Theoretical Investigations of Gas–Solid Interaction Phenomena. IV Spatial and Velocity Distributions

Abstract

A study of the spatial and final velocity distributions for gaseous particles reflected from a solid surface is reported. The three-dimensional classical model previously used by the authors is employed to study the scattering distributions as functions of the incident particle velocity, the velocity distribution in the incident beam, and surface temperature. The broadness of the spatial scattering distributions is predicted to decrease with increasing incident velocity. The spatial distribution peak position is specular for large incident velocity, shifts toward the surface for intermediate velocity, and approaches the surface normal for small incident velocity. The model indicates that the spatial distribution dependence upon initial velocity causes velocity-selected and thermal beams to give similar spatial scattering. For hot surfaces the spatial scattering is indicated to be a rather insensitive function of surface temperature. The velocity distributions of reflected particles are predicted to be non-Maxwellian, bimodal for large incident velocities, and clearly different for velocity-selected and thermal incident beams. The results indicate that final velocity distribution measurements may be the most informative characteristic of gas–solid interaction phenomena.