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 excited x-ray emission to adsorbates has evolved into a powerful technique to investigate the valence electronic structure of adsorbates in an atom-specific 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(2×2)CO/Cu(100) is used as a prototypical system to investigate and rationalize the ground-state interpretation for adsorbates on metal surfaces, employing experimental data and ab initio calculations in different approximations. We conclude that the observed agreement between experiment and the one-electron ground-state interpretation could be due to a cancellation of dynamic core-hole effects and valence-hole relaxation. This hypothesis should be tested further by improving theoretical techniques to include the fully relaxed valence-hole final states, not possible for adsorbate systems at present. An alternative interpretation is that the inelastic x-ray scattering process is a true one-step process without the formation of a relaxed core-hole intermediate state and small differential final-state effects. In any case, resonant inelastic x-ray scattering and resonantly excited x-ray emission applied to adsorbates can be interpreted as an atom-specific and orbital-symmetry-selective projection of the ground-state electronic structure.