations and helium atoms withcenter of mass energy 110 eV are shown in Figure 4.6. Martin et al [40] mea-sured the internal energies of fragmenting anthracene cations and found thatabout 10 eV is needed for statistical fragmentation on microsecond timescales,

26

Figure 4.5: Fragmentation mass-to-charge spectra for He + C14H10 collisions at11 keV (upper panel) and for C14H+

10 + He at 110 eV center-of-mass energy col-lisions (lower panel). Insets show electronic and nuclear stopping contributionsto the molecular excitation energies.

27

which is relevant for their and the present experiments. These results are usedto calculate the total fragmentation cross section by assuming that trajecto-ries associated with larger stopping energies (electronic plus nuclear) than 10eV will lead to fragmentation of anthracene within microseconds. A subset ofthese trajectories will lead to much larger nuclear energy transfers to individualatoms in anthracene and lead to single-carbon knockout i.e. to non-statisticalfragmentation. The threshold energy for statistical fragmentation should scaleroughly with the number of atoms, N, in the PAH

T statth (N) =

3N−63×24−6

×10 eV (4.1)

Here, the value 10 eV is the internal energy needed for fragmentation of an-thracene, C14H10, on the microsecond timescale. This has been measured di-rectly by Martin et al [41]. In eq. (4.1) we scale this result with the numberof degrees of freedom for an arbitrary PAH molecule containing N atoms. Foranthracene N = 24. The scaling relies on the fact that dissociation energies aresimilar for PAHs of different sizes. This was shown to be the case for a rangeof PAHs by Holm et al [27].

We find that the model and experimental total fragmentation cross sectionsare similar for the PAHs considered here. We see from Figure 4.6 that the non-statistical knockout cross section becomes more important for larger PAHs anddominant for circumcoronene. The reason is that the statistical threshold en-ergy Tstat

th increases as a function of PAH size, while the non-statistical thresh-old energy essentially is size independent - the chemical bonds within the PAHmolecules have similar strengths for different PAH sizes.

From Figure 4.6, we see that the total cross sections are measured to besignificantly smaller than the geometrical cross sections. This means thatthe PAHs are partially transparent to helium atoms with energies in the rangearound 100 eV.

In Figure 4.7, we show fragmentation spectra for 9 keV C+60 colliding with

He, Ne, Ar and Xe yielding center of mass energies of 50 eV, 245 eV, 473 eVand 1388 eV, respectively. For the Ne, Ar and Xe targets we detect C+

59 ionsmost likely stemming from prompt carbon knockout processes. In the caseof the He target it was not possible to separate a possible corresponding con-tribution from features due to other processes. The He atom could be eitherscattered or captured to form endohedral He@C60 [11, 60, 73]. The scatteringprocess gives a broad shoulder on the low-energy side of the primary beampeak. This shoulder extends to energy losses of about 200 eV, which corre-sponds to elastic scattering of He in the forward direction (see Paper III fordetails). Interestingly, the single carbon loss peaks are much smaller for C60(Figure 4.7) than for PAHs (lower panel of Figure 4.5). A reason for this could

28

10 20 30 40 50 60 70 80No. of atoms in the Cm Hn

+ molecule, m + n0

1

2

3

4

5

6

Cros

s Se

ctio

n (1

0−15

cm

2)

Circumcoronene

Cm Hn+ - destruction in He, ECM = 110 eV

Non-stat. fragmentation cross section (calc.)Stat. fragmentation cross section (calc.)Total (stat. + non-stat.) cross section (calc.)Geometrical cross sectionTotal cross section (exp.)

d = 18.5 ◦A

Figure 4.6: Statistical, non-statistical, and total fragmentation cross sectionsas calculated using the ZBL potential for PAH-cations colliding with He at 110eV center of mass energy. Inset: Estimate of the geometrical cross section areaof a coronene molecule σ = 0.8π

d2

4 , where the factor 0.8 is due to the randomorientation of the molecule in the collision [22].

29

be that PAHs are planar while fullerenes have truly three dimensional molecu-lar structures.

Figure 4.7: Fragmentation spectra for 9 keV C+60 (energy in the laboratory frame

of reference) in collisions with He, Ne, Ar and Xe. There are more fragmentsfor the heavier atoms where the center-of-mass energies are higher. The He maybe inserted in the fullerene cage to form endohedral He@C60 or scattered. Bothprocesses contributes to the shoulder to the left of the main peak. Inset: A zoom-in with the expected C+

59 position indicated.

4.2 Collisions with clusters of PAH or fullerene molecules

We show a mass spectrum for 22.5 keV He2+ colliding with loosely boundvan der Waals clusters of C60 fullerenes in Figure 4.8. The total time-of-flightspectrum, where C+

60 dominates is shown in the top panel of Figure 4.8. Thelow-intensity peaks to the right of C+

60 are mainly due to intact [C60]n ions(the intensity scales are different for the right and left panels of Figure 4.8).The peaks close to nC/e = 120, where nC is the number of carbon atoms in thefragment, are shown in an expanded scale in Figure 4.9 (first from left) withpositions corresponding to nC/e=119 and 118 etcetera. In the left top panel ofFigure 4.8 there are a few rather weak peaks due to the emission of one and

30

several C2 molecules from C60 monomer cations that have been left sufficientlyhot to fragment. This could occur after several steps of fragmentation of acharged [C60]n-cluster leaving at the end one or several singly charged C60molecules of which a few may fragment further.

We show single stop events, i.e. events where only one charged fragmentis detected, in the middle panel of Figure 4.8. The peaks in this panel areattributed to the most distant electron transfer collisions where the clustersbecome singly and doubly ionized. Here, only low amounts of energies aretransferred to the individual molecules and the fragmentation is weak.

In the lowest panels in Figure 4.8, we show product ions measured in co-incidence with C+

60 ions. These events are mainly due to closer collisions andmultiple ionizations of [C60]n-clusters in which substantial amounts of energyare transferred. A zoom-in on the peak in the lower right panel of Figure 4.9reveals that there are peaks at nC/e = 118, 119 and 120. The latter, could bedue to weakly bound [C60]+2 dimers from decays of larger clusters but mayalso be due to C+

60 + C60 → C+120 covalent bond formation. As there are sub-

stantial barriers involved in the formation of C120 from two unperturbed C60molecules it could be reasonable to assume that this peak instead could be dueto reactions where one of the C60 cages are damaged. This could be caused byan incomplete knockout process where the C-atom does not actually leave thefullerene. The peaks at nC/e = 119 and nC/e = 118 are most likely producedin at low energy C+

59/C+58 + C60 reactions, in which covalently bound C+

119 andC+

118 dumb-bell systems are formed as discussed in detail in Papers I and II.

4.2.1 Molecular growth

We show a zoom-in of the mass region around nC/e=120 in the spectrum dueto Ar2+ + [C60]n collisions in Figure 4.9. There are more peaks than in He2+

case. We have performed electronic and nuclear stopping calculations for 22.5keV He2+ and 12 keV Ar2+ collisions on both fullerene monomers and clus-ters. Highly reactive products (e.g. C+

59, C+58, etc) are produced by prompt

knockout processes due to nuclear stopping processes for the collisions onmonomers and on clusters. The C59 and C58 produced in knockout collisionswith monomers inevitably also become strongly heated due to electronic stop-ping processes. They would therefore decay very quickly - on subpicosecondtime scales if they were isolated. In the clusters however, the electronic excita-tions of those individual molecules that are directly penetrated in the collisionmay share its excitation energy with other (colder) molecules in the cluster. Inthis way, fragments like C59 and C58 may stay intact long enough in order toreact with other (intact) molecules in the cluster (see inset in Figure 4.9, PaperI and II). The knockout cross sections calculated using the nuclear stopping

31

02000400060008000

1000012000140001600018000

28 48 52 56 60 200 300 400 500 600 700

0

50

100

150

200

250

300

0

50

100

150

200 300 400 500 600 7000

10

20

30

40

50

60

700

2000

4000

6000

8000

28 48 52 56 600

200400600800

10001200140016001800

C+

54

C+

58

Cou

nts

Total spectrum C+

60

3+

172+

152+

112+

92+

C+

118-120

11+10+

9+

8+

72+

132+

5+

7+6+

4+

Cou

nts

He2+ + [C60

]n

3+

72+

172+15

2+

132+11

2+9

2+

11+10+

9+

8+7+

6+

5+

4+

Cou

nts

C+

118-120

Number of carbon atoms per charge (nC/e)

Cou

nts

C+

118-120

x50

x100C

+

54

C+

56

C+

58

C+

60

Cou

nts

Single stop events

C+

56

C+

58

C+

60

Cou

nts

Coincidences

with C+

60

x20

52 54 56 58

50 52 54 56 58

50 52 54 56 58

C+56

Figure 4.8: Mass to charge-spectra due to 22.5 keV He2+ + [C60]n collisions.From top to bottom we show the total mass spectrum (all events including single-and multiple-stop events), single-stop events (only one ion registered per colli-sion), and coincidences with only intact C+

60 ions (one or several). There aredifferent intensity scales for the parts of the spectra shown in the right and leftpanels.

32

model with the screened Bohr potential (see Eq. 3.5 and 3.6 in Section 3.2)are displayed in Figure 4.9. The cross sections are calculated assuming thatthe threshold for knocking out a carbon from a C60 molecule is about 15 eV(this was the value used for the analysis in Paper I. We have later found, how-ever, that much larger energy transfers to the molecular system are needed tobreak all bonds of a C-atom in a fullerene). In the He2+ case, single knockoutsare more likely than knockouts of two different C atoms. The situation is theopposite in the case of Ar2+ where the projectile does not need to come asclose to an individual C atom in the molecule to knock it out as was the casewith He2+. This is most likely the reason for the more extensive distributionof peaks due to molecular fusion processes in the case of Ar2+.

110 115 120No. of carbons per charge, nc /e

0

10

20

30

40

50

60

70

80

Counts

C120

C119

C118

He2+ + [C60]n

110 115 120

C120

C119

C118

C116

C114

Ar2+ + [C60]n

54 55 56 57 58 59Cm -fullerene size, m

0.0

0.5

1.0

1.5

2.0

2.5

σ m,

10−15cm

2

He2+

Ar2+

Figure 4.9: Zoom-ins on mass-to-charge spectra due to 22.5 keV He2+ + [C60]n(left panel) and 12 keV Ar2+ +[C60]n collisions (middle panel). The curves showevents recorded in coincidence with one or several C+

60 ions. The peaks are dueto bond forming reactions in which e.g. dumbbell shaped C119 molecules may beformed (an example is shown in the inset in the left panel). The bond formationprocess is most likely initiated by prompt knockouts of one or several carbonatoms in a C60 molecule. The right panel shows calculated absolute cross sectionsfor producing Cm-fullerenes in direct knock-out processes.

In Figure 4.10 we show a few examples of typical reactions leading to for-mations of new molecular structures when pyrene clusters are hit by 12 keVAr2+ projectiles. In all three examples, the reactions are between one pyrenefragment and one intact molecule according to our molecular dynamics sim-ulations [20]. Within a few tens of femtoseconds, bonds are formed by theinteracting fragment and an intact pyrene molecule. The top row shows the

33

Figure 4.10: Snapshots from molecular dynamics simulations of 12 keV Ar+

[C16H10]9 collisions (three different trajectories). The molecular growth pro-cesses are typically started by prompt atom knockouts or bond cleavages. Thehighly reactive fragments then form covalent bonds with neighbouring moleculesin the clusters on the sub-picosecond timescales.

34

case where no atom has been knocked out, but instead a C-C bond has beenspilt open by the projectile. The dangling C atoms react with a neighboringintact molecule before the system emits a C2H2 molecule. The latter is typi-cally one of the lowest energy decay pathways in PAH molecules, and here itresults in a fused C30H+

18 molecule at mass 378 amu. The second row showsthe inclusion of a C-atom (from a knockout process), reaction with an intactpyrene molecule and formation of a C17H+

10 which undergoes a few isomeriza-tion steps during the 1 ps simulation time. In the last row of Fig. 4.10, C31H+

19is formed when a CH group has been knocked out from one pyrene moleculewhich then forms bond with another molecule. A single bond is formed witha neighboring pyrene where one ring in the merged product closes into a pen-tagon.

35

36

5. Concluding remarks

In this work, fragmentation and molecular growth processes following colli-sions between fullerene/PAH molecules or clusters and atoms or atomic ionsare presented. Experiments were performed with the aid of two complemen-tary experimental setups for studies of collisions at high center of mass col-lision energies (> 10 keV) and for low center of mass collision energies (<1 keV). The experimental results have been discussed in view of a collisionmodel, molecular structure calculations, and molecular dynamics simulations.

We show that high energy (keV energies in the center-of-mass system)are dominated by heating of the molecular electron clouds (electronic stop-ping). The excess energy is then statistically distributed among all degreesof freedom before the molecules or molecular clusters cool down by emitingphotons, electrons, atoms or molecules. The typical energy transfers are wellabove the dissociation energy thresholds, which lead to rich fragment massspectra. Molecular structure calculations show that the dissociation energiesare rather independent of PAH size for the energetically most favourable de-cay pathawys. The dissociation energies for H-loss and H2-loss are about 5 eVand 4 eV, respectively. However, there are reaction barriers for H2 emission,which significantly reduce the decay rate. As a consequence, H2-emission isonly important for high internal PAH temperatures (> 2200 K). Such temper-atures are typically reached in the present collisions with keV ions/atoms butnot upon e.g. single UV-photon absorption.

Non-statistical fragmentation refer to the process, in which a single car-bon atom is knocked out directly from the molecule in Rutherford like atom-atom scattering processes. The latter has been observed in experimental stud-ies of collisions between PAH/fullerene molecules and atoms at low center ofmass collision energies (< 1 keV). In general this yields more reactive frag-ments than statistical fragmentation processes. For non-statistical fragmenta-tion, simple scaling laws for estimates of absolute fragmentation cross sectionshave been deduced.

For collision between keV ions and molecular clusters, prompt knockoutprocesses play a key role in the formation of new molecules. The highly reac-tive fragments may then form covalent bonds with neighbouring molecules inthe clusters. The experiments were carried out for collisions between variousatomic ions and clusters of fullerenes or pyrene molecules (C16H10). In the

37

former case, exotic dumbbell shaped molecules (e.g. C119) are formed, whilebroad size distributions with masses corresponding to C16+mHx with m=1-21may be formed with the PAH cluster target.

Isolated PAHs or fullerenes and their clusters are believed to be importantin various astrophysical environments [68, 69]. However, little is known abouthow they e.g. are formed, destroyed or how they react when they are intact orwhen they have been fragmented. It is also not known if molecular hydrogen,H2, may form directly from internally hot native PAHs. One study presented inthis thesis suggests that this may indeed be the case. More detailed informationabout these intriguing aspects are of great interest for the future. This maye.g. be obtained using the experimental setups at the novel DESIREE (DoubleElectroStatic Ion Ring Experiment) facility at SU. Possible future experimentsinvolve studies of molecular growth processes inside size selected clusters atthe standalone beam line for low energy (<100 eV) collisions (see Section4.2), and studies of charge and energy transfer processes between anions andcations of PAHs/fullerenes in DESIREE [61, 67]. In the latter case, differentranges of initial internal ion temperatures may be selected by storing the ionsfor different times before the collisions. By storing for long times the ionswill equilibrate thermally with the ring and its vacuum vessel which may beoperated down to about 12 K. From a theoretcial point of view, calculations ofthe H2 rates from internally heated PAHs would help to gauge the significanceof such processes in astrophysical environments.

38

Appendix: Additional publications

Peer-reviewed articles

1. Ions interacting with planar aromatic molecules: Modeling electrontransfer reactionsB. O. Forsberg, J. D. Alexander, T. Chen, A. T. Pettersson, M. Gatchell,H. Cederquist and H. Zettergren JOURNAL OF CHEMICAL PHYSICS,138, 054306 (2013).DOI: 10.1063/1.4790164

2. Ions colliding with polycyclic aromatic hydrocarbon clustersM. Gatchell, H. Zettergren, F. Seitz, T. Chen, J. D. Alexander, M. H.Stockett, H. T. Schmidt, A. Ławicki, J. Rangama, P. Rousseau, M. Capron,S. Maclot, R. Maisonny, A. Domaracka, L. Adoui, A. Méry, J. Y. Ches-nel, B. Manil, B. A. Huber, and H. Cederquist PHYSICA SCRIPTA,T156, 014062, (2013).DOI: 10.1088/0031-8949/2013/T156/014062

3. First storage of ion beams in the Double Electrostatic Ion-Ring Ex-periment: DESIREEH. T. Schmidt, R. D. Thomas, M. Gatchell, S. Rosén, P. Reinhed, P.Löfgren, L. Brännholm, M. Blom, M. Björkhage, E. BäckstrÃum, J.D. Alexander, S. Leontein, D. Hanstorp, H. Zettergren, L. Liljeby, A.Källberg, A. Simonsson, F. Hellberg, S. Mannervik, M. Larsson, W. D.Geppert, K. G. Rensfelt, H. Danared, A. Paál, M. Masuda, P. Halldén, G.Andler, M. H. Stockett, T. Chen, G. Källersjö, J. Weimer, K. Hansen, H.Hartman and H. Cederquist REVIEW OF SCIENTIFIC INSTRUMENTS,84, 055115 (2013).DOI: 10.1063/1.4807702

4. Formation dynamics of fullerene dimers C+118, C+

119, and C+120

Y. Wang, H. Zettergren, P. Rousseau, T. Chen, M. Gatchell, M. H.Stockett, A. Domaracka, L. Adoui, B. A. Huber, H. Cederquist, M. Al-camí and F. Martín PHYSICAL REVIEW A, 89, 062708, (2014).DOI: 10.1103/PhysRevA.89.062708

5. Ions colliding with mixed clusters of C60 and coronene: Fragmenta-tion and bond formationM. Gatchell, P. Rousseau, A. Domaracka, M. H. Stockett, T. Chen, H.T. Schmidt, J. Y. Chesnel, A. Méry, S. Maclot, L. Adoui, B. A. Huber,H. Zettergren and H. Cederquist PHYSICAL REVIEW A, 90, 022713,(2014).DOI: 10.1103/PhysRevA.90.022713

6. Fragmentation of anthracene C14H10, acridine C13H9N and phenazineC12H8N2 ions in collisions with atomsM. H. Stockett, M. Gatchell, J. D. Alexander, U. Berzinš, T. Chen, K.Farid, A. Johansson, K. Kulyk, P. Rousseau, K. Støchkel, L. Adoui, P.Hvelplund, B. A. Huber, H. T. Schmidt, H. Zettergren and H. Cederquist.PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 16, 21980, (2014).DOI: 10.1039/C4CP03293D

Peer-reviewed conference contributions

1. Bond formation in C+59-C60 collisions

H. Zettergren, P. Rousseau, Y. Wang, F. Seitz, T. Chen, M. Gatchell, J.D. Alexander, M. H. Stockett, J. Rangama, J. Y. Chesnel, M. Capron, J.C. Poully, A. Domaracka, A. Méry, S. Maclot, H. T. Schmidt, L. Adoui,M. Alcamí, A. G. G. M. Tielens, F. Martín, B. A. Huber, and H. Ced-erquist JOURNAL OF PHYSICS CONFERENCE SERIES, 488, 012028(2014).DOI: 10.1088/1742-6596/488/1/012028

2. Modeling electron and energy transfer processes in collisions be-tween ions and Polycyclic Aromatic Hydrocarbon moleculesT. Chen, J. D. Alexander, B. O. Forsberg, A. T. Pettersson, M. Gatchell,H. Cederquist and H. Zettergren JOURNAL OF PHYSICS CONFER-ENCE SERIES, 488, 102015 (2014).DOI: 10.1088/1742-6596/488/10/102015

3. Commissioning of the DESIREE storage rings - a new facility forcold ion-ion collisionsM. Gatchell, H. T. Schmidt, R. D. Thomas, S. Rosén, P. Reinhed, P.Löfgren, L. Brännholm, M. Blom, M. Björkhage, E. BäckstrÃum, J.D. Alexander, S. Leontein, D. Hanstorp, H. Zettergren, L. Liljeby, A.Källberg, A. Simonsson, F. Hellberg, S. Mannervik, M. Larsson, W. D.Geppert, K. G. Rensfelt, H. Danared, A. Paál, M. Masuda, P. Halldén, G.

Andler, M. H. Stockett, T. Chen, G. Källersjö, J. Weimer, K. Hansen,H. Hartman and H. Cederquist EPJ WEB OF CONFERENCES, 488,012040, (2014).DOI: 10.1088/1742-6596/488/1/012040

4. First results from the Double ElectroStatic Ion-Ring ExpEriment,DESIREEM. Gatchell, H. T. Schmidt, J. D. Alexander, G. Andler, M. Björkhage,M. Blom, L. Brännholm, E. BäckstrÃum, T. Chen, W. D. Geppert, P.Halldén, D. Hanstorp, F. Hellberg, A. Källberg, M. Larsson, S. Leontein,L. Liljeby, P. Löfgren, S. Mannervik, A. Paál, P. Reinhed, K. G. Rens-felt, S. Rosén, F. Seitz, A. Simonsson, M. H. Stockett, R. D. Thomas,H. Zettergren, and H. Cederquist EPJ WEB OF CONFERENCES, 488,092003, (2014).DOI: 10.1088/1742-6596/488/9/092003

5. DESIREE: Physics with cold stored ion beamsR. D. Thomas, H. T. Schmidt, M. Gatchell, S. Rosén, P. Reinhed, P.Löfgren, L. Brännholm, M. Blom, M. Björkhage, E. BäckstrÃum, J.D. Alexander, S. Leontein, D. Hanstorp, H. Zettergren, L. Liljeby, A.Källberg, A. Simonsson, F. Hellberg, S. Mannervik, M. Larsson, W. D.Geppert, K. G. Rensfelt, H. Danared, A. Paál, M. Masuda, P. Halldén, G.Andler, M. H. Stockett, T. Chen, G. Källersjö, J. Weimer, K. Hansen, H.Hartman and H. Cederquist EPJ WEB OF CONFERENCES, 84, 01004,(2015).DOI: 10.1051/epjconf/20158401004

Acknowledgements

This work was accomplished with the help of many people, without them myjourney would have been a lot more difficult. I would therefore like to thankthe following people:

Henrik Cederquist, my main supervisor and the leader of the group. Hehelped create an excellent environment for conducting my research. His guid-ance has helped develop the DESIREE and ISLab projects, and introduce meto the wider scientific community via external collaborations as well as en-abling me to participate in conferences, etc. All of these led to fruitful resultsand helped me grow in many different ways.

Henning Zettergren, my co-advisor and life teacher. I have learned fromhim not only how to do research, search literature, give talks, publish articles,but also how to behave like a father, husband and how to defend myself againstpollen.

Henning Schmidt, as my co-advisor and principle of study, he gave methe opportunity to teach, which not only improved my teaching skills but alsohelped my family improve the financial problems in the beginning when I hadutbildningsbidrag for three people in the family.

Wolf Geppert, as the director of the AstroBiology Centre (ABC), he orga-nized many memorable events, which helped me to know all the aspects aboutastrobiology and to expand my horizons. I feel very happy and pround to bepart of the ABC.

John Alexander, my previous roommate, I had the pleasure of collaboratingwith him on several projects. He is not only an encyclopedia but also myenglish and swedish teacher. He is a nice person in all dimensions, and I liketo chat with him about everything.

Fabian Seitz, another previous roommate with whom I had a lot of inter-esting conversations. He knows a lot of things about Sweden, sometimes I feelthat he knows more than most Swedes. Thanks to Fabian I took part in my firsttriathlon (10 km run) and tasted my first feuerzangenbowle.

Michael Gatchell and Mark Stockett for providing the experimental dataand explaining the experimental setup. As english native speakers, their helpin correcting my papers, thesis, etc is very much appreciated. Special thanksto Mark, it is always fun to talk with him, he brings a lot of positive energyto the group. I like his presentation style and I consider him one of the best

speaker I ever met.Tor Kjellsson and Viktoria Larsson, my trainer and the organizer of many

activities. Thanks to them I now have good health. I took part in lots of mem-orable events with them: the 1.6×4 relay, AlbaNova bench press competition,climbing classes, 12 km runs around lappis and the lake near AlbaNova, etc.

Michael Wolf, Emma Anderson, and Sifiso Nkambule - my current room-mates. I appreciate the relaxed and creative atmosphere they make; it is themost joyful time chatting with them everyday.

Muhammad Umair, thanks for providing lots of useful information, andthe supertasty pakistani-flavor pies and cakes.

There are many others who have contributed to various aspects of my PhD,e. g. Elisabet Oppenheimer, Petra Nodler, Wolf Geppert, Albert Siegbahn, Per-Erik Tegnér, Sven Mannervik ... but due to limited space, I have to draw theline here.

Deepest thanks to my mother and mother-in-law, they dedicate themselfto help my family. It was a huge challenge for them to travel 7000 km to acompletely different culture at their age. I am so proud to have parents likethem.

Finally, I would like to thank my significant other Xiaoyu Li who has beentaking care of the other part of my life day by day, year by year. Her willing-ness to come to Sweden to help me further my education is very much appreci-ated. Her constant supports and encouragements has helped me through bothmy PhD and adapting to Sweden. I can not imagine what my life would bewithout her. Since coming to Sweden, she has also given me the greatest giftof my son Ethan. His presence always lightens my day...

Thanks again to all the people who helped me to complete this work, andwho made my life easier and happier.

Tao ChenMarch, 2015 Stockholm

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