High energy resolution X-ray spectroscopy at the actinide M _4,5 edges: experimental challenges and theoretical advances

Actinide systems continue to raise many unanswered questions on topics involving the number of electrons in valence states, the degree of f-electron localisation, and the character of their chemical bonding. Their partially filled 6d and 5f valence shells are responsible for most of their complex behaviour. Understanding the intricate electronic structure of actinides requires the use of advanced experimental and theoretical techniques. Among the experimental approaches, resonant inelastic X-ray scattering (RIXS) and X-ray absorption near edge structure (XANES) in the high energy resolution fluorescence detection (HERFD) mode at the actinide M4,5 edges have proven to be powerful techniques for investigating their electronic structure. Here, we review the fundamentals of these X-ray spectroscopies and the theoretical advances in electronic structure calculations using data recorded on 5f electron systems at the An M4,5 edges (An = Th, U, Np, Pu).