Photoionization of adsorbates on sites with non-axial local symmetry. An example of the symmetry group

Abstract

We present an analysis of the influence of the local adsorption site symmetry on the angular distribution of photoelectrons ejected from adsorbed atoms or molecules. This analysis applies to photoelectron spectra obtained with photons of any energy in the VUV (photoelectron spectroscopy) or x-ray (XPS) regions, provided that the associated wave of the electron `sees' the neighbouring surface atoms. The adsorbate - substrate system is modelled by a cluster of symmetry group consisting of an adsorbate and several atoms of the substrate. The analysis applies also to clean surfaces of symmetry and to molecules of the symmetry group fixed in space. This local symmetry analysis is an alternative to models of photoelectron diffraction and backscattering of photoelectrons from the surface. To unravel specific symmetry properties of the system we use the expansion of continuous spectrum wavefunctions in symmetry-adapted harmonics instead of expansion in spherical harmonics. The particular cases of circularly and linearly polarized light beams incoming along the rotation axis are considered. Terms appearing in the analytical expressions for the differential cross section and for linear or circular dichroism in the angular distribution of photoelectrons are either common to and axial symmetry groups and or specific to the group. The presence of the latter terms allows identification of the symmetry of the adsorption site and the position of the molecule on the surface. We illustrate the analytical expressions by modelling the CO molecule adsorbed on a Ni(100) surface.