Scattering of Low-Energy Helium Beams from Clean Surfaces of Sodium-Fluoride Lithium-Fluoride and Graphite.

An examination of helium beam scattering from the basal plane of graphite and from {001} surfaces of two alkali halide crystals is presented. Results are given for both ('4)He and ('3)He. Incident beam energies used range from 2.9 meV to 17.3 meV, chosen to probe the physisorption regime. Bound state energy levels are measured for ('4)He and ('3)He adsorbed on graphite, NaF and LiF. Matrix elements of the atom-surface potential Fourier components are measured for ('4)He on graphite and LiF. A brief discussion of how this information is used to determine the details of the gas-atom - solid surface interaction are given. In the case of ('4)He and ('3)He on graphite, comparison is made with thermodynamic adsorption data for submonolayer films and excellent agreement is found. Resonance lineshapes are studied for ('4)He scattering from LiF and from graphite as the incident angles and energy are varied. The LiF results are seen to agree well with recent theoretical predictions. For graphite, some phenomena exhibit conformance to elastic theory while others appear to contradict it. Experimental evidence that the latter are due to inelastic scattering mechanisms is presented.