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Ions colliding with Polycyclic Aromatic Hydrocarbons and Fullerenes
Stockholm University, Faculty of Science, Department of Physics. (Atomfysik)
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis a series of experiments on collisions between atomic projectile ions at keV energies and target vapors of either isolated molecules or van der Waals clusters is presented and analyzed. The atomic ions are produced in an Electron Cyclotron Resonance (ECR) ion source, accelerated and guided into the target volume. The charged target collision products are mass-to-charge analyzed in a time-of-flight spectrometer. The Polycyclic Aromatic Hyrdrocarbons (PAHs) Anthracene (C14H10), Coronene (C24H12), two C16H10 isomers, Pyrene and Fluoranthene, and the fullerene C60 are examined.

For projectile ions in low charge states, small impact parameter collisions dominate, which leads to internal heating of the target. With isolated molecules as targets, this typically results in ionization and often also in fragmentation. For cluster targets energy and charge are rapidly distributed among the cluster building blocks. This is followed by cluster evaporation and very limited fragmentation of the individual molecules. C119+ and C118+ are observed as products. These are due to the formation of the reactive C58/59+ ions by direct knockout processes, which react with another C60 of the cluster to form dumb-bell shaped molecules.

For projectile ions of high charge (Xe20+) larger impact parameters dominate, leading to little internal heating. For isolated molecule targets, intact molecular ions are the main collision products. Charged fragments stem mostly from multifragmentation following ionization to high charge states. For cluster targets, the collision products consist mainly of singly charged monomers. Fragmentation of the individual molecules is comparatively strong. This suggests a quick distribution of charges followed by a Coulomb explosion, which leads to internal heating.

The results show that weakly bound clusters do not sustain the impact of keV-ions and that it is possible to form new molecular structures.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2013. , 118 p.
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-88427ISBN: 978-91-7447-649-1 (print)OAI: oai:DiVA.org:su-88427DiVA: diva2:611177
Public defence
2013-04-30, lecture hall FD5, AlbaNova universitetscentrum, Roslagstullbacken 21, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

At the time of doctoral defense, the following papers were unpublished and had a status as follows: Paper 5: Submitted. Paper 6: Submitted.

Available from: 2013-04-08 Created: 2013-03-14 Last updated: 2014-03-28Bibliographically approved
List of papers
1. Ions Colliding with Cold Polycyclic Aromatic Hydrocarbon Clusters
Open this publication in new window or tab >>Ions Colliding with Cold Polycyclic Aromatic Hydrocarbon Clusters
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2010 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, no 21, 213401- p.Article in journal (Refereed) Published
Abstract [en]

We report the first experimental study of ions interacting with clusters of polycyclic aromatic hydrocarbon (PAH) molecules. Collisions between 11.25 keV He-3(+) or 360 keV Xe-129(20+) and weakly bound clusters of one of the smallest PAH molecules, anthracene, show that C14H10 clusters have much higher tendencies to fragment in ion collisions than other weakly bound clusters. The ionization is dominated by peripheral collisions in which the clusters, very surprisingly, are more strongly heated by Xe20+ collisions than by He+ collisions. The appearance size is k = 15 for [C14H10](k)(2+).

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-51342 (URN)10.1103/PhysRevLett.105.213401 (DOI)000284407400012 ()
Note
authorCount :16Available from: 2011-01-11 Created: 2011-01-10 Last updated: 2017-12-11Bibliographically approved
2. Multiple ionization and fragmentation of isolated pyrene and coronene molecules in collision with ions
Open this publication in new window or tab >>Multiple ionization and fragmentation of isolated pyrene and coronene molecules in collision with ions
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2011 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 83, no 2, 022704- p.Article in journal (Refereed) Published
Abstract [en]

The interaction of multiply charged ions (He2+, O3+, and Xe20+) with gas-phase pericondensed polycyclic aromatic hydrocarbon (PAH) molecules of coronene (C24H12) and pyrene (C16H10) is studied for low-velocity collisions (v <= 0.6 a.u.). The mass spectrometric analysis shows that singly and up to quadruply charged intact molecules are important reaction products. The relative experimental yields are compared with the results of a simple classical over-the-barrier model. For higher molecular charge states, the experimental yields decrease much more strongly than the model predictions due to the instabilities of the multiply charged PAH molecules. Even-odd oscillations with the number of carbon atoms, n, in the intensity distributions of the CnHx+ fragments indicate a linear chain structure of the fragments similar to those observed for ion-C60 collisions. The latter oscillations are known to be due to dissociation energy differences between even-and odd-n Cn-chain molecules. For PAH molecules, the average numbers of H atoms attached to the CnHx chains are larger for even-n reflecting acetylenic bond systems.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-63730 (URN)10.1103/PhysRevA.83.022704 (DOI)000287076800006 ()
Available from: 2011-10-27 Created: 2011-10-27 Last updated: 2017-12-08Bibliographically approved
3. Ionization and fragmentation of polycyclic aromatic hydrocarbon clusters in collisions with keV ions
Open this publication in new window or tab >>Ionization and fragmentation of polycyclic aromatic hydrocarbon clusters in collisions with keV ions
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2011 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 84, no 4, 043201- p.Article in journal (Refereed) Published
Abstract [en]

We report on an experimental study of the ionization and fragmentation of clusters of k polycyclic aromatic hydrocarbon (PAH) molecules using anthracene, C14H10, or coronene, C24H12. These PAH clusters are moderately charged and strongly heated in small impact parameter collisions with 22.5-keV He2+ ions, after which they mostly decay in long monomer evaporation sequences with singly charged and comparatively cold monomers as dominating end products. We describe a simple cluster evaporation model and estimate the number of PAH molecules in the clusters that have to be hit by He2+ projectiles for such complete cluster evaporations to occur. Highly charged and initially cold clusters are efficiently formed in collisions with 360-keV Xe20+ ions, leading to cluster Coulomb explosions and several hot charged fragments, which again predominantly yield singly charged, but much hotter, monomer ions than the He2+ collisions. We present a simple formula, based on density-functional-theory calculations, for the ionization energy sequences as functions of coronene cluster size, rationalized in terms of the classic electrostatic expression for the ionization of a charged conducting object. Our analysis indicates that multiple electron removal by highly charged ions from a cluster of PAH molecules rapidly may become more important than single ionization as the cluster size k increases and that this is the main reason for the unexpectedly strong heating in these types of collisions.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-63732 (URN)10.1103/PhysRevA.84.043201 (DOI)000295712700013 ()
Available from: 2011-10-27 Created: 2011-10-27 Last updated: 2017-12-08Bibliographically approved
4. Polycyclic aromatic hydrocarbon-isomer fragmentation pathways: Case study for pyrene and fluoranthene molecules and clusters
Open this publication in new window or tab >>Polycyclic aromatic hydrocarbon-isomer fragmentation pathways: Case study for pyrene and fluoranthene molecules and clusters
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2011 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 135, no 6, 064302- p.Article in journal (Refereed) Published
Abstract [en]

We report on measurements of the ionization and fragmentation of polycyclic aromatic hydrocarbon (PAH) targets in Xe(20+) + C(16)H(10) and Xe(20+) + [C(16)H(10)](k) collisions and compare results for the two C(16)H(10) isomers: pyrene and fluoranthene. For both types of targets, i.e., for single PAH molecules isolated in vacuum or for isomerically pure clusters of one of the molecules, the resulting fragment spectra are surprisingly similar. However, we do observe weak but significant isomer effects. Although these are manifested in very different ways for the monomer and cluster targets, they both have at their roots small differences (<2.5 eV) between the total binding energies of neutral, and singly and multiply charged pyrene and fluoranthene monomers. The results will be discussed in view of the density functional theory calculations of ionization and dissociation energies for fluoranthene and pyrene. A simple classical over-the-barrier model is used to estimate cross sections for single-and multiple-electron transfer between PAHs and ions. Calculated single and multiple ionization energies, and the corresponding model PAH ionization cross sections, are given.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-68323 (URN)10.1063/1.3622589 (DOI)000293955000012 ()
Note

authorCount :17

Available from: 2012-01-13 Created: 2012-01-03 Last updated: 2017-12-08Bibliographically approved
5. Formation of dumb-bell C118 and C119 inside clusters of C60 -molecules
Open this publication in new window or tab >>Formation of dumb-bell C118 and C119 inside clusters of C60 -molecules
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(English)Article in journal (Refereed) Submitted
Abstract [en]

We report highly selective covalent bond-modifications in collisions between keV alpha particles and van der Waals clusters of C60-fullerenes. Surprisingly, C119+ and C118+ are the dominant molecular fusion products. We use Molecular Dynamics simulations to show that C59+ and C58+ ions - effectively produced in prompt knock-out processes with He2+ - react rapidly with C60 to form dumb-bell C119+ and C118+ . Ion impact on molecular clusters in general is expected to lead to efficient secondary reactions of interest for astrophysics. These reactions are different from those induced by photons.

National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-88422 (URN)
Available from: 2013-03-14 Created: 2013-03-14 Last updated: 2013-03-21Bibliographically approved
6. Ions colliding with clusters of fullerenes-Decay pathways and covalent bond formations
Open this publication in new window or tab >>Ions colliding with clusters of fullerenes-Decay pathways and covalent bond formations
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2013 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 139, no 3, 034309Article in journal (Refereed) Published
Abstract [en]

We report experimental results for the ionization and fragmentation of weakly bound van der Waals clusters of n C-60 molecules following collisions with Ar2+, He2+, and Xe20+ at laboratory kinetic energies of 13 keV, 22.5 keV, and 300 keV, respectively. Intact singly charged C-60 monomers are the dominant reaction products in all three cases and this is accounted for by means of Monte Carlo calculations of energy transfer processes and a simple Arrhenius-type [C-60](n)(+) -> C-60(+) + (n - 1)C-60 evaporation model. Excitation energies in the range of only similar to 0.7 eV per C-60 molecule in a [C-60](13)(+) cluster are sufficient for complete evaporation and such low energies correspond to ion trajectories far outside the clusters. Still we observe singly and even doubly charged intact cluster ions which stem from even more distant collisions. For penetrating collisions the clusters become multiply charged and some of the individual molecules may be promptly fragmented in direct knock-out processes leading to efficient formations of new covalent systems. For Ar2+ and He2+ collisions, we observe very efficient C-119(+) and C-118(+) formation and molecular dynamics simulations suggest that they are covalent dumb-bell systems due to bonding between C-59(+) or C-58(+) and C-60 during cluster fragmentation. In the Ar2+ case, it is possible to form even smaller C-120-2m(+) molecules (m = 2-7), while no molecular fusion reactions are observed for the present Xe20+ collisions.

National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-92922 (URN)10.1063/1.4812790 (DOI)000322203000028 ()
Funder
Swedish Research Council, 621-2008-3773Swedish Research Council, 621-2009-3468Swedish Research Council, 621-2011-4047EU, European Research Council, 246976
Note

AuthorCount:23;

Available from: 2013-08-30 Created: 2013-08-26 Last updated: 2017-12-06Bibliographically approved

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