We have measured absolute cross sections for ultrafast (fs) single-carbon knockout from Polycyclic Aromatic Hydrocarbon (PAH) cations as functions of He-PAH center-of-mass collision energy in the range 10-200 eV. 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 \emph{in vacuo}. We further deduce semi-empirical (SE) and MD displacement energies --- \emph{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 semi-empirical results compare favorably with measured displacement energies for graphene Tdisp=23.6 eV [Meyer \emph{et al.} Phys. Rev Lett. \textbf{108} 196102 (2012) and \textbf{110} 239902 (2013)].
Cite
@article{arxiv.1511.05742,
title = {Threshold Energies for Single Carbon Knockout from Polycyclic Aromatic Hydrocarbons},
author = {M. H. Stockett and M. Gatchell and T. Chen and N. de Ruette and L. Giacomozzi and M. Wolf and H. T. Schmidt and H. Zettergren and H. Cederquist},
journal= {arXiv preprint arXiv:1511.05742},
year = {2015}
}