Elastic-Scattering Effects in Characteristic Electron-Energy-Loss Spectra of Ni(110) Adsorbate Surfaces

The sensitivity of electron-energy-loss spectra to primary-beam energy is found to derive from elastic-scattering effects associated with ordered structure characterizing the selvedge of a solid-vacuum interface. These effects are demonstrated in the clean Ni (110) spectra and in sulfur adsorbate spectra for the Ni (110)-c(2×2)S structure. In the disordered adlayer, the elastic effects are essentially quenched leaving characteristic loss effects as shown in nitrogen-adsorbate spectra for the Ni (110)-(disordered)-N adlayer. It is shown that characteristic loss spectra for an ordered-surface structure result from a two-step scattering process involving an inelastic and an elastic event. The sequence of these events can be determined from N(Eₚ)|_EL loss spectra. Several energy losses have been found in the 4-10-eV range for a clean Ni (110) surface which have been shown to correlate with interband excitations. The identification of elastic-scattering effects in electron-energy-loss processes introduces a new consideration into electron scattering with characteristic energy loss, namely, the role of directionality of the internal propagating electron.