Elastic and Inelastic Effects in the Scattering of Low-Energy Helium Beams from Solid Surfaces.

Low-energy ('4)He atom scattering was used to study the basal plane surface of graphite and the LiF (001) surface. Experiments involving the selective adsorption line shapes and other scattering features revealed a variety of effects, some consistent with elastic and some with inelastic scattering processes. Theoretical predictions of the nature of the elastic and inelastic scattering features were tested. A high-resolution, time-of-flight energy analyzer for the direct study of inelastic effects was designed and constructed. Calculations were done of the analyzer resolution relative to other existing systems. Preliminary tests were completed which revealed changes necessary to make this analyzer operational. An ultra-low-energy ('4)He beam was used in the ('4)He/graphite work. A continuously adjustable beam energy in the range from less than 3 meV and up to 63 meV was used in several experiments which looked for inelastic effects. The behavior of the scattering features as beam energy was varied and the inconsistency of several features with an elastic theory provided the major evidence of inelastic effects. A 17 meV energy beam was used in the LiF work, and several diffracted beams in addition to the specular were measured. Two theoretical predictions of selective adsorption line shapes were tested, and, except for a few cases, good agreement was found. Deviations from expected elastic behavior were observed as either surface temperature or age was varied, indicating the presence of inelastic processes in this scattering system also.