Counting the 5f electrons of the actinides

The actinides have a complex electronic structure and, as a result, are amongst the elements with the least understood chemical and physical properties in the periodic table. The occupation and role in covalency of the 5f and 6d orbitals is central to understanding the electronic structure of actinides. Advanced experimental tools, able to obtain deep insights into the electronic structure and binding properties of the actinides, are highly desirable. Here, we present two highly sensitive spectroscopic tools. The first is capable of ‘counting’ the number of 5f electrons localized on an actinide element. The second is sensitive to the level of covalent character of the actinide-ligand bonding. Both tools are based on the multiplet structure present in actinide M4 edge core-to-core resonant inelastic X-ray scattering (CC-RIXS) maps. The spectral intensity of different many-body final-state multiplets directly depends on the local many-electron ground-state symmetry including the local 5f spin configuration and electron occupation. By comparing U M4 edge CC-RIXS data for total of 21 U, Np, Pu and Am compounds, we theoretically and experimentally demonstrate how this type of spectroscopy can be used to compare the number of 5f electrons across the actinide series or bond covalency for different ligands bound to one actinide element.