SCATTERING OF MOLECULAR BEAMS BY METALLIC SURFACES

Modulated molecular beam techniques were employed to measure the spatial distribution of rare gases reflected or scattered from polycrystalline platinum. The density distribution of the molecules about the target was measured with a movable detector and molecular velocity changes which occurred during collisions between the beam and target at different temperatures were determined through observation of shifts in the phase of the modulated beam signals. The results are that diffuse scattering is obtained from ''dirty'' surfaces, while the reflections obtained after surface cleaning by heating at a pressure of 10 to the minus 9th power torr are specular in character. The reflected beams are not, however, found at the true specular angle, and the dependence of this angular deviation on experimental parameters was studied. From the data of collision induced molecular velocity changes, energy accommodation coefficients are calculated, and the results are correlated with changes in scattering distributions. Accommodation coefficients obtained with a clean platinum surface agree with literature values for clean metallic surfaces as determined by a hot filament technique. (Author) Presented at the International Symposium on Rarefied Gas Dynamics (4th), Toronto, Canada, July 14-17, 1964.