Low Energy Electron Microscopy and Normal Incidence Vleed

Abstract

To achieve an understanding of the diffraction contrast similar to that already available in high energy transmission electron microscopy (TEM) in low energy electron microscopy (LEEM), it is necessary first to understand very low energy electron diffraction (VLEED), which requires both experimental and theoretical studies. VLEED and LEEM are intimately connected, both theoretically and experimentally. Because of the importance of LEEM for understanding surfaces, in particular their microstructure and the kinetics of surface processes, a rapid development of the theory of VLEED is of utmost importance. This chapter discusses some of similar studies that have been conducted in researchers' laboratories, mainly in the experimental direction, in the hope to stimulate more theoretical work. Recently, the realization of LEEM has revived the interest in (nearly) normal incidence VLEED, although it has been introduced a long time ago. VLEED is important for LEEM because one of the most important contrast mechanisms in LEEM is diffraction contrast, and because at present the most useful energy range in LEEM is from 0 to about 20 eV. In this energy range several of the approximations usually made in standard LEED analysis are not valid: the influence of the surface barrier becomes important, correlation and exchange play an increasing (energy-dependent) role in the scattering phase shifts, and the real part V or of the mean inner potential, and the imaginary part V oi of the inner potential can vary strongly with energy depending upon the excitation spectrum of single particle excitations of the material. Collective excitations are, in general, beyond the energy range of interest but interband transitions are not limited any longer to direct processes.