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Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physics. Aarhus University, Denmark.
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physics.
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Number of Authors: 16
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.

Place, publisher, year, edition, pages
2015. Vol. 92, no 5, 050702
National Category
Physical Sciences
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-124759DOI: 10.1103/PhysRevA.92.050702ISI: 000364807900001OAI: oai:DiVA.org:su-124759DiVA: diva2:893266
Available from: 2016-01-12 Created: 2016-01-04 Last updated: 2016-05-25Bibliographically approved
In thesis
1. Molecular Hole Punching: Impulse Driven Reactions in Molecules and Molecular Clusters
Open this publication in new window or tab >>Molecular Hole Punching: Impulse Driven Reactions in Molecules and Molecular Clusters
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

When molecules are excited by photons or energetic particles, they will cool through the emission of photons, electrons, or by fragmenting. Such processes are often thermal as they occur after the excitation energy has been redistributed across all degrees-of-freedom in the system. Collisions with atoms or ions may also lead to ultrafast fragmentation in Rutherford-like scattering processes, where one or several atoms can literally be knocked out of the molecule by the incoming projectile before the energy can be completely redistributed. The resulting fragmentation pathways can in such knockout processes be very different from those in thermal processes.

This thesis covers extensive studies of collisions between ions/atoms and isolated Polycyclic Aromatic Hydrocarbon (PAH) molecules, isolated fullerene molecules, or clusters of these. The high stabilities and distinct fragmentation channels make these types of molecules excellent test cases for characterizing knockout-driven fragmentation and the reactions that these processes can lead to. I will present experimental measurements for a wide range of energies and compare them with my own molecular dynamics simulations and quantum chemical calculations. In this thesis, I present an in-depth study of the role of knockout in the energetic processing of molecules and clusters. The competition between knockout and thermally driven fragmentation is discussed in detail.

Knockout-driven fragmentation is shown to result in exotic fragments that are far more reactive than the intact parent molecules or fragments from thermal processes. When such reactive species are formed within molecular clusters efficient molecular growth can take place on sub-picosecond timescales. The cluster environments are crucial here because they protect the newly formed molecules by absorbing excess energy. This is a possible pathway for the growth of large PAHs, fullerenes, and similar carbonaceous complexes found in, for instance, the interstellar medium.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2016. 74 p.
Keyword
PAHs, Fullernes, Reactions, Clusters, Interstellar Medium, Fragmentation, Non-Statistical Fragmentation, Collisions, Experiments, Molecular Dynamics, Density Functional Theory
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-129523 (URN)978-91-7649-436-3 (ISBN)
Public defence
2016-06-10, FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 1: Submitted.

Available from: 2016-05-18 Created: 2016-04-25 Last updated: 2016-05-19Bibliographically approved
2. Carbon backbone stability of Polycyclic Aromatic Hydrocarbons
Open this publication in new window or tab >>Carbon backbone stability of Polycyclic Aromatic Hydrocarbons
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:nbn:se:su:diva-130533 (URN)
Opponent
Supervisors
Available from: 2016-05-25 Created: 2016-05-25 Last updated: 2016-05-25Bibliographically approved

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Gatchell, MichaelStockett, Mark H.de Ruette, NathalieChen, TaoGiacomozzi, LindaNascimento, Rodrigo F.Wolf, MichaelAnderson, Emma K.Schmidt, Henning T.Zettergren, HenningCederquist, Henrik
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