Theoretical study of the dynamics of hyperthermal collisions of Ar with a fluorinated alkanethiolate self-assembled monolayer

Abstract

We present a classical-trajectory study of the dynamics of energy exchange in collisions between hyperthermal Ar (6-12 eV collision energy) and a fluorinated self-assembled monolayer (SAM). Product translational-energy, polar-angle, and azimuthal-angle distributions as a function of collision energy and incidence angle are presented to provide a detailed description of the gas/surface energy exchange dynamics. Our results indicate that while the properties of the scattered Ar atom at normal and 30 degrees incidence are notably similar and essentially independent of collision energy in the 6-12 eV range, the dynamics of energy exchange when Ar impinges at 60 degrees are remarkably different and depend on collision energy. This behavior is understood via analysis of the microscopic mechanism of the collisions. Three main collision mechanisms-direct collisions without surface penetration, direct collisions involving surface penetration, and surface-penetrating non-direct collisions-are found to govern the dynamics, and the ratio of these mechanisms determines the properties of the scattered Ar atom. Our study also reveals that the Ar atoms that penetrate the organic monolayer do not desorb following a direct-ejection mechanism proposed in recent studies of Xe collisions with a hydrogenated SAM. The vast amount of energy transferred to the surface found in the calculations lends support to recent experiments showing degradation of fluorinated surfaces in collisions with hyperthermal Ar.