Scattering of helium nozzle beams from LiF(001) and NaCl(001) crystal surfaces: I. Elastic and inelastic transitions

Abstract

Nozzle-type atomic beams and time-of-flight detection methods are employed in studies of the scattering of thermal-energy 4He atoms from LiF (001) and NaCl (001) surfaces under conditions where diffraction is observed and where substantial inelastic scattering occurs. Intensity and speed-distribution measurements are obtained for the portion of the scattering within the plane of incidence. For these measurements, incident-beam orientation with respect to target (polar angle and azimuth), incident-beam de Broglie wavelength (0.38–0.60 Å), and crystal temperature (300–1000 K) are varied. For LiF, the measured intensity distributions exhibit sharp peaks comprised essentially of elastically scattered atoms and corresponding to up to third-order diffraction, inelastic scattering in the form of local dispersion about each of the elastic peaks, and a component of nominally diffuse scattering distributed below and between the peaks. For NaCl, the same general types of scattering are observed, with elastic scattering less prevalent, intensity peaks not as sharp, and dispersions of inelastic scattering harder to resolve. Even so, scattering from NaCl is highly structured and diffuse scattering is of minor magnitude. For both LiF and NaCl, because quantitative intensity data are obtained, the fractions of incident-beam flux scattered into the elastic beams can be estimated. Values of these flux fractions measured versus incident-beam most probable wavelength and incidence angle are compared with predictions of a particular theoretical model. From specular intensity data obtained at differing crystal temperatures, the following estimates of effective surface Debye temperature for crystal-atom motions normal to the surface are obtained : 350 ± 50 K for LiF and 250 ± 30 K for NaCl. For NaCl, adsorbate builds up on the surface below 350 K and causes the scattering to become essentially diffuse. For LiF at or above 300 K and NaCl at or above 400 K, under present test conditions (background pressure about 10 −7 torr), there are substantial indications that the surfaces are reasonably clean.