The branching ratio of core-valence transitions in electron energy-loss spectroscopy and x-ray absorption spectroscopy is linearly related to the expectation value of the spin-orbit operator of the valence states. Here, we analyze the branching ratio of the N(4,5) edges in the actinides and find that the spin-orbit sum rule gives an accurate result without the need to include the core-valence interactions. The branching ratio is not only useful to study the variations in the 5f spin-orbit interaction, it also allows us to constrain the 5f count for given angular-momentum coupling conditions.

Synchrotron-radiation-based x-ray absorption, electron energy-loss spectroscopy in a transmission electron microscope, multielectronic atomic spectral simulations, and improved first-principles calculations ͑general-ized gradient approximation in the local density approximation͒ have been used to investigate the electronic structure of the light actinides: ␣-Th, ␣-U, and ␣-Pu. It will be shown that the spin-orbit interaction can be used as a measure of the degree of localization of valence electrons in a material. The spin-orbit interaction in the light actinide metals ␣-Th, ␣-U, and ␣-Pu, has been determined using the branching ratio of the white line peaks of the N 4,5 edges, which correspond to 4d → 5f transitions. Examination of the branching ratios and spin-orbit interaction shows that the apparent spin-orbit splitting is partially quenched in ␣-U, but is strongly dominant in ␣-Pu. These results are fully quantified using the sum rule. This picture of the actinide 5f electronic structure is confirmed by comparison with the results of electronic structure calculations for ␣-Th, ␣-U, and ␣-Pu, which in turn are supported by a previous bremsstrahlung isochromat spectroscopy experiment.

Using high energy-electron energy loss spectroscopy, transmission electron microscopy, and synchrotron-radiation-based x-ray absorption spectroscopy, we provide the first experimental evidence that Russell-Saunders (LS) coupling fails for the 5f states of Pu. These results support the assumption that only the use of jj or intermediate coupling is appropriate for the 5f states of Pu. High energy-electron energy loss spectroscopy experiments were performed by use of a transmission electron microscope and are coupled with image and diffraction data; therefore, the measurements are completely phase specific.

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Photoelectron spectroscopy, synchrotron-radiation-based x-ray absorption, electron energy loss spectroscopy, and density-functional calculations within the mixed-level and magnetic models, together with canonical band theory, have been used to study the electron configuration in Pu. These methods suggest a 5f n occupation for Pu of 5 n < 6, with n = 6, contrary to what has recently been suggested in several publications. We show that the n = 6 picture is inconsistent with the usual interpretation of photoemission, x-ray absorption, and electron energy loss spectra. Instead, these spectra support the traditional conjecture of a 5f 5 occupation in Pu as is obtained by density-functional theory. We further argue, based on 5f-band filling, that an n = 6 hypothesis is incompatible with the position of Pu in the actinide series and its monoclinic ground-state phase.