The M4,5 edges HERFD XANES: approaches to calculations

X-ray Absorption Near-Edge Structure (XANES) in the High-Energy-Resolution Fluorescence Detected Mode (HERFD) is a very powerful technique for actinide systems. The M4,5 edges are of particular interest because they probe directly 5f states. However, before the advent of HERFD they were scarcely used because the large core-hole lifetime broadening results in broad and featureless spectra. With the improved resolution of the HERFD mode, the characterristic edge shift of different oxidation states is well resolved and several spectral features are observed in M4,5 HERFD XANES of actinides. The richness of physicochemical information coded in the spectra are hard to extract due to the complexity of XANES interpretaion and drawing conclusions on the system under study is not trivial.

In this regards, spectral calculations are fundamental for a correct interpretation. Calculating HERFD XANES on actinide systems is however particularly challenging. Relativistic effects, spin-orbit and interelectronic interactions and the influence of the atomic environment are all relevant and need to be considered in calculations. In last years, despite several approaches has been used with promising results, we still miss a theoretical framework that can address the complexity of M4,5 HERFD XANES on actinide systems.

In this presentation we will report the results we obtained with a DFT-based approach on the M4 edge of U6+ systems. Our results will be compared with works done with other approaches in order to give an overview of the level of agreement with experimental data that can be reached today. Special emphasis will be given to the investigation of covalency of 5f states.