Quadrupolar transitions and medium-range-order effects in metal K-edge x-ray absorption spectra of 3d transition-metal compounds -: art. no. 033104

We have investigated the pre-edge features in x-ray absorption spectra of transition metal (TM) monoxides at the metal K edge. By comparing the calculated dipolar and quadrupolar spectra with experiments we assign the first peak at the lowest energy to a direct quadrupolar transition (due to the more effective attraction of the core hole). The following peaks in this region are mainly dipolar in character and reflect the density of states due to the medium range order as long as the radius of the cluster equals the cation-cation shortest distance plus the nearest neighbor cation-anion bond, representing a band-like effect arising from the hybridization of the 4p orbitals of central TM atoms with the 3d-based octahedral molecular orbitals of the higher-neighboring cations.