Photoelectron spectra of transition-metal carbonyl complexes: comparison with the spectra of adsorbed CO

The uv and x-ray photoelectron spectra of carbon monoxide and transition-metal (TM) carbonyl complexes have been studied. The systematic changes in these spectra were recorded as the number of metal atoms in the complexes was increased and as the bonding configuration of the CO changed. The observations on TM carbonyl complexes are compared to the spectra of CO adsorbed on corresponding TM surfaces. We show this comparison for the electron binding energies, the shake-up spectra, the relative peak intensities, and the Auger-peak kinetic energies, as a function of the number of metal atoms in the TM carbonyl complexes. A single-metal-atom carbonyl complex reproduces nearly all of the features of the photoelectron spectra of adsorbed CO. However, there are subtle features of the photoelectron spectra of adsorbed CO, associated with molecular orbitals of CO which participate in the bonding to the substrates, which can only be reproduced by multimetal carbonyl complexes. Three or four metal atoms in a carbonyl complex are sufficient to reproduce the spectra associated with CO adsorbed on the corresponding semi-infinite TM solid.