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Carbon backbone stability of Polycyclic Aromatic Hydrocarbons
Stockholm University, Faculty of Science, Department of Physics.
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis I present results from Collision-Induced Dissociation (CID) experiments of Polycyclic Aromatic Hydrocarbons (PAHs) colliding with a stationary target gas at center-of-mass collision energies in the 20–200 eV range. In this energy region nuclear stopping processes dominate, i.e. energy transfer due to nuclear scattering processes in the molecule are much more important than interactions with the electrons (electronic stopping). If the energy deposited in the molecule by the collision is redistributed among all degrees of freedom before the decay, dissociation often happens statistically through the lowest dissociation energy channels. However, in the collisions that we study, billiard-like, prompt knockout of a single carbon atom from the PAH can also be observed as a form of non-statistical fragmentation.

Here I present measurements of the center-of-mass collision energy dependence for single carbon knockout. I further report results on two key properties. The first is the target dependent threshold energy—the minimum center-of-mass collision energy required for knocking out a single carbon atom. The second is the target independent displacement energy—the kinetic energy a single carbon atom must receive to be permanently removed from the PAH. I further present CID experiments on hydrogenated pyrene and compare them to molecular dynamics simulations for atomic knockout. I specifically show that statistical fragmentation is the dominant contribution to the single carbon loss channel for hydrogenated species of pyrene due to their lower dissociation energies.

Place, publisher, year, edition, pages
Stockholm University, 2016.
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-130533OAI: oai:DiVA.org:su-130533DiVA: diva2:930776
Opponent
Supervisors
Available from: 2016-05-25 Created: 2016-05-25 Last updated: 2016-05-25Bibliographically approved
List of papers
1. Threshold Energies for Single-Carbon Knockout from Polycyclic Aromatic Hydrocarbons
Open this publication in new window or tab >>Threshold Energies for Single-Carbon Knockout from Polycyclic Aromatic Hydrocarbons
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2015 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 6, no 22, 4504-4509 p.Article in journal (Refereed) Published
Abstract [en]

We have measured absolute cross sections for ultrafast (femtosecond) single-carbon knockout from polycyclic aromatic hydrocarbon (PAR) cations as functions of He-PAR center-of-mass collision energy in the 10-200 eV range. Classical molecular dynamics (MD) simulations cover this range and extend up to 105 eV. The shapes of the knockout cross sections are well-described by a simple analytical expression yielding experimental and MD threshold energies of E-th(Exp) = 32.5 +/- 0.4 eV and E-th(MD) = 41.0 +/- 0.3 eV, respectively. These are the first measurements of knockout threshold energies for molecules isolated in vacuo. We further deduce semiempirical (SE) and MD displacement energies, i.e., the energy transfers to the PAH molecules at the threshold energies for knockout, of T-disp(SE) = 23.3 +/- 0.3 eV and T-disp(MD) = 27.0 +/- 0.3 eV. The semiempirical results compare favorably with measured displacement energies for graphene (T-disp = 23.6 eV).

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-124747 (URN)10.1021/acs.jpclett.5b02080 (DOI)000365460700010 ()
Available from: 2016-01-12 Created: 2016-01-04 Last updated: 2016-05-25Bibliographically approved
2. Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation
Open this publication in new window or tab >>Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation
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2015 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 92, no 5, 050702Article in journal (Refereed) Published
Abstract [en]

A recent study of soft x-ray absorption in native and hydrogenated coronene cations, C24H12+m + m = 0-7, led to the conclusion that additional hydrogen atoms protect (interstellar) polycyclic aromatic hydrocarbon (PAH) molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002 (2014)]. The present experiment with collisions between fast (30-200 eV) He atoms and pyrene (C16H10+m +, m = 0, 6, and 16) and simulations without reference to the excitation method suggests the opposite. We find that the absolute carbon-backbone fragmentation cross section does not decrease but increases with the degree of hydrogenation for pyrene molecules.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-124759 (URN)10.1103/PhysRevA.92.050702 (DOI)000364807900001 ()
Available from: 2016-01-12 Created: 2016-01-04 Last updated: 2016-05-25Bibliographically approved
3. Hydrogenated pyrene: Statistical single-carbon loss below the knockout threshold
Open this publication in new window or tab >>Hydrogenated pyrene: Statistical single-carbon loss below the knockout threshold
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2016 (English)In: European Physical Journal D: Atomic, Molecular and Optical Physics, ISSN 1434-6060, E-ISSN 1434-6079, Vol. 70, no 4, 85Article in journal (Refereed) Published
Abstract [en]

An ongoing discussion revolves around the question of what effect hydrogenation has oncarbon backbone fragmentation in polycyclic aromatic hydrocarbons (PAHs). In order to shedmore light on this issue, we have measured absolute single carbon loss cross sections incollisions between native or hydrogenated pyrene cations (C16H+ 10+m , m = 0, 6, 16) and He as functions of center-of-massenergies down to 20 eV. Classical molecular dynamics (MD) simulations give further insightinto energy transfer processes and also yield m-dependent threshold energies for prompt(femtoseconds) carbon knockout. Such fast, non-statistical fragmentation processesdominate CH x -loss for native pyrene (m = 0), while much slowerstatistical fragmentation processes contribute significantly to single-carbon loss for thehydrogenated molecules (m =6 and m =16). The latter is shown by measurements of large CH x -loss crosssections far below the MD knockout thresholds for C16H+ 16 and C16H+ 26.

National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-129527 (URN)10.1140/epjd/e2016-60735-3 (DOI)000375296200002 ()
Available from: 2016-04-26 Created: 2016-04-25 Last updated: 2016-06-07Bibliographically approved

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