Theory of core-hole-decay dynamics of adsorbates on metal surfaces: Role of the 2 pi * level of a chemisorbed CO molecule.

Abstract

A comparative study of core-hole-decay dynamics is made for an adsorbate on a metal surface. On the basis of the available experimental results for a CO molecule chemisorbed on Cu(110) as a prototype system, we investigate the elementary processes of the core-hole decay via various new Auger channels open for adsorbates, i.e., the participation of an electron in the initially unoccupied level of the adsorbate is populated either by charge transfer from the substrate before core-hole decay or resonant core excitation and the crossed Auger transition involving a metal electron and valence electron of the adsorbate. We calculate the deexcitation spectrum following resonant core-to-bound excitation in the cases where the excited electron may become a spectator, or participate in, or escape before the Auger decay of the core hole. For a spectator decay leading to the final state composed of two valence holes and one electron in the initially occupied and empty levels, respectively, it is demonstrated that when the final-state configuration is unstable due to the decay into the metal the resultant deexcitation spectrum exhibits resonant-enhanced photoemission whose intensity is proportional to the population of the resonance state. We also calculated the deexcitation spectra due to a participant decay (autoionization) leading to a single valence-hole state and Auger spectra involving a screening electron transferred from the metal before the Auger decay of the core hole. These spectra are compared with the direct valence photoemission, in particular with a single valence-hole state screened by charge transfer from the metal. Absence of shakeup satellites in the deexcitation spectrum following resonant core-to-bound (e.g., C 1s\ensuremath{\rightarrow}2${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ of a CO molecule) level excitation and the apparent mismatch of the binding energy of the screened final state between direct photoemission and normal Auger spectra and deexcitation spectra are explained in terms of the relaxation processes of the excited state before the Auger decay of the core hole.