ENERGY-DEPENDENCE OF UPS PEAK INTENSITIES OF PHYSISORBED SF6 LAYERS - INFLUENCE OF THE IMFP

The energy dependence of the electron inelastic mean free path (IMFP) or, more precisely, of the attenuation length (AL), lambda(E), may drastically influence the relative intensities of valence peaks, even within the UPS He(II) spectrum of a single species. This is shown for the well-characterized adsorbate system SF, on Ni(111), for which five valence peaks are observed in a relatively small energy range of 8 eV. With He(II) radiation the kinetic energies of these peaks range between 23 and 31 eV, and their intensities vary by up to 60% for adsorbate layers of different thicknesses. This potentially confusing effect can be used to determine lambda and its energy dependence in the low kinetic energy range (<35 eV). We find lambda values between 3.5 and 7.5 monolayers; their energy dependence differs from that expected from the Seah and Dench equation.