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 ${\text{5f}}^3$ 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, $E^1\text{\hspace{0.17em}=\hspace{0.17em}3185}$ , $E^2\text{\hspace{0.17em}=\hspace{0.17em}18.50}$ , $E^3\text{\hspace{0.17em}=\hspace{0.17em}323.2}$ , and the spin–orbit parameter $\text{ζ\hspace{0.17em}=\hspace{0.17em}2009}$ in cm⁻¹. A six‐parameter calculation that includes the effect of configuration interaction yields the parameters $E^1\text{\hspace{0.17em}=\hspace{0.17em}3160}$ , $E^2\text{\hspace{0.17em}=\hspace{0.17em}17.59}$ , $E^3\text{\hspace{0.17em}=\hspace{0.17em}323.2}$ , $\text{ζ\hspace{0.17em}=\hspace{0.17em}1983}$ , $\text{α\hspace{0.17em}=\hspace{0.17em}22.44}$ , and $\text{β\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}575.9}$ 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 $B_4\text{\hspace{0.17em}=\hspace{0.17em}−\hspace{0.17em}410}{\mathrm{cm}}^{\text{−1}}$ and $B_6\text{\hspace{0.17em}=\hspace{0.17em}50.4}{\mathrm{cm}}^{\text{−1}}$ predict the positions of the crystal quantum states in good agreement with experiment.