Origin of the Boltzmann translational energy distribution in the scattering of hyperthermal Ne atoms off a self-assembled monolayer

Abstract

A classical trajectory simulation is performed to study the dynamics of Ne-atom scattering off an n-hexyl thiolate self-assembled monolayer (SAM). The energy distribution of the scattered Ne-atoms may be deconvoluted into a Boltzmann component based on the surface temperature and a remaining non-Boltzmann high energy component. The former component becomes as large as 100% at low Ne-atom incident energies and appears to assume a high incident energy asymptotic value of ∽0.2–0.3 for incident angles less than 50°. This Boltzmann component does not arise from a Ne–SAM intermediate, since the vast majority of the collisions are direct with only one inner turning point in the Ne+SAM motion. Instead, it has varying contributions from trajectories which directly scatter from and those that penetrate the SAM. The translation (T)→vibration (V) energy transfer mechanism for the former class of trajectories may have similarities to exit-channel T→V coupling in models of unimolecular dissociation. SAM penetration becomes more important as the initial translational energy Ei is increased, resulting in both efficient and inefficient energy transfer. The latter results from repulsive forces as the Ne-atom is ‘‘ expelled’’ from the SAM. For incident angles less than 50° the size of the Boltzmann component scales with the total incident energy Ei reflecting equivalent energy transfer probabilities from the normal and parallel components of Ei. Such a result is consistent with the corrugated SAM surface. Penetration of the SAM scales with the normal component of Ei for small incident angles θi and Ei . Both the Boltzmann component to the transitional energy distribution of the scattered Ne atom, P(Ei) and SAM penetration become negligible for an increase in θi from 45° to 60°, suggesting an abrupt transition in the collision dynamics. The angular distributions for the scattered Ne-atoms are random for low initial energies Ei and θi, reflecting the surface corrugation and not the presence of an actual Ne–SAM intermediate. At high Ei and θi the scattering is non-random, preferring to remain in the incident plane with the parallel component of Ei conserved. This study shows there are ambiguities in associating a trapping desorption intermediate with statistical gas–surface scattering attributes, such as a Boltzmann component, in the translational energy distribution and a random angular distribution.