EXTRA-ADSORBATE SCREENING IN XPS OF C-1S CORE LEVELS OF CO/CU

Recent high resolution measurements of the XPS C 1s and O 1s core level ionization and corresponding NEXAFS excitation energies of CO/Cu adsorption complexes have revived the discussion about the nature of relaxation and screening of adsorbate core holes in XPS and NEXAFS of these systems. The presently available experimental data, as well as the theoretical estimates of the magnitude of the intra-adsorbate core hole-valence electron interactions, call for a reassessment of the efficiency of charge transfer processes in screening of C 1s core holes in weak chemisorption systems CO/Cu. Hence, we have reexamined the role which the complementary mechanism of extra-adsorbate or dynamic image screening may play in the XPS of C 1s levels in CO/Cu(100) by using a realistic surface response function of the substrate. A good agreement between the calculated and experimental spectral shapes indicates that the extra-adsorbate screening mechanism may indeed give an important contribution to the overall final state relaxation following the C Is core ionization of CO/Cu.