Investigating the complex 6p/5f valence region of uranium with M4,5-edge valence-to-core RIXS

Actinides exhibit unique behaviour and unpredictable bonding trends thanks to a complex valence region which facilitates both 5f and 6d orbital participation in bonding, as well as, in certain cases, the "pseudo-core" 6p orbitals. Experimental access to the 6p orbitals is limited, and there have been no direct experimental observations of 6p metal-ligand bonding. We have identified that U M4,5-edge valence-to-core (V2C) resonant inelastic X-ray scattering (RIXS) at ID26 gives direct access to the 6p orbitals, showing sensitivity to their participation in bonding as well as probing the extent of 5f hybridisation with ligand valence orbitals. We propose a comprehensive V2C RIXS investigation into both 6p and 5f bonding across a series of molecular complexes including the highly covalent linear dioxo uranyl ion [O=U=O]2+, which is predicted by theory to exhibit significant 6p contributions to bonding.