Auger line-shape studies of the bonding in transition metal carbonyls and nitrosyls

Abstract

The C, N, and O core-valence-valence Auger spectra of CO, NO, Co(CO)/sub 3/NO, Fe(CO)/sub 5/, and Mo(CO)/sub 6/ were theoretically analyzed in this report. These spectra are shown to have a qualitative chemical interpretation in terms of ground-state molecular orbital (MO) descriptions. The involvement of both core and valence electrons in the Auger process causes a sensitivity to the valence electron density in the immediate vicinity of the atom containing the initial core hole. As a result, the variation in relative intensity of transitions for different atomic sites in a molecule is a measure of the polarization of the orbital electron density distribution. The nitrogen and oxygen spectra of NO are far more similar than the carbon and oxygen spectra of CO, reflecting a more equal electron density distribution of the valence MO's. Another major difference between the spectra of NO and CO is the presence of peaks corresponding to transitions involving the 2..pi.. level of NO. A common fingerprint carbon Auger spectrum is obtained for the metal carbonyls that is independent of the central metal atom. Theoretical analysis of the carbonyl Auger spectra, which are directly sensitive to the local-valence charge density, indicates that ab initio Hartree-Fock calculations performedmore » with double-zeta basis sets predict an electron density that is in agreement with the experimental Auger results. The assignment of the lowest energy and highest amplitude C(ls/sup -1/) core-hole state in carbonyls (the subject of some recent debate) was checked as different Auger initial states lead to very different Auger line shapes. It is found that the adiabatically relaxed state (as opposed to a shakeup state) dominates.« less