HIGHER EXCITED-STATES IN X-RAY-ABSORPTION SPECTRA OF ADSORBATES

The development of higher excited x-ray-absorption final states from the free atom or molecule to physisorbed and chemisorbed species has been studied. These states are shown to correspond to Rydberg and multielectron excited states of the free species, modified by the interaction with the substrate. For physisorbed systems, the main influence of the substrate is a lowering of the ionization limit. The Rydberg-derived levels below the new ionization limit are only slightly perturbed relative to the gas phase, while those above are proposed to form lifetime-broadened continuum resonances. For chemisorbed systems, the adsorbate-substrate hybridization causes strong broadening of the Rydberg-derived states. These are seen to partially overlap with the pi resonance, and may even have to be considered in the description of the chemisorption bond. The multielectron states are only weakly affected by the adsorption. Due to the orientation of the adsorbed molecules, the symmetry of the multielectron states can be probed. The importance of the final state in x-ray absorption is demonstrated by the similarity between the C*O and NN* core-excited final states and NO.