Threshold electronic structure at the oxygen K edge of 3 d -transition-metal oxides: A configuration interaction approach

It has been generally accepted that the threshold structure observed in the oxygen K-edge x-ray absorption spectrum in $3d$-transition-metal oxides represents the electronic structure of the $3d$ transition metal. There is, however, no consensus about the correct description. We present an interpretation which includes both ground-state hybridization and electron correlation. It is based on a configuration-interaction cluster calculation using an MO₆ cluster. The oxygen K-edge spectrum is calculated by annihilating a ligand hole in the ground state, and is compared to calculations representing inverse photoemission experiments in which a $3d$ transition-metal electron is added. Clear differences are observed related to the amount of ligand holes created in the ground state. Two ``rules'' connected to this are discussed. A comparison with experimental data of some early transition-metal compounds is made, and shows that this simple cluster approach explains the experimental features quite well.