Molecular dynamics simulations to explore the effect of projectile size on the ejection of organic targets from metal surfaces

Experiments have shown that cluster projectiles as compared to atomic projectiles enhance the secondary ion emission of organic molecules. The yield depends nonlinearly on the number of constituent atoms in the primary ion cluster. In this paper, we describe molecular dynamic simulations aimed at determining the fundamental mechanisms responsible for the enhancement in emission yield. Our model system, a biphenyl adsorbate on a Cu(001) surface, is chosen as a prototype of the experimental systems of interest. Cu atoms and Cu_n clusters with n = 2-4 and kinetic energies from 0.050 to 0.100 keV per atom are brought in at 45° incidence. The emission yield of the biphenyl molecule increases nonlinearly with the number of atoms in the cluster. Several parts of the biphenyl molecule must be hit in order for it to be ejected intact. A monatomic projectile initially strikes one atom in the top surface layer, while a polyatomic projectile initially strikes two or more atoms. Therefore, with the cluster projectile, there will be a greater probability of two or more collision cascades that are adjacent in time and space and can collaborate to eject the molecule from the surface.