Theoretical Investigations of Gas—Solid Interaction Phenomena. I

Abstract

A classical study of gas—solid surface interactions is reported. A two‐dimensional, independent oscillator lattice (IOL) is used as a surface model with harmonic potentials connecting each lattice site. Morse‐type potential functions are assumed to operate between the incident gaseous atom and each lattice atom. Using Monte Carlo techniques and numerical integration of the 16 differential equations describing the system, energy accommodation coefficients (EAC) and spatial distributions have been calculated for a (He/Ni) system. The dependence of these quantities upon incidence angle, gaseous beam temperature, surface temperature, and various interaction parameters has been investigated. The results indicate that the EAC increases with increasing beam temperature and beam velocity; decreases with increasing lattice force constant; increases with increasing attractive interaction between gas and surface; decreases with increasing incidence angle; and decreases as the surface and gaseous temperatures approach one another. In general, these results are in qualitative to semiquantitative agreement with experiment. The calculated spatial distributions depend strongly upon incidence angle. For perpendicular incidence (θ_i=0°) a diffuse distribution is predicted. For θ_i=37.5° the distribution is peaked at an angle subspecular by 18°. As the attractive interactions between gas and surface increase, the maximum is shifted toward the surface normal. Similar behavior is observed with respect to an increase in surface temperature. Comparison with experiment indicates the He scattering to be specular rather than 18° subspecular. The calculated distributions are also too diffuse due probably to the two‐dimensional character of the model. The predicted trends with regard to attractive interactions and surface temperature are in accord with observation. The over‐all behavior indicates that the ``normal‐component model'' of energy transfer is oversimplified and that transfer of energy in the parallel component of motion can be of importance.