Rampant changes in 5f5/2 and 5f7/2 filling across the light and middle actinide metals:: Electron energy-loss spectroscopy, many-electron atomic spectral calculations, and spin-orbit sum rule

We examine the branching ratio of the N-4,N-5 (4d -> 5f) spectra of Th, U, Np, Pu, Am, and Cm metals using electron energy-loss spectroscopy (EELS) in a transmission electron microscope, together with many-electron atomic spectral calculations and the spin-orbit sum rule. Our results show the following. (1) The actinide metals Pu, Am, and Cm exhibit intermediate coupling. (2) The intermediate coupling values for the 5f states as calculated using a many-electron atomic model are correct for the actinides; this being proven by our results for curium. (3) The EELS branching ratio is sensitive to the degree of 5f electron delocalization, which is illustrated by the transition from LS to intermediate coupling between U and Pu.