In the analysis of rarefied gas flows, much uncertainty exists regarding the boundary condition on the solid surface. This is mainly due to a lack of understanding of the micro scale dynamics of gas-surface interaction under various conditions, such as mass ratio, incident angle and wall temperature. In this paper, the behavior of a Xe gas molecule which collides with the clean Pt(111) surface of very thin film is computed by the molecular dynamics method. A clean surface of Pt(111) is formed in the thermal equilibrium state and then a Xe molecule starts to approach it. The calculated initial-trapping probabilities agree with the experimental data. The calculated results reveal that the average normal momentum of the gas molecule after the first collision has a good correlation with the incident normal momentum.