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 E_i 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 E_i reflecting equivalent energy transfer probabilities from the normal and parallel components of E_i. Such a result is consistent with the corrugated SAM surface. Penetration of the SAM scales with the normal component of E_i for small incident angles θ_i and E_i . Both the Boltzmann component to the transitional energy distribution of the scattered Ne atom, P(E_i) and SAM penetration become negligible for an increase in θ_i 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 E_i and θ_i, reflecting the surface corrugation and not the presence of an actual Ne–SAM intermediate. At high E_i and θ_i the scattering is non-random, preferring to remain in the incident plane with the parallel component of E_i 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.