Molecular-Beam Scattering from the (111) Plane of Silver

Abstract

The spatial distributions of molecular beams of CH₄, NH₃, He, Ne, Ar, and Xe scattered from the (111) plane of an epitaxially grown Ag single crystal have been examined both during growth of the Ag crystal and immediately afterward, and no distinction between these modes was observable. The scattering of Ne, CH₄, and NH₃ has been compared to illustrate the important role of the gas—metal interaction energy; NH₃, the most strongly interacting gas, gives rise to nearly diffuse scattering, while Ne is nearly specularly directed. Although the scattering of He is essentially specularly directed at all incident angles and temperatures, the heavier gases (Ar and Xe) exhibit scattering maxima lying between the specular direction and the tangent to the surface (supraspecular scattering), with the location of the maxima depending in a complex manner on the angle of incidence of the beam and the beam energy. Since a necessary condition for supraspecularity is that the beam temperature be greater than the surface temperature, an energy‐transfer step seems to be involved in determining the particle trajectory. Similar complexities in the intensity of the scattered‐beam maxima are observed; however, no simple explanation seems to suffice in this case.