Determination of the surface excitation parameter for oxides: TiO2, SiO2, ZrO2 and Al2O3

The angular distribution of the surface excitation parameter (SEP) was theoretically determined for four oxide materials (TiO2, SiO2, ZrO2, Al2O3) and for electron energies of 300, 1000 and 3400 eV. We use our previous definition of the SEP [N. Pauly, S. Tougaard, Surf. Sci. 601 (2007) 5611], as the change in excitation probability of an electron caused by the presence of the surface in comparison with an electron moving in an infinite medium. The calculation procedure is based on analysis of energy-differential inelastic electron scattering cross sections determined from the QUEELS software (quantitative analysis of electron energy losses at surfaces) valid for reflection-electron-energy-loss spectroscopy (REELS). SEP results are found to be noticeably smaller than those previously found for metals. This is shown to be mainly attributed to the energy band gap which reduces the range of channels for surface excitations.