Licentiate Thesis: Statistical and Non-statistical fragmentation of large molecules in collisions with atoms
Friday 11 April 2014
to 15:00 at
Tao Chen (Stockholm University, Department of Physics)
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.