Experimental and Theoretical Studies of Fine Structure in Polarized Ni K X‐Ray Absorption Spectra of NiO Single Crystals

Polarized Ni K XANES of single crystal NiO is studied experimentally and theoretically for two crystal orientations relative to the polarization and wave vectors of incident X-ray beam. The spectra calculated by the full multiple scattering method for a spin-dependent cluster potential and incomplete screening of the core hole appear to be in an excellent agreement with the experiment. The low energy pre-edge peak is shown to be caused by the quadrupole transitions of the Ni 1s electron to an exciton-type state shifted down by ∼4 eV from the empty eg high-energy conduction band under the influence of the core hole potential. The feature caused by the Ni 1s → e g band dipole transitions is also recognized in the spectrum. It is shown that the same mechanisms, which are responsible for the formation of the pre-edge fine structure in the oxides of early 3d-metals in 3d° state, work in the considered case as well. A comparative analysis outlining the significance of different mechanisms for the pre-edge structure formation in cases of early and late 3d-metal spectra is presented. The antiferromagnetic order of the absorber is taken into account within one-electron approximation.