Self-consistent field calculations of X-ray emission spectra of surface adsorbates: CO/Cu(100)

We have used CO adsorbed on a copper(100) surface as a critical test case for calculations of soft X-ray emission spectra of surface adsorbates. Energies and intensities are obtained from separate state self-consistent held wave functions and modelling the surface by clusters of copper atoms using effective core potentials. The X-ray intensities have also been evaluated from population analysis and from ground state frozen orbitals in order to test the validity of the local selection- (one-center) and the initial and the final state rules. The calculations show that the most important surface effect on the X-ray emission spectrum is a reordering of the outer 1 pi and 5 sigma derived bands and an addition of metal-CO binding states appearing weakly on the high energy side of the spectra. For large clusters the calculations predict the intensity for relaxation-induced X-ray transitions from the pi* band representing a local occupied part of the 2 pi-3d adsorbate-substrate bond. The one-center and limited basis set approximations are found to have the same significance for the cluster adsorbate spectra as for the free molecular spectra. The calculations indicate that, considering the current experimental resolution, the choice between the initial and final state rules is not very crucial for interpreting the main part nor the spectra, neither for the surface adsorbate system nor for the free molecule.