Electronic Spectra of Cesium Fluoride Complexes of Pentavalent Neptunium

The absorption spectra of magenta-colored CsNpF6 both as a mull of the crystalline solid and as a solution in CsF·2HF were obtained under various conditions. The room-temperature spectra were unique to the electronic energy levels of the seven Russell–Saunders states for the 5f2 configuration, split by spin–orbit coupling into 13 free-ion levels; this confirms the chemical evidence for the presence of pentavalent neptunium. At liquid-nitrogen temperatures the spectra of CsNpF6 mulls were interpreted in terms of the free-ion f–f transitions of Np5+ split by an Oh crystal field. Term assignments to the following experimental energy levels obtained at room temperature were made on the basis of the observed Stark splitting in the low-temperature spectra, and agreement with calculated energy levels (in cm−1): F23 = 4337, 3H5 = 8425, 3F3 = 8797, 3F4 = 9701, 3H6 = 13 738, 3P0 = 16 129, 1D2 = 16 736, 1G4 = 18 265, 3P1 = 19 531, 1I6 = 20 619, 3P2 = 26 667. All levels above the H43 ground state were observed except S01. A least-squares fit of these experimental energy levels was made to the Racah Coulomb interaction parameters, E1 = 2807 cm−1, E2 = 19.98 cm−1, E3 = 284.1 cm−1; the spin–orbit coupling parameter ζ = 2316 cm−1; and the configuration-interaction parameters, α = 2.7 cm−1 and β = 52.9 cm−1. The position of the 5f2 levels of Np5+ on the intermediate coupling diagram compared to the f2 levels of the isoelectronic series Pr3+, Nd4+, Th2+, U4+, and Pu6+ (in PuF6) was intermediate between U4+ and Pu6+ as expected.