Scattering of 3He, 4He, and the Hydrogenic Molecules from the (111) Plane of Ag

Abstract

Molecular beams of the isotopes of He and H have been reflected from the (111) plane of epitaxially grown Ag crystals. The spatial scattering distributions of ³He and ⁴He are compared with those of H₂, HD, and D₂ at various angles of incidence and primary beam temperatures from 80° to 1500°K. Deviations in the specularity of these gases are interpreted in terms of the exchange of rotational state energy of the diatomic species at the surface. Comparisons of number density and flux measurements indicate a significant dependence of the mean speed of the reflected beam on ${\theta }_r$ . It is proposed that this variation is also enhanced by the degree of rotational state interactions, which decreases with increasing beam temperature. Finally, it is suggested that the appearance of a truly specular component in these scattering distributions is related to an apparent surface roughness through the relation $\mathrm{\lambda \hspace{0.17em}>\hspace{0.17em}h}\cos {\theta }_i$ , where $\text{0.8 Å\hspace{0.17em}<\hspace{0.17em}h\hspace{0.17em}<\hspace{0.17em}1.7 Å}$ .