Interpretation of a threshold effect on Al(001) in terms of the discontinuity of non-specular beam intensities at grazing emergence

Abstract

A widely known threshold effect in very low energy electron diffraction (VLEED) is the appearance of rapid oscillations of diffracted intensities just before the emergence of one or several diffracted beams. Their maxima and minima are arranged in a Rydberg-like series due to the coulombic long range behaviour of the surface potential barrier. Another curious consequence of this behaviour is the discontinuity of non-specular beam intensities at their emergence threshold. Contrarily to the prediction of the classical LEED theory, non-specular beams appear in vacuum with a non-zero current. Although this discontinuity has already been theoretically predicted, it has never been noticed in measured LEED spectra. However, measurements of current transmitted from crystal to ground performed by Henrich on Al(001) near the normal incidence show the existence of such a discontinuity. At about 18.5 eV, four new diffracted beams emerge from the crystal and thus the backscattered electron discontinuity induces a discontinuity in the transmitted current. In this paper, VLEED spectra on Al(001) near normal incidence are reinvestigated in details. In order to take into account the energy spread of the incident beam, calculated VLEED spectra are convoluted with a gaussian distribution. In this way, they can be compared with experimental spectra. The rapid Rydberg-like oscillations are completely smoothed and the discontinuity effects subsist alone in convoluted spectra. These effects are actually large at normal incidence for which four beams simultaneously emerge. In accordance with Henrich's observations, they disappear as soon as the incident beam deviates from normal incidence.