Multiplet coupling and band structure in L_2,3-edge XAS through multi-channel multiple scattering theory

Using the recently developed multi-channel multiple scattering (MCMS) method we have calculated the x-ray absorption spectra (XAS) at the L_2,3-edge of transition metal compounds. The MCMS method is an ab initio scheme which combines an accurate description of the band structure of the material with a correlated many-electron wave function on the absorber atom. Thereby configuration interaction in the XAS final state, in particular multiplet effects, can be taken into account. In the present implementation, we use an electron-hole wave function and treat the interaction with all other electrons on a mean-field level. The calculated spectra agree well with experiment for the early transition metals (Ti,V), where the L2–L3 configuration mixing is particularly strong. We present a detailed study of titanium oxides with nominal Ti⁴⁺ ground state, namely rutile and anatase TiO₂ and perovskite SrTiO₃. The XAS spectra display a rich fine structure with marked differences between the three systems despite the almost identical octahedral coordination of the Ti atom. The calculated spectra are in excellent agreement with the experimental data of all three compounds. As an example for a late 3d-metal we have studied the L_2,3-XAS and the x-ray magnetic circular dichroism (XMCD) of iron. In agreement with recent time-dependent density functional studies, we find that also for the late 3d elements, the particle-hole description can yield satisfactory L3/L2 branching ratios and lineshapes of the isotropic XAS spectrum if the screened monopole term of particle-hole Coulomb interaction is properly taken account for. The XMCD spectra of the same calculation are, however, less good than both atomic multiplet and one-electron spectra indicating clear limitations for the application of particle-hole theories to late 3d elements.