X-Ray absorption spectroscopy at the L2,3 edges of transition metal oxides by relativistic time-dependent density functional calculations

Theoretical simulation of the electronic spectra needs for computational approaches capable to correctly describe the excited states of the system. The time dependent density functional theory (TDDFT) allows the treatment of the electronic excited states and its relatively computational economy makes it a good candidate to treat large systems. X-Ray absorption Spectroscopy (XAS) has proven to be a powerful local probe technique for the investigation of both electronic and geometric structure of condensed matter. The computational strategy adopted to describe the XAS spectra of solid samples is based on the TDDFT method, properly extended to the computation of core electron excitations, and the finite size cluster model for the simulation of the bulk. Cluster model appears suitable for the calculation of core excitations since the excited electron is localized near the core hole therefore the description of a limited region would be sufficient to describe it accurately. Here we present an application of the TDDFT core scheme at the relativistic spin-orbit (SO) level to the simulation of the XAS spectra of bulk transition metal oxides V2O5 and TiO2 at the V and Ti L2,3 edges. It is well known that the SO coupling plays an important role in the spectral shapes of L3 and L2 edges of transition metal oxides and strongly influence their intensity distribution, giving rise to distinct spectral features converging to the L3 and L2 ionization thresholds. Calculations at this level of accuracy of the 2p metal edge absorption spectra of solid systems represent a novelty for condensed systems. The very good agreement between calculated and experimental features show the reliability of the method to take into account the complex effects involved in the spectroscopy of transition metal compounds where the correlation effects are important and the spin-orbit coupling large.