Prediction and Interpretation of Free-Ion and Crystal-Field Spectra of the Actinides. The Intermediate Crystal-Field Diagram for 5f2 Np5+ in Octahedral CsNpF6

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Abstract
The problem of identification and characterization of the many oxidation states of the actinide elements which give rise to f–f electronic spectra is treated both for free‐ion and crystal‐field spectra. For the free‐ion, nonrelativistic self‐consistent‐field calculations on f n configurations of the actinides have yielded values for the Coulomb and spin–orbit interaction parameters from which electronic spectra may be predicted. Comparison of these calculated energy levels with the spectra observed for several actinides has been made. Crystal‐field splitting of the free‐ion levels was examined in detail for the case of 5f 2 Np 5+ in CsNpF6. A complete intermediate crystal‐field coupling diagram for this f 2 configuration ion under O h point‐group symmetry was constructed and the observed spectrum was fit to the diagram confirming the predominant octahedral structure of NpF6 −. The combined Coulomb, spin–orbit, and octahedral crystal‐field spectroscopic parameters for NpF6 − were: E 1 = 3144 , E 2 = 16.75 , E 3 = 251.9 , ζ = 2230 , B 0 4 = 1071 , and B 0 6 = 551 cm −1 . Application of these techniques to other electron configurations and point‐group symmetries is suggested.

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