Elementary surface excitations and electron energy loss spectroscopy

Abstract

Energy loss spectroscopy of electrons reflected from solid surfaces has become a major source of information about a variety of elementary excitations of the surface. These excitations comprise plasmons of free carriers in metals and semi-conductors, interband transitions, electronic excitations of adsorbed molecules and, with high resolution spectroscopy, vibrations of adsorbates and the substrate. Our understanding of surface chemistry is greatly improved by the spectroscopy of adsorbate vibrations since decomposition reactions of adsorbed molecules on surfaces are easily analysed by the characteristic eigenmodes of the products. In many cases new and unexpected surface species have been identified. One of the advantages of electron energy loss spectroscopy as opposed to other techniques is that different scattering mechanisms may be employed which have different selection rules with respect to the symmetry elements of adsorbate complexes. Recently even surface phonon spectroscopy has become possible, and the dispersion of Rayleigh waves has been measured throughout the Brillouin zone.