X-ray Absorption Linear Dichroism at the Ti K-edge of TiO2 anatase single crystals
Anatase TiO2 (a-TiO2) exhibits a strong X-ray absorption linear dichroism with the X-ray incidence angle in the pre-edge, the XANES and the EXAFS at the titanium K-edge. In the pre-edge region the behaviour of the A1-A3 and B peaks, originating from the 1s-3d transitions, is due to the strong $p$-orbital polarization and strong $p-d$ orbital mixing. An unambiguous assignment of the pre-edge peak transitions is made in the monoelectronic approximation with the support of ab initio finite difference method calculations and spherical tensor analysis in quantitative agreement with the experiment. It is found that A1 is mostly an on-site 3d-4p hybridized transition, while peaks A3 and B are non-local transitions, with A3 being mostly dipolar and influence by the 3d-4p intersite hybridization, while B is due to interactions at longer range. Finally, peak A2 which was previously assigned to a transition involving pentacoordinated titanium atoms exhibits a quadrupolar angular evolution with incidence angle. These results pave the way to the use of the pre-edge peaks at the K-edge of a-TiO2 to characterize the electronic structure of related materials and in the field of ultrafast XAS where the linear dichroism can be used to compare the photophysics along different axes.