Threshold Energies for Single-Carbon Knockout from Polycyclic Aromatic Hydrocarbons

We have measured absolute cross sections for ultrafast (femtosecond) single-carbon knockout from polycyclic aromatic hydrocarbon (PAH) cations as functions of He–PAH center-of-mass collision energy in the 10–200 eV range. Classical molecular dynamics (MD) simulations cover this range and extend up to 10⁵ eV. The shapes of the knockout cross sections are well-described by a simple analytical expression yielding experimental and MD threshold energies of E_th^Exp = 32.5 ± 0.4 eV and E_th^MD = 41.0 ± 0.3 eV, respectively. These are the first measurements of knockout threshold energies for molecules isolated in vacuo. We further deduce semiempirical (SE) and MD displacement energies, i.e., the energy transfers to the PAH molecules at the threshold energies for knockout, of T_disp^SE = 23.3 ± 0.3 eV and T_disp^MD = 27.0 ± 0.3 eV. The semiempirical results compare favorably with measured displacement energies for graphene (T_disp = 23.6 eV).