MANY-BODY CALCULATIONS OF THE CORE EXCITATION-SPECTRA OF CO AND NICO - DISAPPEARANCE OF THE GIANT SHAKE-UP SATELLITE

The carbon and oxygen Is core excitation spectra of free CO and NiCO are calculated by a initio 1h1p/1h1p and 2h2p/2h2p configuration interaction (CI) method using an extended basis set. We employed the ground state as well as core-hole relaxed orbitals. For free CO, we obtain a reasonably good description of the electron energy loss spectroscopy (EELS) spectra. The present interpretation of the spectra agrees with others. For NiCO, we obtain a reasonably good description of the near edge x-ray absorption fine structure (NEXAFS) spectra of the CO/Ni(100) system and that of the electron energy loss spectroscopy (EELS) spectra of the gas phase Ni(CO)4. We show the existence of the Rydberg-derived additional excited states in the NEXAFS spectra of the chemisorbed molecule and give an interpretation of these states. The disappearance of the giant shake-up satellite in the NEXAFS spectra of the adsorbate is explained in terms of the hindrance of the cooperative core-hole screening mechanism in the pi* resonantly excited state. The core-hole screening mechanism in the sigma* resonantly excited state is also investigated.