The main peaks of the angular distribution of atoms inelastically reflected from a crystal surface are shown to be associated with analytical critical points of the surface phonon dispersion curves, particularly those of Rayleigh and Lucas modes. Because of such a relation, the frequencies and wave vectors of these modes could be measured by analyzing the shift of the peaks under a change of the angle of incidence.

• Received 19 February 1975

DOI:https://doi.org/10.1103/PhysRevLett.35.234