Collision cascade and sputtering process in a polymer

The particle, induced fragmentation and sputtering of a similar to7.5 kilodalton organic sample is modeled using molecular dynamics (MD) simulations. The model system consists of a polystyrene coil containing 61 styrene repeat units adsorbed on Ag(111). It is bombarded by 500 eV Ar projectiles. To obtain a realistic picture of the dynamics for an organic material, we used the new adaptative AIREBO potential developed by Stuart, Tutein, and Harrison, which includes long-range van der Waals forces in the reactive potential created by Brenner (REBO) for hydrocarbon systems. Significant differences between the results obtained with and without the long-ranged interaction are identified. The development of the collision cascade in the organic medium is analyzed in detail using collision trees and movies of the results from the simulation. In addition to fast atomic collision processes, we show the existence of long-lived vibrational excitations and demonstrate their importance for the emission of kilodalton chain segments. Recombined and rearranged fragments are emitted, but their contribution to the mass spectrum is insignificant beyond 40 Da. Delayed emission via vibration-induced bond scission is also observed. Finally, we compare the MD results with new ToF-SIMS measurements performed in the context of this study.