SHAPE RESONANCES IN PHOTOEMISSION FOR CO MOLECULES ADSORBED ON METALLIC SURFACES - A MODEL INCLUDING BACKSCATTERING

The photoionization differential cross section and the circular dichroism in the angular distribution (CDAD) have been calculated for the case of a theoretical model of on-top chemisorption of CO molecules on Ni(111) and Ni(100) surfaces. The results show a stability with respect to the atomic basis set used provided one includes the polarization functions and are sensitive to orthogonalization to the first σ or π valence virtual orbitals having, respectively, predominant 4s character of the Ni atom or 2π* character of the CO molecule. The backscattering of photoelectrons modifies the results only slightly. This theoretical model gives, for valence shell ionization of adsorbed CO on metals, 4σ and 5σ excitation cross section and 4σ/5σ branching ratio in agreement with experiment. For s polarization of light the σ transitions to the well known σ resonance are forbidden and we show that an unknown π shape resonance, corresponding in particular to the 5σ→π transition, can explain the experimental spectrum. This interpretation is in contradiction with the usual picture based on σ→σ resonant and 1π→π nonresonant transitions, corresponding to orthogonal electron emission directions. The selection rules for the transition of oriented-in-space molecules should explicitly incorporate the presence of two degenerate σ and π continua. We also show that the normalization procedure used in the measurements of CDAD can sometimes give spurious results. © 1995 The American Physical Society.