Theoretical simulations of atomic and polyatomic bombardment of an organic overlayer on a metallic substrate

Our previous molecular dynamics simulations on initial test systems have laid the foundation for understanding some of the effects of polyatomic bombardment. In this paper, we describe simulations of the bombardment of a more realistic model system, an overlayer of sec-butyl-terminated polystyrene tetramers on a Ag{1 1 1} substrate. We have used this model system to study the bombardment with Xe and SF₅ projectiles at kinetic energies ranging from 0.50 to 5.0 keV. SF₅ sputters more molecules than Xe, but a higher percentage of these are damaged rather than ejected intact when the bombarding energy is greater than 0.50 keV. Therefore, at energies comparable to experimental values, the efficiency, measured as the yield-to-damage ratio, is greater with Xe than SF₅. Stable and intact molecules are generally produced by upward moving substrate atoms, while fragments are produced by the upward and lateral motion of reflected projectile atoms and fragments from the target molecule. SF₅ is ineffective on this model system because of the densely packed lattice and the high mass of the substrate atoms. Experiments have determined that enhancements in yield with polyatomic projectiles are smaller on thin organic films compared to those found on thick organic targets.