High-resolution x-ray absorption spectroscopy as a probe of crystal-field and covalency effects in actinide compounds

Applying the high-energy-resolution uorescence-detection (HERFD) mode of x-ray absorption spectroscopy (XAS) we were able for the first time to probe the crystalline electric field (CEF) splittings of the 5f shell directly in the HERFD-XAS spectra of actinides. Using ThO2 as an example, the data measured at the Th 3d edge were interpreted within framework of the Anderson impurity model. Since the charge-transfer satellites were also resolved in the HERFD-XAS spectra, their analysis revealed that ThO2 is not an ionic compound as previously believed. The Th 6d occupancy in the ground state was estimated to be as twice as much compared to that for the Th 5f states. We demonstrate that HERFD-XAS now allows for characterization of the CEF interaction and degree of covalency in the ground state of actinide compounds as it is extensively done for 3d transition metal systems.