Approaches to HERFD XANES calculations of U M4 edge

The M4 edge XANES of actindies acquired in the High-Energy-Resolution Fluorescence Detected (HERFD) mode is very powerful since it porbes directly the 5f manifold with sufficient resolution to resolve the relevant spectral features. Theoretical calculations of such spectra are then fundamental for a correct interpretation and to guide the insight into actinides electronic structure.
We recently explored the use of a DFT-based code for X-ray spectroscopies, i.e. FDMNES,1 to calculate the M4 edge HERFD XANES of U6+ systems.2 The DFT approach is suitable to calculate closed-shell systems and our results are in good agreement with the experimental data. Our findings reveal the high sensitivity of the M4 edge to the local environment of the An ion and the suitability of the DFT approach to reproduce closed-shell systems. In particular, we were able to reproduce the spectral variations observed in 4 different U6+ systems with a local geometry from uranate to uranyl. We are currently expanding our investigation to open-shell systems, in particular to U(III) halides. For this case we are using atomic multiplet theory3 and investigating how scaling of atomic parameters affects the spectra in order to reproduce the effects observed.
We will present the results from both approaches, discuss the assignment of spectral features as emerging from the analysis of the projected density of states (DOS), underline the critical points that suggest further developments in the calculation approach.