Flux and Speed Distributions of Molecular Beams after Scattering by Metal Surfaces

Abstract

High‐intensity, nearly monoenergetic, nozzle‐type molecular beams together with a sensitive time‐of‐flight detection system have been employed to study the scattering of argon and nitrogen from polycrystallinenickel and stainless‐steel surfaces. The apparatus has allowed the precise measurement of essentially complete flux and speed distributions of the particles reflected in the plane of the incident beam and the target normal. Incident energy (0.065–0.24 eV), surface temperature (300°–1000°K), angle of incidence, and angle of reflection have been varied. At room temperature, prior to any heat treatment in the apparatus, these unpolished targets exhibit essentially the classical diffuse scatteringpattern with almost complete thermal accommodation. With the Nisurfaces tested, outgassing and annealing at approximately 900°K resulted in highly lobular scattered flux patterns for target temperatures between 550° and 950°K. These patterns are accompanied by a marked angular variation of the reflected‐particle speed distribution, which differs the more from the fully accommodated Maxwellian distribution the farther the angle of scattering is from the surface normal. These lobular flux patterns are in qualitative agreement with the predictions of the so‐called “hand‐cube” model, but for the scattered particles the angular variation of the mean speed is smaller, and the angular variation of the relative spread in kinetic energies is greater, than predicted. Stainless steel exhibits similar but less pronounced scattering lobes accompanied by somewhat subdued angular variations of the speed distributions.