Angle and Energy Dependent Electron Impact Vibrational Excitation of Adsorbates

Abstract

In a previous chapter in this volume, NEWNS [1] has outlined the electron scattering conditions under which image dipole theory is generally applicable, viz: scattering into a narrow angle about the specularly reflected beam direction produces electron energy losses arising from the vibrational excitation of modes that possess a net change in dipole moment perpendicular to a metal surface. In this chapter, we will be concerned with the limitations of this theory. In particular, we will investigate circumstances which lead to a break down in this “surface dipole selection rule” to give wide angle scattering from adsorbate modes vibrating parallel, as well as perpendicular, to the surface. The importance of this wide angle scattering (due to the short range part of the electron scattering potential) is that it provides detailed information on the structure of the surface adsorbate system. The number of vibrational modes, as revealed by the angle dependence of the differential inelastic scattering cross sections, relates to the point group symmetry of the adsorbate’s site and geometry [2]. That is, the cross section for any given mode should serve to distinguish between scattering from modes with displacements parallel as opposed to perpendicular to the surface plane. The impact energy dependence of the cross sections provides information concerning the scattering mechanism, as well as the adsorbate surface structure.