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 EthExp = 32.5 ± 0.4 eV and EthMD = 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 TdispSE = 23.3 ± 0.3 eV and TdispMD = 27.0 ± 0.3 eV. The semiempirical results compare favorably with measured displacement energies for graphene (Tdisp = 23.6 eV).