Kiloelectronvolt particle-induced emission and fragmentation of polystyrene molecules adsorbed on silver: Insights from molecular dynamics

Polystyrene oligomers adsorbed on Ag{111} have been used as a model system for a molecular dynamics study of polyatomic fragment ejection from large o rganic adsorbates on metals. The simulation of the interaction between the incident 500 eV Ar atoms and the sample predicts the nature and kinetic ene rgy of the characteristic fragments observed in the low-mass range of the e xperimental polystyrene secondary ion mass spectrum (SIMS). This agreement confirms that characteristic molecular fragments can be the result of a col lisional emission process. Moreover, the results indicate that the ejection of intact molecules occurs frequently. To understand the mechanisms of emi ssion of fragments and parent molecules, a representative set of trajectori es has been analyzed in detail. The ejection of characteristic fragments is primarily due to the direct interaction between the primary particle and t he organic adsorbate. In contrast, desorption of intact molecules is induce d by collision cascades in the sample surface. The emission of energetic in tact molecules is best explained by a cooperative uplifting mechanism in wh ich substrate atoms with similar momenta push the molecule upward. For comp arison to the 500 eV bombardment conditions, simulations conducted with 5 k eV primary particles show that large-scale simultaneous motions in the subs trate may also occur, inducing the emission of many silver atoms and organi c molecules in a single high action event.