Multistage gas–surface interaction model for the direct simulation Monte Carlo method

A multistage (MS) gas–surface interaction model for a monatomic/diatomic gas molecule interacting with a solid surface is presented, based on the analysis of molecular dynamics (MD) simulations and a model equation derived from the classical theory of an ellipsoid hitting a hard cube. This model is developed for use with the direct simulation Monte Carlo (DSMC) method and belongs to the thermal scattering regime. The molecular dynamics method is used for the molecular-level understanding of the scattering of $O2,$ $N2,$ and Ar from a graphite surface. The basic idea of the model is to separate the collision into three stages. At stages 1 and 2, the energy and scattering direction are determined by the model equation. At stage 3, according to the translational energy, the molecule is determined to scatter, re-enter or be trapped by the surface. Re-entering molecules return to stage 1. The model parameters are determined from our MD database. Experiments are also performed by scattering a supersonic $O2$ molecular beam from a clean graphite surface in an ultrahigh vacuum chamber. The in-plane scattering distribution, out-of-plane scattering distribution, and in-plane velocity distribution of the model show good agreement with those of molecular beam experiments. A model equation was included in the MS model to maintain thermal equilibrium between a gas and a surface at the same temperature when applied to DSMC simulations and the results are also shown. The high accuracy of the model clearly shows that such multiple-scale analysis can lead to the development of realistic models of the gas–surface interaction.