PNNL

Abstract

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 abinitio relativistic wavefunctions for an embedded MnO6 cluster model of MnO. These wavefunctions 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 4-component spinors used in these wavefunctions are optimized separately for the ground and for the 2p and 3p-hole configurations. This orbital relaxation leads to a “closed-shell” inter-atomic screening of the Mn core-hole. The different orbital sets optimized for the ground and core-ionized configurations mean that mutually, non-orthogonal 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 non-orthogonality. 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, semi-empirical 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.