Molecular dynamics simulation of energy accommodation of internal and translational degrees of freedom at gas–surface interfaces

Abstract

The exchange of energy and momentum during the collision between molecules of a diatomic gas with translational and rotational degrees of freedom and solid surfaces is studied by classical trajectory calculations. The gas molecules are regarded as rigid rotators and the (semi-)infinite surface is approximated by the detailed molecular simulation of a few surface atoms which are embedded in a temperature bath, realised by the use of the Langevin differential equation. For the system nitrogen and platinum this surface model is used to compute large numbers of trajectories of gas molecules interacting with the surface, for directly scattered molecules as well as adsorbed and desorbed molecules. The simulation shows a strong coupling between the translational and rotational degrees of freedom for the directly scattered molecules. The sticking coefficient shows exponential dependencies on the kinetic energy as well as on the rotational energy of the incoming molecules. Molecules leaving the surface after physisorption exhibit the effect of rotational cooling at higher surface temperatures.