Beam threshold and surface resonance effects in low energy electron diffraction from Ni(100)

Abstract

A systematic study has been made of the appearance of minima in the diffracted intensities from a nickel (100) surface in the primary energy range 50–950 eV, and the possible identification of these minima with beam threshold and surface state resonance effects has been investigated. Two different types of constant energy experiments are reported here: observations of the intensities of various diffracted beams as a function of the angle of incidence of the primary beam at constant azimuthai angle, and measurements of the total integrated specular intensity as a function of azimuthal angle for constant angle of incidence. The experimental arrangements are described and the results given. Based on a comparison of the experimental diffraction geometries for which minima occur with the theoretical conditions for the excitation of a grazing emergent diffracted beam, many of the minima observed in all measurements can be correlated to the excitation of grazing emergent diffracted waves and may therefore be beam threshold effects. Some of the minima, which also are correlated to the grazing emergence condition, but which appear at a slightly lower energy, are believed to be due to surface state resonances. Lack of a better angular resolution prevents a definite distinction from being made between these two effects. Contrary to previous observations in LEED from other materials or at high energies, no intensity enhancements that could be identified with the beam emergence condition could be observed.