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Ionization and fragmentation of polycyclic aromatic hydrocarbon clusters in collisions with keV ions
Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physics.
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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.

Place, publisher, year, edition, pages
2011. Vol. 84, no 4, 043201- p.
National Category
Physical Sciences
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-63732DOI: 10.1103/PhysRevA.84.043201ISI: 000295712700013OAI: oai:DiVA.org:su-63732DiVA: diva2:452031
Available from: 2011-10-27 Created: 2011-10-27 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Ionization and Fragmentation of Complex Molecules and Clusters: Biomolecules and Polycyclic Aromatic Hydrocarbons
Open this publication in new window or tab >>Ionization and Fragmentation of Complex Molecules and Clusters: Biomolecules and Polycyclic Aromatic Hydrocarbons
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This work deals with ionization and fragmentation of biomolecules and polycyclic aromatic hydrocarbon (PAH) molecules. They are studied in the gas phase both as isolated molecules and as weakly bound clusters. The purpose of the experimental and theoretical investigations are to elucidate charge and energy transfer and related redistribution processes, as well as fragmentation behaviors.

The first part of this thesis presents results from studies on biomolecular ions, in particular nucleotides and peptides, which are primarily examined in electron capture induced dissociation processes. These investigations are relevant for the better understanding of radiation damage to DNA and processes involved in the sequencing of proteins. It is found that the immediate environment have a decisive influence on the fragmentation behaviors. Evaporation of surrounding molecules protect the biomolecules, but their effect on the electronic structure may also enhance or suppress different fragmentation channels.

In the second part of the thesis, results from studies on PAH molecules are presented. Experimentally, their properties are mainly probed through collisions with atomic ion projectiles having kilo-electronvolt kinetic energies. As a widespread pollutant on Earth, and as a family of abundant molecules in space, PAHs are not only relevant from an environmental and health perspective, but they are also important for the understanding of the universe. The present results relate to the stabilities of these molecules, both in isolated form and in clusters, when heated or multiply ionized. It is found to be easier to remove several electrons from clusters of PAH molecules than from isolated PAHs, and fission processes determine their ultimate stabilities. Heated low-charge state clusters of PAHs undergo long evaporation sequences once these have started. For isolated and heated PAHs, internal structural rearrangements are demonstrated to be important in the fragmentation processes.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2011. 168 p.
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-63733 (URN)978-91-7447-399-5 (ISBN)
Public defence
2011-12-02, lecture room FB53, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2011-11-10 Created: 2011-10-27 Last updated: 2011-11-01Bibliographically approved
2. Ions colliding with Polycyclic Aromatic Hydrocarbons and Fullerenes
Open this publication in new window or tab >>Ions colliding with Polycyclic Aromatic Hydrocarbons and Fullerenes
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:nbn:se:su:diva-88427 (URN)978-91-7447-649-1 (ISBN)
Public defence
2013-04-30, lecture hall FD5, AlbaNova universitetscentrum, Roslagstullbacken 21, Stockholm, 10:15 (English)
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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

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Johansson, Henrik A. B.Zettergren, HenningHolm, Anne I. S.Seitz, FabianSchmidt, Henning T.Cederquist, Henrik
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