Comparisons of the Reflection and Diffraction of 3He, 4He, H2, and D2 from the (001) Surface of LiF

Abstract

Comparisons of the reflection and diffraction of thermal energy beams of ³He, ⁴He, H₂, and D₂ from the (001) surface of LiF are reported. Intense diffracted beams, including those of the second order, result from a freshly cleaved LiF surface without heating, and large exposures of H₂O vapor do not reveal any measurable change in either the intensity, normalized to constant incident beam intensity, or location of such beams. The intensity and location of the individual beams are examined, taking into account the influence of adjacent beams. The locations of beams not so influenced display good agreement with the predictions of simple diffraction theory. The shapes and intensities of the diffracted beams are compared for the four gases with reference to possible inelastic effects. The decreasing intensities of diffracted beams for the same incident de Broglie wavelength are found to be in the order ⁴He, H₂, and D₂, i.e., in the order of increasing rotational energy coupling with the surface; ³He diffracted beams have intensities approximately the same as for ⁴He. The order of decreasing specular intensity, on the other hand, changes, depending on angle of incidence, but ³He is consistently more intense than for ⁴He, implying a mass collisional dependence. Preferentially scattered beams, similar to those reported for the heavier rare gases, are also evident although the presence of these beams only becomes apparent when conditions are such that the influence of other beams (diffracted and specular) is small. Selective adsorption studies reveal the presence of one or more bound energy states for each of the gases. The energy states for ³He and ⁴He are found to be significantly different, which is consistent with the results for the adsorption of these gases on charcoal. The energy states for H₂ and D₂ are identical, within the experimental error, and considerably greater in magnitude than for either ³He or ⁴He.