Licentiate Thesis: Statistical and Non-statistical fragmentation of large molecules in collisions with atoms

by Tao Chen (Stockholm University, Department of Physics)

Friday 11 Apr 2014, 13:00 → 15:00 Europe/Stockholm

FB53

In this work, I present a study of the fragmentation of Polycyclic Aromatic Hydrocarbon (PAH) molecules and fullerenes following collisions with atoms or atomic ions. The study is partly based on experiments at two different center of mass energy regimes. At the higher collision energy (~ 10 keV), the molecules are mainly excited through interactions between the fast ion/atom and the electron cloud (electronic stopping processes). The excitation energy is then rapidly distributed across the molecules' vibrational degrees of freedom. The lowest energy dissociation channels, H- and C2H2-loss from PAHs and C2-loss from fullerenes, are then statistically favoured. This type of decay is referred to as statistical fragmentation. For the lower center of mass collision energies (~ 100 eV), single atoms may be knocked out in close atom-atom collisions. Such nonstatistical fragmentation processes are very fast and they are due to nuclear stopping processes. I will show that non-statistical fragmentation processes become dominant for isolated PAHs with more than about 50 carbon atoms in the 100 eV regime. Prompt atom knockout gives highly reactive fragments which may form covalent bonds with other molecules and atoms. Dumbbell shaped C119 + molecules are detected following collisions between 22.5 keV He2+ or 12 keV Ar2+ ions and clusters of C60 molecules. This molecular fusion process is most likely due to single atom knock out and C59 + + C60 collisions inside the fragmenting cluster. Knockout of single carbon atoms from PAHs could, e.g., be a first step in forming nitrogen containing PAHs - so called PANHs. The theoretical part of the work is carried out with the aid of Monte Carlo simulations. The energy loss due to nuclear stopping is calculated using two different potentials describing atom-atom interactions. The energy loss due to electronic stopping is calculated with the aid of friction coefficients for atoms interacting with PAH or fullerene electron clouds. Based on such simulations I present a simple scaling formula for total non-statistical fragmentation cross sections for H + PAH and He + PAH collisions in the 50 eV to 10 keV enrgy range.