Surface excitation probability of medium energy electrons in metals and semiconductors

Reflection energy electron loss spectra (REELS) have been measured for several metals and semiconductors {Be, Al, Si, V, Fe, Co, Ni, Cu, Ge, Mo, Pd, Te, Ta, W, Au, Pb) in the medium energy range {150-3400 eV) for normal incidence and an emission direction of 60 degrees with respect to the surface normal. The ratio of the number of electrons that induced a surface excitation to the intensity of the elastic peak was extracted from each spectrum providing the so-called surface excitation parameter {SEP). This quantity is equal to the average number of surface excitations an electron experiences when it crosses the surface once. For the nearly free-electron materials the results agree reasonably with free-electron theory while significant deviations are observed for the other materials. In all cases the SEP is found to be inversely proportional to the speed of the probing particle. It is generally found that the surface excitation parameter decreases with the generalized plasmon energy. A simple predictive formula to estimate the surface excitation parameter for medium energy electrons entering or leaving an arbitrary material is proposed. (C) 2001 Elsevier Science B.V. All rights reserved.