Energy transfer in rare gas collisions with self-assembled monolayers

A molecular dynamics simulation is presented of a beam of neon atoms scattering off a n-hexyl thiolate self-assembled monolayer adsorbed on a Au {111} surface. Ab initio QCISD(T)/ 6-311++G** calculations, for a model system consisting of a neon atom and a methane molecule, were used to derive an accurate interaction potential between the neon projectile and the monolayer. Four initial translational energies of 1.2, 5, 20, and 40 kcal/mol and five incident angles (with respect to the surface normal) of 10, 30, 45, 60, and 80 degrees were investigated in the simulations. Both trapping desorption and direct inelastic scattering collisional events were observed. The fraction of trapping desorption decreases as the initial translational energy and/or the angle of incidence increases. For high initial translational energy, trapping desorption may involve penetration of the monolayer. The overall energy transfer probability and the fraction of trapping desorption are in good agreement with previous experiments [J. Chem. Phys. 99, 7056 (1993)] and computational studies [J. Chem. Phys. 100, 8408, 6500 (1994)] of neon atom beams colliding with liquid squalane surfaces. There is a small but measurable change in the energy transfer efficiency for neon atoms colliding parallel versus perpendicular to the "tilt angle" of the SAM. The distributions of the change in the azimuthal angle and the scattering angle between the neon final velocity vector and surface normal provide additional information about the collision dynamics. A general discussion of these simulation results is given, in the context of existing models and theories for gas/surface collisions, to achieve a more fundamental understanding of the dynamics of interfaces.