Dipole and quadrupole contributions to polarized Cu K x-ray absorption near-edge structure spectra of CuO

Polarized Cu K x-ray absorption near-edge structure spectra of CuO are measured and analyzed with the aim of extracting quadrupole and dipole partial spectral components. Theoretical spectral components are calculated within the framework of real-space multiple-scattering technique, relying on a non-self-consistent muffin-tin potential. A local coordinate system x^′y^′z^′ suitable for polarization analysis is defined by the sides of the CuO₄ quasirectangle rather than by directions of the Cu-O bonds. An exclusively quadrupole nature of the prepeak is established both experimentally and theoretically. By analyzing the experimental data, we find that those states which give rise to the prepeak lie within the CuO plane, and have mainly a d_x^′y^′ character. The theory correctly reproduces gross features of the polarized structure generated by dipole transitions, apart from a spurious peak at 8983 eV for the p_z^′ component. A partial agreement between theory and experiment in the shoulder region suggests that this structure may arise partly from one-electron states and partly from many-body processes.