Molecular Dynamics Simulation Study of Molecular Ejection Mechanisms:  keV Particle Bombardment of C₆H₆/Ag{111}

Molecular dynamics simulations have been performed to gain microscopic insight into the factors that lead to molecular ejection after ion bombardment of an organic overlayer on a metal surface. The specific system modeled is benzene (C 6 H 6 ) adsorbed on Ag{111}. The kinetic energy and angular distributions of C 6 H 6 molecules obtained from the simulations match well with the experimentally measured distributions. The angular distributions of C 6 H 6 molecules show both normal and off-normal components. Analysis of individual trajectories reveal that the off-normal ejection arises from single collisions between substrate Ag atoms and C 6 H 6 molecules, while multiple collisions result in low-energy ejection along the surface normal. To separate issues of rotational and vibrational excitation from translational motion, calculations are also performed on an atomic adsorbate with a mass similar to that of C 6 H 6.