INITIAL AND FINAL-STATE RULES IN X-RAY SPECTROSCOPIES OF ADSORBATES

The so-called ''initial and final state rules'' for X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) are applied to CO on Ni(1 0 0). In XAS the final state core hole strongly modifies the distribution of unoccupied valence states from that of the ground state, whereas in XES the final state reflects the ground state (without core hole) occupied density of states. The intensities of the X-ray transitions are related to the electronic state occupancy in the initial state. This is used to derive electronic state populations from XAS since the initial state corresponds to the ground state.