Effects of covalency on the p-shell photoemission of transition metals:: MnO

The effect of the solid-state environment for an Mn cation in MnO on the Mn 2p- and 3p-shell x-ray photoemission spectra (XPS) has been investigated using ab initio relativistic wave functions for an embedded MnO6 cluster model of MnO. These wave functions include many-body effects due to the angular momentum coupling and recoupling of the open-shell electrons. They also include the covalent mixing of the metal d orbitals with ligand p orbitals. The treatment of covalency has not been included previously in ab initio theoretical studies of the 2p-shell XPS of transition-metal complexes. In this work, covalent interactions between the metal and ligands are treated on an equal footing with spin-orbit splittings. The four-component spinors used in these wave functions are optimized separately for the ground and for the 2p- and 3p-hole configurations. This orbital relaxation leads to a "closed-shell" interatomic screening of the Mn core hole. The different orbital sets optimized for the ground and core-ionized configurations mean that mutually nonorthogonal orbital sets are used to determine the matrix elements for the XPS relative intensities. Our treatment of the transition intensities is rigorous, and no approximations are introduced to handle the orbital nonorthogonality. The closed-shell screening is important because changes in the XPS obtained for the MnO6 cluster from that obtained for an isolated Mn2+ cation can be directly linked to this screening and to the consequent reduction in the values of certain exchange integrals. The present work is compared to prior, semiempirical calculations; these comparisons allow us to identify unresolved questions about the origin of certain features of the MnO XPS and to suggest further calculations to resolve these questions.