Influence of the symmetry on the circular dichroism in angular resolved core-level photoemission

The model for angular resolved photoemission from adsorbed atoms is extended to account for the non-axial symmetry of atoms in a crystal field. In particular, circular dichroism in the angular distribution (CDAD) is theoretically investigated. A special emphasis is put on the case when incident photons are propagating along the principal axis of an atom in C_nv symmetry. The model, although mainly developed for adsorbates, may also be used as a base for emission from solid surfaces. An extension to simple bulk symmetries, like D_6h for hexagonal or O_h for cubic crystals, is included. The CDAD for normal incidence does not vanish in the extended model and reflects the symmetry of the adsorption site. Scattering induced final state effects are discussed for alkali metal adsorption. A numerical calculation of the emission from the shallow 4p core level of Rb atoms adsorbed in a (√3×√3)−R30° structure on a Pt(111) surface is presented. In this case even the extended photoemission model predicts the absence of CDAD. The appearance of CDAD is only possible due to the scattering of photoelectrons from the neighbouring atoms of the solid.