Excitation Dispersion from Inelastic Low-Energy Electron Diffraction

Abstract

Combined inelastic scattering and diffraction (inelastic diffraction) provide a means of observing low-energy electrons which have produced electronic excitation in bulk single crystals. In principle, the energy-versus-momentum dispersion of the excitations can be inferred, provided the momentum supplied by the lattice can be determined. The present work shows the feasibility of extracting dispersion data from inelastic diffraction measurements. A close analogy is found between inelastic and elastic diffraction (LEED) intensities at normal and at varying incidence, respectively. This identifies the diffracting reciprocal-lattice element and its momentum contribution in the inelastic case. Dispersion data in the 11 azimuth of a tungsten (110) surface are compared with the free-electron volume and surface-plasmon dispersion relations. Discrepancies are found indicating departure from free-electron-like behavior. Also, the excitation which has been previously assigned to a surface plasmon fails to show the appropriate dispersion characteristics. Inner potential corrections are discussed, and the equivalence of the inner potential for elastic and inelastic diffraction is pointed out.