Direct measurement of velocity distributions in argon beam-tungsten surface scattering

Abstract

Velocity distributions for Ar atoms scattering from a clean, polycrystalline tungsten surface have been measured for a wide range of incident supersonic beam energies 300 K < (1/2)m〈υ2〉/k < 2000 K, and surface temperatures 350 K < Ts < 1900 K. This work studies directly the nature of the scattering process of an intermediate mass atom on a clean metal surface over a very wide range of conditions. Direct inelastic scattering involving a single encounter of the gas atom with the surface is the most important process. No distinct elastic or quasielastic scattering occurs. Only at the lowest temperatures is a trapping-desorption scattering process observed. The direct inelastic scattering process is characterized by the linear proportional relationship 〈KEe〉 = 0.83 〈KEi〉 + 0.20 〈KETs〉 over the entire range of energies and temperatures for 45° angle of incidence and observation in the specular direction (KEe, KEi, and KETs are the kinetic energy of the exiting Ar, the incident Ar, and the Ar in equilibrium at the surface temperature, respectively). The proportionality relationship is nearly the same for observation in the normal direction. The collision-caused spread in velocities is linearly proportional to the surface temperature, and substantially independent of incident energy. Tangential momentum accommodation is also discussed. © 1980 American Institute of Physics.