Licentiate thesis: Collisions between keV ions and Polycyclic Aromatic Hydrocarbon (PAH) molecules or clusters

by Fabian Seitz (Stockholm University, Department of Physics)

Friday 21 Oct 2011, 14:00 → 16:00 Europe/Stockholm

FA32

This thesis summarizes the results from a series of experiments with polycyclic aromatic hydrocarbon (PAH) clusters and monomers in collisions with positively charged ions. With low charge state projectiles we detect singly and doubly charged intact molecules, ionization with losses of H-atoms or C2Hx molecules, and smaller CnHx + fragments. The intensity distribution indicates that this fragmentation is mainly thermally driven. With high charge state projectiles, H- and C2Hx-loss processes become very weak. These results and smaller-n CnHx + fragments compared to the low projectile charge state case suggest that charge-driven fragmentation processes are important here. In experiments with pure PAH clusters the fragment spectra are dominated by the singly charged intact monomer both for low and high charge state projectiles. Long series of singly and doubly charged product clusters of decreasing intensities with increasing size appear with low charge state projectiles and we conclude that the induced charge and the locally induced excitation energies redistribute quickly in PAH clusters before fragmentation. We find that hot singly charged PAH clusters then must evaporate slowly by ejecting single molecules, as we describe with a simple evaporation model. Highly charged ions colliding with PAH clusters eventually give strong internal heating of the individual molecules. This effect is suggested to be due to efficient transfer of the Coulomb energy to the internal degrees of freedom in the molecules in highly charged clusters during charge-driven fragmentation processes. Finally, we compare the ionization and fragmentation of the C16H10-isomers pyrene and fluoranthene. We then find subtle but distinct differences which may be rationalized by means of quantum-chemical calculations of monomer ionization energies and the classical over-the-barrier model for target ionization.