Angular Distribution of Scattered and Desorbed Atoms from Specular Surfaces

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Abstract
The angular distribution of atoms and molecules leaving a cleaved‐crystal surface, on which the conditions for specular reflection are fulfilled, is analyzed. Starting from published experimental data concerning the reflecting power vs glancing‐angle dependence (He–LiF system) and from some general considerations, the existence of a deviation from the cosine law of the angular distribution of atoms, leaving a specular surface after an inelastic collision, is pointed out. It is shown that this deviation is correlated with the shape of the reflecting power vs glancing‐angle curve, belonging to the atoms which undergo a specular reflection on this surface. A tentative model for the specular reflection of helium on LiF cleaved‐crystal surfaces is proposed; it considers, in analogy with the neutron case, the specular reflection of atoms as total reflection on a surface of positive average interaction potential. The proposed model leads to a reflecting power vs glancing‐angle curve which fits rather well the experimental data. Moreover, it explains qualitatively the correlation between the shape of the reflecting power vs glancing‐angle curve and the characteristics of the deviation from the cosine law of the angular distribution for atoms having undergone an inelastic collision. Experiments for the check‐up of the general conclusions, as well as of the tentative model, are proposed.