The apparent absence of chemical sensitivity in the 4d and 5d X-ray absorption spectroscopy of uranium compounds

Recently, X-ray absorption spectroscopy (XAS) and related derivative measurements have been used to demonstrate that the Pu 5f states are strongly relativistic and have a 5f occupation number near 5. Owing to the success in this regime, it has been argued that the XAS measurements should be a powerful tool to probe 5f occupation variation, both as a function of elemental nature (actinide atomic number) and as a function of physical and chemical perturbation, e.g., oxidation state. It will be shown here that XAS and its related measurements fail in this latter aspect for a wide variety of uranium compounds and materials. Possible causes will be discussed.