Molecular Beam Study of the Scattering of Rare Gases from the (110) Face of a Tungsten Crystal

Abstract

The scattering of He, Ne, Ar, and Kr from a (110) tungsten surface has been investigated for surface temperatures, $T_s$ , in the range 300°–2100°K and for angles of incidence, ${\theta }_i$ , of 40, 50, and 60 deg. The apparatus consists of a nozzle‐type molecular beam modulated at 2650 Hz so that lock‐in detection may be used to determine both the mean speed and density patterns of the scattered gas. The majority of the data are for $T_s\text{\hspace{0.17em}=\hspace{0.17em}2100°K}$ since thermal desorption of contaminants should be sufficiently rapid at this temperature to insure that the surface is essentially clean. (For $T_s\text{\hspace{0.17em}<\hspace{0.17em}1000°}\mathrm{K}$ , the surface became contaminated by residual gases (primarily CO) before measurements could be obtained.) At $T_s\text{\hspace{0.17em}=\hspace{0.17em}2100°}\mathrm{K}$ , the maxima of the density patterns for ${\theta }_i\text{\hspace{0.17em}=\hspace{0.17em}40, 50, and 60}\mathrm{deg}$ are at the specular angles for He, whereas they are increasingly subspecular for Ne, Ar, and Kr. Both the mean speed and density patterns are compared with the theoretical predictions of the hard‐cube and soft‐cube models.