Ground-state interpretation of x-ray emission spectroscopy on adsorbates: CO adsorbed on Cu(100)

The application of resonant inelastic x-ray scattering and resonantly excit ed x-ray emission to adsorbates has evolved into a powerful technique to in vestigate the valence electronic structure of adsorbates in an atom-specifi c and orbital-symmetry-selective way. A surprisingly simple interpretation of spectral features in a one-electron ground-state interpretation has been found empirically. In this work c(2 x 2)CO/Cu(100) is used as a prototypic al system to investigate and rationalize the ground-state interpretation fo r adsorbates on metal surfaces, employing experimental data and ab initio c alculations in different approximations. We conclude that the observed agre ement between experiment and the one-electron ground-state interpretation c ould be due to a cancellation of dynamic con-hole effects and valence-hole relaxation. This hypothesis should be tested further by improving theoretic al techniques to include the fully relaxed valence-hole final states, not p ossible for adsorbate systems at present. An alternative interpretation is that the inelastic x-ray scattering process is a true one-step process with out the formation of a relaxed core-hole intermediate state and small diffe rential final-state effects. In any case, resonant inelastic x-ray scatteri ng and resonantly excited x-ray emission applied to adsorbates can be inter preted as an atom-specific and orbital-symmetry-selective projection of the ground-state electronic structure.