Explanation of the anomalous peak observed in the He/Ag(111) atom scattering

The inelastic He/Ag(111) cross section has been evaluated in the one-phonon distorted-wave Born approximation. The authors use an atom-surface potential made up by a repulsive part derived from a superposition of atomic charges and by the attractive van der Waals interaction. The surface phonon density of states is evaluated within a force-constants parametrization of the bulk dynamics. By changing the surface force constants the authors are able to explain all the salient features appearing in the experimental time-of-flight spectra.