ABSORPTION SPECTRUM OF NP4+ IN SINGLE-CRYSTAL THORIUM OXIDE

The optical absorption spectrum of Np4+ in single-crystal ThO2 has been investigated at 4°K, 77°K, and room temperature between 0.3 and 2.5 μ. Infrared spectra were recorded between 2.5 and 300 μ. Intermediate-coupling wavefunctions and eigenvalues for the ground-state electronic configuration 5f3 were generated by an IBM 360-67 program using the combined Coulombic and spin-orbit Hamiltonian. The best over-all agreement between calculated free-ion J levels and the observed spectrum of Np4+ are obtained with the Racah parameters, E1=3185, E2= 18.50, E8=323.2, and the spin-orbit parameter ζ=2009 in cm-1. A six-parameter calculation that includes the effect of configuration interaction yields the parameters E1=3160, E2= 17.59, E3=323.2, ζ=1983, α=22.44, and β= -575.9 in cm-1. Intermediate-coupling wavefunctions were used to calculate operator equivalent factors. A first-order crystal-field analysis was carried out for the six lowest J manifolds, where the separation of adjacent J levels is large compared to the crystal-field splitting of each J level. For Np4+ in an eightfold cubic environment, the crystal-field parameters B4=- 410 cm-1 and B4=50.4 cm-1 predict the positions of the crystal quantum states in good agreement with experiment.