Angular dependence of the surface excitation probability for medium energy electrons backscattered from Al and Si surfaces

Reflection electron energy loss spectra have been measured for medium energy electrons backscattered from Al and Si surfaces. Angular distributions were obtained for emission angles between 15° and 90° with respect to the surface normal as well as for incidence angles in the same range. The surface excitation parameter (SEP), i.e., the average number of surface excitations an electron experiences when it crosses a surface once, was extracted from each spectrum by fitting the raw data to theory and determining the ratio of the surface loss peak to the elastic peak intensity. No difference in the SEP for incoming and outgoing electrons could be detected in the data. The SEP was found to depend linearly on the time an electron spends in the vicinity of the surface. Both the energy and angular dependence of the SEP can be accurately described by free-electron theory when the electron momentum is rescaled by a material-dependent parameter. The value of the scaling parameter is given for Al and Si so that the SEP in these materials can accurately be predicted for medium energies and arbitrary experimental configurations.