Comparative study of CuK-edge x-ray-absorption and Cu 2px-ray photoelectron spectra in copper oxide compounds

We have measured polarization-dependent x-ray-absorption spectra (XAS) at the Cu K edge in magnetically-aligned powder samples of La₂{CuO}₄$, ${Nd}₂CuO₄, Bi₂{CuO}₄$, ${Ca}0.85Sr0.15{CuO}₂$, and ${SrCuO}₂$, all of which contain Cu coordinated by four coplanar O nearest neighbors. We have also measured Cu 2${p}3/2$ x-ray photoelectron spectra (XPS) for all except the last compound. The measurements have been analyzed in terms of unrestricted Hartree-Fock calculations for ${CuO}₆^{10{{-}}}$ and ${CuO}₁₂^{22{{-}}}$ clusters, with energies and Coulomb parameters, including interactions with the core hole, for the Cu 3${d}ₓ²$-${y}²$, 4s, and 4p orbitals determined in an ab initio fashion. It is well known that the intensity ratio of the two XPS peaks is sensitive to the degree of hybridization between Cu 3${d}ₓ²$-${y}²$ and O 2${p}_{{{σ}}}$ orbitals in the ground state. We show both by empirical correlations and model calculations that the relative shift of the continuum absorption threshold for {ε}∥z (where z is perpendicular to the plane of the CuO₄ units) and the shift of the main (3d¹⁰L) XPS peak are also useful measures of the covalency. The Cu 4pz-like quasibound state observed in XAS is sensitive as well to 4pz--O 2p hybridization. In the cluster calculations, good agreement with the experimental features is obtained with use of a single adjustable parameter, the energy of the O 2p_σ level (measured relative to Cu 3dₓ²-y²). We show that this energy varies linearly with the difference in Madelung energies for the O and Cu sites.