BACKSCATTERING OF ELECTRONS IN PHOTOIONIZATION OF AN ADSORBED CO MOLECULE

A simple physical model, based on a step potential, is developed to study the backscattering of electrons from photoionization of adsorbed molecules on surfaces. We used a two-step approach: first, the calculation of photoionization of oriented-in-space molecules, second, the introduction of backscattering as an a posteriori correction. The three valence shell ionization channels (5sigma)-1, (1pi)-1 and (4sigma)-1 were calculated in the case of CO adsorbed on a Ni(111) metallic surface. Backscattering drastically changes the (5sigma)-1 shape resonance but it is not sufficient to explain the discrepancy between theory and experiment. Chemisorption, continuum-continuum interaction and/or autoionization and breaking of the axial symmetry should be responsible for this disagreement. The (4sigma)-1 shape resonance favorably compares with the experiment. The relation between the backscattering and circular dichroism is also considered.