Advanced X-Ray Spectroscopic Tools for Probing 5F Electron Participation in Actinide Bonding

Abstract The question of the participation of actinide (An) 5f and 6d electrons in bond covalency, and their role in the formation, bond stability, and chemical reactivity, as well as how this participation changes across the actinide series, is controversially discussed and a hot topic in actinide chemistry. We obtained advanced understanding by developing and applying state-of-the-art experimental and theoretical tools for in-depth studies of actinide-ligand bond covalency in various actinide systems.
We will discuss how An M4,5 edge core-to-core resonant inelastic X-ray scattering (CC-RIXS), high energy resolution X-ray absorption near edge structure (HR-XANES) and valence band-RIXS (VB-RIXS) spectroscopic tools can be useful to obtain advanced insights into the An-ligand chemical bonding and bond covalency.1, 2 An M4,5 edge CC-RIXS and VB-RIXS are sensitive to the “effective“ 5f electron occupation numbers. The former counts the number of 5f open-shell electrons that is localized on the actinide ion and constitutes a large part of the effective occupation number, and the latter determines the degree of delocalization of the 5f orbitals as results of donation effects to the surrounding ligands (i.e. ... mehr5f participation in the formation of covalent bonds). We will show that when combining HR-XANES, CC-RIXS and VB-RIXS experimental techniques and computations, a precise characterization of 5f valence electron density with different level of localization in a chemical bond is possible and can be used to benchmark theoretical approaches.1, 2 Development of two new experimental tools based on a satellite peak in An M4 edge CC-RIXS spectra for the characterization of the 5f electrons on the actinide atoms will be discussed.1 It will be demonstrated that the intensity of the satellite peak is sensitive to the number of 5f localized electron density on the An atom when measured at 90° scattering geometry. Moreover, it will be shown that the intensity of this satellite peak increases and probes variations in bond covalency when recorded in near backscattering geometry. This development was possible by CC-RIXS study of 21 different U, Np. Pu and Am compounds in solid and liquid state and novel quantum chemical computations based on multiplet ligand/crystal field theory and ligand field density function theory (LFDFT).1
References
[1] Schacherl, B.; Tagliavini, M.; Kaufmann-Heimeshoff, H.; Göttlicher, J.; Mazzanti, M.; Popa, K.; Walter, O.; Pruessmann, T.; Vollmer, C.; Beck, A.; Ekanayake, R. S. K.; Branson, J. A.; Neill, T.; Fellhauer, D.; Reitz, C.; Schild, D.; Brager, D.; Cahill, C.; Windorff, C.; Sittel, T.; Ramanantoanina, H.; Haverkort, M. W.; Vitova, T. (2025). Resonant inelastic X-ray scattering tools to count 5 f electrons of actinides and probe bond covalency. Nat Commun 16: 1221
[2] Bajaj, A. R., H.; Schacherl, B.; Schenk, S.; Prüßmann, T.; Tasi, A.; Fellhauer, D.; Thompson, A.; Terry, J.; Wang, X.; Zurek, E.; Vitova, T.; S. Bagus, P.; Autschbach, J. (2025). Actinide 5f Occupations: The Case of PuO2. ChemRxiv, doi:10.26434/chemrxiv-2025-72jhl