Physical aspects of relaxation and shake-up effects in x-ray photoemission spectroscopy and core →2π/emph>absorption spectra of CO chemisorbed on Ni(111)

The physical origin of the peculiar relaxation shifts and spectral shapes appearing in x-ray-induced core-to-valence excitation and core-level photoemission spectra (XPS) of CO chemisorbed on Ni(111) are discussed and interpreted within a unified framework. Within the model presented, the electronic transitions in core-to-valence excitation spectroscopy and XPS are shown to give rise to drastic electronic rearrangements within the adsorption system and to the charge shake-up in the CO 2${\pi }^{\mathrm{*}}$ derived resonance partly filled via the back-donation mechanism. Such singular relaxation processes, common to both spectroscopies, are closely related and can be treated on the same footing. This makes it possible to establish unified relaxation shifts and spectral characteristics for two seemingly different experimental situations. The use of this formalism in analyzing the experimental data enables one to estimate and distinguish between the extra-adsorbate (image or nonbonding) and intra-adsorbate (chemically induced) screening of the core holes created either by x-ray-induced core-to-valence electronic transitions or core-level photoionization in CO/Ni(111).