Cooperative core hole screening mechanism in adsorbates: its hindrance in the NEXAFS spectra of the CO/Ni and N 2/Ni systems

Abstract

We introduce a new concept “cooperative core hole screening mechanism” to explain in a systematic manner the strong spectral feature changes in the XPS core hole spectra of adsorbates with variations of the adsorbate/substrate systems. The presence of the resonantly excited π* electron hinders the cooperative core hole screening mechanism of adsorbates. As examples we consider the case of the K edge NEXAFS (near edge X-ray absorption fine structure) spectra of the CO/Ni and N 2/Ni systems. We calculate the 1s core excitation spectra of NiCO and NiN 2 by ab initio 1h1p/1h1p (one hole one particle) CI (configuration interaction) and 2h2p/2h2p CI methods, using an extended basis set, employing neutral ground state as well as core hole relaxed orbitals. We obtain a reasonably good description of the NEXAFS spectra. We show that the π* resonant peak splitting for the N 2/Ni system is due to the inequivalent N atoms and that there is a reduction of the splitting energy from that of the core ionization peaks. We obtain good agreement with available data for the intensity of the resonant absorption line and its reduction from the free molecule value, and discuss the reversal of the intensity ratio between inequivalent nitrogen atoms brought about by relaxation. We show the existence of the Rydberg-derived additional excited states in the NEXAFS spectra of the chemisorbed molecules and give an interpretation of the states. The disappearance of the giant shakeup satellites in the NEXAFS spectra of adsorbates is explained in terms of the hindrance of the cooperative core hole screening mechanism in the π* resonantly excited state. The shakeup satellite features of the NEXAFS spectra as well as those of the XPS core hole spectra of adsorbates depend on the local metal ground state configuration of the metal atom at the adsorption site.