Isomer effects in fragmentation of Polycyclic Aromatic Hydrocarbons

We have observed significant differences in the fragmentation patterns of isomeric Polycyclic Aromatic Hydrocarbon (PAH) cations following collisions with helium atoms at center-of-mass energies around 100 eV. This is in contrast to the situation at other collision energies or in photo-absorption experiments where isomeric effects are very weak and where the lowest-energy dissociation channels (H- and C₂H₂-loss) dominate in statistical fragmentation processes. In the 100 eV range, non-statistical fragmentation also competes and is uniquely linked to losses of single carbon atoms (CH_x-losses). We find that such CH_x-losses are correlated with the ionic ground state energy within a given group of isomers. We present results for three C₁₆H₁₀⁺, four C₁₈H₁₂⁺ and five C₂₀H₁₂⁺ isomers colliding with He.