Observed Intensity and Speed Distributions of Thermal-Energy Argon Atoms Scattered from the (111) Face of Silver

The scattering of thermal-energy monatomic gases from metal surfaces is studied using the Ar–Ag system. The argon atoms are incident to the surface in the form of nearly monoenergetic beams from a gas-dynamic nozzle source. The target surface is the polished (111) face of a monocrystalline silver disk. In studying this interaction, essentially complete maps of the relative intensities and speeds of the scattered atoms are obtained using a time-of-flight method. The argon beams have energies ranging from 0.06 to 0.17 eV and are incident at angles varying from 40° to 60° from the target normal. The surface temperature ranges from 550 to 790°K. Due to surface roughness, approximately 80% of the incident atoms are scattered diffusely while the remainder are scattered into a crest-shaped lobe centered subspecularly within the plane of incidence. To a first approximation, it is shown that the speed distributions of the diffusely scattered components are independent of scattering angle and that they are nearly Maxwellian in form and exhibit mean speeds corresponding to high thermal accommodation to the surface. The lobularly scattered atoms show a clear tendency to conserve their momentum tangential to the surface. Moreover, it appears that the characteristics of the two component beams are relatively independent of the fraction scattered into each. General properties of the scattered beams are discussed and comparisons with applicable studies are offered.