Calculations based on the inelastic collision model for LEED: II. Ni (111) and (110)

Abstract

Intensity-voltage profiles are measured for several angles of incidence for the specular beam on the Ni (111) surface and the results compared with calculations based on the Inelastic Collision Model for LEED. Good correlation is found between theory and experiment, and the theory is also found to account qualitatively for most of the striking features of the data of Park and Farnsworth for several non-specular beams on the (111) and (110) surfaces. The same values are used throughout for the mean free path and the ion-core scattering cross section, the only two adjustable parameters in the theory. It is found necessary to augment the s-wave scattering amplitude artificially to produce sufficiently strong isotropic scattering by the ion cores, and the resulting violation of unitarity leads to some artefacts. It is suggested that a consistent description of the ion-core scattering should be incorporated in the Inelastic Collision Model, and that this may remove the main discrepancies between theory and experiment. It is found that while no inner potential correction is needed for the (110) data or for the data on the specular beam on the (111) face, the data on non-specular beams on the latter face require an inner potential correction of about 20 eV. This curious result is found to lend support to the contention that the inner potential and mean free path as normally used in the field of LEED are grossly over-simplified concepts in that they take no account of the fact that the electrons are never in a “uniform” region of the crystal. The whole question of polarisation and scattering in a strongly non uniform system requiries thorough investigation.