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Ions colliding with molecules and molecular clusters: fragmentation and growth processes
Stockholm University, Faculty of Science, Department of Physics. (Atomic physics)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this work we will discuss fragmentation and molecular growth processes in collisions of Polycyclic Aromatic Hydrocarbon (PAH) molecules, fullerenes, or their clusters with atoms or atomic ions. Simple collision models as well as molecular structure calculations are used to aid the interpretations of the present and other experimental results. Fragmentation features at center-of-mass collision energies around 10 keV are dominated by interactions between the fast ion/atom and the electron cloud in the molecules/clusters (electronic stopping processes). This electronic excitation energy is rapidly distributed on the vibrational degrees of freedom of the molecule or of the molecules in a cluster and may result in fragmentation. Here, the fragmentation is statistical and favors the lowest-energy dissociation channels which are losses of intact molecules from clusters, H- and C2H2-losses from isolated PAHs, and C2-loss from fullerene monomers. We will also discuss the possibility of formation of molecular H2 direct from native PAHs which reach high enough energies when interacting 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 highly reactive fragments which may form covalent bonds with other molecules in a cluster on very short time scales (picoseconds). This process may be important when considering the formation of new species. For collision between 12 keV Ar2+ and clusters of pyrene (C16H10) molecules, new molecules, e.g. C17H10+, C30H18+, C31H19+, etc are detected. We also observe molecular fusion processes for He and Ar ions colliding with clusters of C60 molecules. These and related molecular fusion processes may play a key role for understanding molecular growth processes under certain astrophysical conditions.

Place, publisher, year, edition, pages
Stockholm University, 2015. , 38 p.
Keyword [en]
PAH, H2, C60, fragmentation
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
URN: urn:nbn:se:su:diva-117114ISBN: 978-91-7649-063-1 (print)OAI: oai:DiVA.org:su-117114DiVA: diva2:810583
Public defence
2015-06-11, FD5, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2015-05-20 Created: 2015-05-07 Last updated: 2015-05-22Bibliographically approved
List of papers
1. Formations of Dumbbell C-118 and C-119 inside Clusters of C-60 Molecules by Collision with alpha Particles
Open this publication in new window or tab >>Formations of Dumbbell C-118 and C-119 inside Clusters of C-60 Molecules by Collision with alpha Particles
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2013 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, no 18, 185501Article in journal (Refereed) Published
Abstract [en]

We report highly selective covalent bond modifications in collisions between keV alpha particles and van der Waals clusters of C-60 fullerenes. Surprisingly, C-119(+) and C-118(+) are the dominant molecular fusion products. We use molecular dynamics simulations to show that C-59(+) and C-58(+) ions-effectively produced in prompt knockout processes with He2+-react rapidly with C-60 to form dumbbell C-119(+) and C-118(+). 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
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-91298 (URN)10.1103/PhysRevLett.110.185501 (DOI)000319019300008 ()
Funder
Swedish Research Council, 621-2008-3773, 621-2009-3468, 621-2011-4047EU, European Research Council, 246976
Note

AuthorCount:22;

Available from: 2013-06-27 Created: 2013-06-24 Last updated: 2017-12-06Bibliographically approved
2. Nonstatistical fragmentation of large molecules
Open this publication in new window or tab >>Nonstatistical fragmentation of large molecules
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2014 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 89, no 3, 032701Article in journal (Refereed) Published
Abstract [en]

We present experimental evidence for the dominance of prompt single-atom knockout in fragmenting collisions between large polycyclic aromatic hydrocarbon cations and He atoms at center-of-mass energies close to 100 eV. Such nonstatistical processes are shown to give highly reactive fragments. We argue that nonstatistical fragmentation is dominant for any sufficiently large molecular system under similar conditions.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-102212 (URN)10.1103/PhysRevA.89.032701 (DOI)000332340800004 ()
Funder
Swedish Research Council, 621-2012-3662, 621-2012-3660, 621-2011-4047
Note

AuthorCount: 18;

Available from: 2014-03-28 Created: 2014-03-28 Last updated: 2017-12-05Bibliographically approved
3. 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
4. Non-statistical fragmentation of PAHs and fullerenes in collisions with atoms
Open this publication in new window or tab >>Non-statistical fragmentation of PAHs and fullerenes in collisions with atoms
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2014 (English)In: International Journal of Mass Spectrometry, ISSN 1387-3806, E-ISSN 1873-2798, Vol. 365, 260-265 p.Article in journal (Refereed) Published
Abstract [en]

Non-statistical fragmentation processes may be important when Polycyclic Aromatic Hydrocarbon molecules (PAHs), fullerenes, or other large complex molecules collide with atoms and atomic ions. For collisions with hydrogen or helium this occurs for center-of-mass energies between a few tens to a few hundreds of electron volts and typically results in losses of single atoms. In such processes one forms much more reactive fragments than in statistical fragmentation, which instead are dominated by losses of C2- or C2H2-molecules (H-atoms) from fullerenes and PAHs, respectively. An enhanced reactivity has recently been demonstrated for van der Waals clusters of C60 molecules where prompt knockouts of single C-atoms from one of the fullerenes yield highly reactive C59+ fragments, which easily form covalent bonds with a C60 molecule inside the clusters

Keyword
Fullerene, PAH, Knockout, Ion collisions, Molecular fusion, Non-statistical fragmentation
National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-102211 (URN)10.1016/j.ijms.2013.12.013 (DOI)000338622200042 ()
Available from: 2014-03-28 Created: 2014-03-28 Last updated: 2017-12-05Bibliographically approved
5. Absolute fragmentation cross sections in atom-molecule collisions: Scaling laws for non-statistical fragmentation of polycyclic aromatic hydrocarbon molecules
Open this publication in new window or tab >>Absolute fragmentation cross sections in atom-molecule collisions: Scaling laws for non-statistical fragmentation of polycyclic aromatic hydrocarbon molecules
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2014 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 140, no 22, 224306Article in journal (Refereed) Published
Abstract [en]

We present scaling laws for absolute cross sections for non-statistical fragmentation in collisions between Polycyclic Aromatic Hydrocarbons (PAH/PAH(+)) and hydrogen or helium atoms with kinetic energies ranging from 50 eV to 10 keV. Further, we calculate the total fragmentation cross sections (including statistical fragmentation) for 110 eV PAH/PAH(+) + He collisions, and show that they compare well with experimental results. We demonstrate that non-statistical fragmentation becomes dominant for large PAHs and that it yields highly reactive fragments forming strong covalent bonds with atoms (H and N) and molecules (C6H5). Thus nonstatistical fragmentation may be an effective initial step in the formation of, e. g., Polycyclic Aromatic Nitrogen Heterocycles (PANHs). This relates to recent discussions on the evolution of PAHNs in space and the reactivities of defect graphene structures.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-106342 (URN)10.1063/1.4881603 (DOI)000337806100023 ()
Note

AuthorCount:15;

Available from: 2014-08-08 Created: 2014-08-04 Last updated: 2017-12-05Bibliographically approved
6. Formation of H2 from internally heated polycyclic aromatic hydrocarbons: Excitation energy dependence
Open this publication in new window or tab >>Formation of H2 from internally heated polycyclic aromatic hydrocarbons: Excitation energy dependence
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2015 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 142, no 14, 144305Article in journal (Refereed) Published
Abstract [en]

We have investigated the effectiveness of molecular hydrogen (H-2) formation from Polycyclic Aromatic Hydrocarbons (PAHs) which are internally heated by collisions with keV ions. The present and earlier experimental results are analyzed in view of molecular structure calculations and a simple collision model. We estimate that H-2 formation becomes important for internal PAH temperatures exceeding about 2200 K, regardless of the PAH size and the excitation agent. This suggests that keV ions may effectively induce such reactions, while they are unlikely due to, e.g., absorption of single photons with energies below the Lyman limit. The present analysis also suggests that H-2 emission is correlated with multi-fragmentation processes, which means that the [PAH-2H](+) peak intensities in the mass spectra may not be used for estimating H-2-formation rates.

National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-117139 (URN)10.1063/1.4917021 (DOI)000352969600023 ()
Available from: 2015-05-08 Created: 2015-05-08 Last updated: 2017-12-04Bibliographically approved
7. Molecular growth inside polycyclic aromatic hydrocarbon clusters induced by ion collisions
Open this publication in new window or tab >>Molecular growth inside polycyclic aromatic hydrocarbon clusters induced by ion collisions
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2015 (English)In: Journal of Physical Chemistry Letters, ISSN 1948-7185, E-ISSN 1948-7185, Vol. 6, no 9, 1536-1542 p.Article in journal (Refereed) Published
Abstract [en]

The present work combines experimental and theoretical studies of the collision between keV ion projectiles and clusters of pyrene, one of the simplest polycyclic aromatic hydrocarbons (PAHs). Intracluster growth processes induced by ion collisions lead to the formation of a wide range of new molecules with masses larger than that of the pyrene molecule. The efficiency of these processes is found to strongly depend on the mass and velocity of the incoming projectile. Classical molecular dynamics simulations of the entire collision process-from the ion impact (nuclear scattering) to the formation of new molecular species-reproduce the essential features of the measured molecular growth process and also yield estimates of the related absolute cross sections. More elaborate density functional tight binding calculations yield the same growth products as the classical simulations. The present results could be relevant to understand the physical chemistry of the PAH-rich upper atmosphere of Saturn’s moon Titan.

Keyword
poycyclic aromatic hydrogen, ion collisions, density functional tight binding molecular dynamics simulations, classical molecular dynamics
National Category
Atom and Molecular Physics and Optics
Research subject
Physics
Identifiers
urn:nbn:se:su:diva-117140 (URN)10.1021/acs.jpclett.5b00405 (DOI)000355014900006 ()
Available from: 2015-05-08 Created: 2015-05-08 Last updated: 2017-12-04Bibliographically approved

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