INTERFACE AND BULK EFFECTS IN THE ATTENUATION OF LOW-ENERGY ELECTRONSTHROUGH CAF2 THIN-FILMS

We have studied for low kinetic electron energies the attenuation of t he Si 2p core-level photoemission line through epitaxial CaF2 layers d eposited on Si(111). Using an exponential attenuation model we have se parated bulk and interface effects, which are, respectively, comprised within energy-dependent bulk attenuation length and interface transmi ssion probability. The attenuation length has basically a constant val ue of similar to 23 Angstrom for kinetic energies above similar to E-F +15 eV, whereas the transmission probability has a maximum at similar to 23 eV above E-F. The latter effect is consistent with the presence of a large density of bulk Lambda(1) states in the conduction band of CaF2 around 23.5 eV. Such a large density of states is obtained in a b and calculation using the local-density approximation, and it is also detected in the background of secondaries of the photoemission spectra .