High yield events of molecular emission induced by kiloelectronvolt particle bombardment

In an effort to improve our understanding of large molecule emission in organic SIMS, the sub-kiloelectronvolt and kiloelectronvolt Ar atom-induced sputtering of polystyrene tetramers adsorbed on Ag{111} is modeled using classical molecular dynamics. Above 1 keV, in addition to trajectories consisting of successive collisions between individual atoms, we observe a significant number of high action trajectories where several hundreds of substrate atoms are moving simultaneously. These events can generate unusually high emission yields of substrate atoms, clusters, and polystyrene molecules. Because the probability of high yield events strongly depends on the primary particle energy, representative sets of trajectories are calculated at 150 eV, 500 eV, 1500 eV, and 5 keV. These simulations indicate four main scenarios of action and sputtering. Plots of the energetic parts of the cascade ("collision trees") show that high sputtering yields occur when most of the primary particle energy is quickly dissipated among silver atoms belonging to the top silver layers. In addition, it is shown that high emission events influence not only the yield but also the kinetic energy distributions of ejected polystyrene molecules. Finally, we discuss the relevance of these high yield mechanisms for the ejection of organic species with a mass of several kilodaltons such as biomolecules and synthetic polymers. Our results show that trajectories falling in the "high yield" category are also capable of desorbing large intact molecules of similar to 2000 Da.