Molecular dynamics simulation of polymerlike thin films irradiated by fast ions: A comparison between FENE and Lennard-Jones potentials

Abstract

In this paper, the surface effects of individual heavy ions impacting thin polymerlike films were investigated, using molecular dynamics simulations with the finite extensible nonlinear elastic (FENE) potential to describe the molecular chains. The perturbation introduced by the ions in the lattice was modeled assuming that the initial excitation energy in the ion track is converted into an effective temperature, as in a thermal spike. The track was heated only within the film thickness $h$, leaving a nonexcited substrate below. The effect of decreasing thickness on cratering and sputtering was evaluated. The results were compared to experimental data of thin polymer films bombarded by MeV--GeV ions and to simulations performed with the Lennard-Jones potential. While several qualitative results observed in the experiments were also seen in the simulations, irrespective of the potential used, there are important differences observed on FENE films. Crater dimensions, rim volume, and sputtering yields are substantially reduced, and a threshold thickness for molecular ejection appears in FENE simulations. This is attributed to the additional restrictions on mass transport out of the excited track region imposed by interchain interactions (entanglements) and by the low mobility of the molten phase induced by the spike.