ANGULAR DISTRIBUTION OF SCATTERED AND DESORBED ATOMS FROM SPECULAR SURFACES

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.