Surface excitation probability of medium energy electrons in metals and semiconductors

Reflection energy electron loss spectra (REELS) have been measured for seve ral metals and semiconductors {Be, Al, Si, V, Fe, Co, Ni, Cu, Ge, Mo, Pd, T e, Ta, W, Au, Pb) in the medium energy range {150-3400 eV) for normal incid ence and an emission direction of 60 degrees with respect to the surface no rmal. The ratio of the number of electrons that induced a surface excitatio n to the intensity of the elastic peak was extracted from each spectrum pro viding the so-called surface excitation parameter {SEP). This quantity is e qual to the average number of surface excitations an electron experiences w hen it crosses the surface once. For the nearly free-electron materials the results agree reasonably with free-electron theory while significant devia tions are observed for the other materials. In all cases the SEP is found t o be inversely proportional to the speed of the probing particle. It is gen erally found that the surface excitation parameter decreases with the gener alized plasmon energy. A simple predictive formula to estimate the surface excitation parameter for medium energy electrons entering or leaving an arb itrary material is proposed. (C) 2001 Elsevier Science B.V. All rights rese rved.