X-ray spectroscopy has become a powerful tool for the analysis of the electronic structure of actinides, based upon the original ground breaking work of (1) Veal and coworkers using X-ray Photoelectron Spectroscopy (XPS); (2) Naegele and others employing the lower energy Photoelectron Spectroscopy (PES) and references therein; (3) Baer et al. utilizing Bremstrahlung Isochromat Spectroscopy (BIS); (4) Chauvet and Baptist, with the lower energy Inverse Photoelectron Spectroscopy (IPES); and Kalkowski, Kaindl, Brewer and Krone, who did a pioneering X-ray Absorption Spectroscopy (XAS) study.

A particularly effective approach has been to measure the Branching Ratios (BR) in the N_4,5 X-ray Absorption Spectroscopy, where BR = I_4d5/2 /(I_4d5/2 + I_4d3/2), I_x being the intensity of the respective level x. (Figure 1) The combination of strong electric dipole selection rules in the d to f transitions and a firm theoretical foundation for localized systems provided by van der Laan and Thole has permitted the determination of the quantity of 5f electrons (n) in a number of localized actinide systems. While this may seem trivial, it was in fact a topic of serious disagreement for a number of years. For example, early on there was a viewpoint that championed 6d filling instead of 5f filling and the reconciliation between the accepted n = 6 in atomic Pu and n = 5 in solid Pu was only realized completely in the recent past. Nevertheless, there remained a very troublesome “fly in the ointment”: the indistinguishability of the BR for the localized n = 2 U systems and the delocalized n = 3 of uranium metal, as shown in Figure 2. This issue was first reported by Kalkowki, Kaindl, Brewer and Krone in 1987 and addressed more recently elsewhere.

From other measurements and analyses, it has long been accepted that α-U has three 5f electrons and that there is significant delocalization in the 5f states. An example of this is shown below in Figure 3. In Figure 3, the BIS of Baer et al. is compared to the calculations of Kutepov. The agreement between the BIS and the Unoccupied density of states (UDOS) derived from Kutepov’s calculations is very good. Moreover, the underlying j-specific calculations demonstrate the admixture of the 5f_5/2 and 5f_7/2 states, consistent with strong delocalization.

The essence of the problem is that the BR approach works very well for localized systems but cannot rule out the possibility of delocalization with a slightly different number of 5f electrons. However, there now appears to be a solution to this “blind spot.” Recently, it has been shown that the M_4,5 X-ray Emission Spectroscopy of UF₄ exhibits results consistent with the Intermediate Coupling Model of van der Laan and Thole. Normalizing to the 6p → 3d peaks, it can be seen that there is a tremendous differential in the intensities of the 5f peaks in the two spectra, a factor of 5. Figure 4 (The X-ray Emission Spectroscopy (XES) results for UO₂ are very similar to those of UF₄ and are omitted here.) It has been shown that this is consistent with the almost pure n_5/2 nature of the two 5f electrons in UF₄ and strong, albeit not complete, adherence to electric dipole select rules. It will be demonstrated here that this type of data will provide (1) a direct pathway to the experimentally driven discernment of the localized n = 2 and delocalized n = 3 cases in particular and (2) the degree of delocalization in 5f systems in general.