Angular-Dependence in the K Pre-Edge XANES of Cubic Crystals : The Separation of the Empty Metal e_g and t_2g States of NiO and FeO

Abstract
It is well known that by applying the angular dependence of the dipole transition matrix elements on the K XANES of single-crystal samples, the different pm-like DOS components (m = x,y,z) of the empty states can be well resolved. Analyzing quadrupole transitions in the pre-edge absorption region allows to fulfill a symmetry resolution of the different d_m-states (m = xy ,xz, yz,x²-y²,z²). This has been done for the first time at the cubic single-crystals NiO and FeO, where we could differentiate between e_g and t_2g components of the empty states using the orientation dependence of the absorption spectra. The Cu K XANES of tetragonal Bi₂Sr₂CaCU₂O₈ could be also resolved into the nondegenerated p_m- and d_m-like components. The method will be demonstrated and results for NiO, FeO and Bi₂Sr₂CaCU₂O₈ are presented and discussed.