PHOTOIONIZATION OF ADSORBATES ON SITES WITH NONAXIAL LOCAL SYMMETRY -AN EXAMPLE OF THE C-4V SYMMETRY GROUP

We present an analysis of the influence of the C-4 upsilon local adsor ption site symmetry on the angular distribution of photoelectrons ejec ted from adsorbed atoms or molecules. This analysis applies to photoel ectron spectra obtained with photons of any energy in the VUV (photoel ectron spectroscopy) or x-ray (XPS) regions, provided that the associa ted wave of the electron 'sees' the neighbouring surface atoms. The ad sorbate-substrate system is modelled by a cluster of C-4 upsilon symme try group consisting of an adsorbate and several atoms of the substrat e. The analysis applies also to clean surfaces of C-4 upsilon symmetry and to molecules of the C-4 upsilon symmetry group fixed in space. Th is local symmetry analysis is an alternative to models of photoelectro n diffraction and backscattering of photoelectrons from the surface. T o unravel specific symmetry properties of the system we use the expans ion 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 C-4 r otation axis are considered. Terms appearing in the analytical express ions for the differential cross section and for linear or circular dic hroism in the angular distribution of photoelectrons are either common to C-4 upsilon and axial symmetry groups C-infinity v and D-infinity h or specific to the C-4 upsilon group. The presence of the latter ter ms allows identification of the symmetry of the adsorption site and th e position of the molecule on the surface. We illustrate the analytica l expressions by modelling the CO molecule adsorbed on a Ni(100) surfa ce.