Relation between x-ray photoemission spectroscopy binding energies and absorption resonance energies for CO adsorbates

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Abstract
We present a comparison of x-ray photoemission spectroscopy (XPS) binding energies and x-ray absorption resonance energies (1s→2π) for various CO states, in particular adsorbates. We show that the energetic position of the bound π resonance of adsorbed CO molecules is equal for weakly chemisorbed CO on Cu(100) and on Cu(110), and that it coincides with the lowest-energy peak observed in photoemission. This is at variance with earlier results and theoretical predictions for CO adsorbates, and it is different from the results obtained for strongly chemisorbed CO species, which show a 1–2 eV lower XPS threshold energy. The new results will be discussed with emphasis on the influence of charge-transfer screening on the relation between peak positions in near-edge x-ray absorption fine-structure (NEXAFS) and x-ray-photoemission spectroscopy. Important conclusions are that only for strongly chemisorbed CO, not for CO/Cu, the lowest-energy XPS peak represents the ‘‘adiabatic’’ final state of the core-hole system, and that XPS threshold energies should not be generally taken as reference for NEXAFS peaks.