Multielectron excitations in high-energy photoelectron spectra of CO adsorbed on Ni(100)

High-resolution x-ray-excited core-level shake-up and valence-electron spectra of CO adsorbed on Ni(100) in the c(2×2) structure are presented. A number of newly resolved structures are reported. The C 1s and O 1s shake-up spectra show pronounced differences. The shake-up transitions are treated as local molecular excitations in the presence of the core hole. It is proposed that transitions from the 2${\pi }^{\mathrm{*}}$ screening orbital into 3p and other higher states with Rydberg character give rise to the most intense shake-up peaks in adsorbed CO. In the C 1s spectrum a low-energy satellite is identified as an excitation from bonding to antibonding 2${\pi }^{\mathrm{*}}$-3d combinations. The high-energy shake-off continua reveal several broad structures which are interpreted as autoionizing or shape resonances in the shake-off continua. These features are compared with thresholds observed in photon-stimulated-ion-desorption spectra for adsorbed CO. The difference spectrum between the valence region of Ni with and without adsorbed CO reveals several features over an energy range of 40 eV. The 3σ state and the correlation satellites are compared with features in the autoionizing spectra at the core-level thresholds.