Energy Levels of Uranium Ions in Calcium Fluoride Crystals

The crystal-field-split levels of divalent, trivalent, and tetravalent uranium ions have been studied at 20 °K in selected crystals having single charge-compensation sites. Complete calculations have been made for the free-ion energy levels of ${\mathrm{U}}^{4+}$, with the pertinent constants determined as $F_0=13\mathrm{}175$, $F_2=200\mathrm{}$, $F_4=40\mathrm{}$, $F_6=8.\mathrm{}05$, ${\zeta }_{5f}=1660\mathrm{}$, and $\alpha =26\mathrm{}$. Also calculated crystal-field-split levels of ${\mathrm{U}}^{4+}$ in $C_{3v}$ symmetry are compared to experimental values, and the various "best fit" values of the crystal-field constants determined. ESR results on trigonal Ca${\mathrm{F}}_2$: ${\mathrm{U}}^{4+}$ are correlated with predicted values derived from optical measurements. Experimental studies of the ${\mathrm{U}}^{2+}$ ion in calcium fluoride are related to the probable ${\mathrm{U}}^{2+}$ configuration, $f^{3}s$. Crystal-field splittings of the lower levels of Ca${\mathrm{F}}_2$: ${\mathrm{U}}^{2+}$ are determined from measurements of absorption and fluorescence at 20°K. The calculated value of ${\zeta }_{5f}$ for ${\mathrm{U}}^{2+}$, based on approximate $L-S$ coupling conditions for the lowest levels, is 1560 ${\mathrm{cm}}^{-1\mathrm{}}$, and is compared with a similarly calculated value for ${\mathrm{U}}^{3+}$ of 1520 ${\mathrm{cm}}^{-1\mathrm{}}$. Experimental crystal-field-split levels of tetragonal Ca${\mathrm{F}}_2$: ${\mathrm{U}}^{3+}$ are shown to fit symmetry requirements, and various laser wavelengths of Ca${\mathrm{F}}_2$: ${\mathrm{U}}^{3+}$ are related to the level system derived from absorption and fluorescence measurements. Temperature shifts of crystal-field-split levels of uranium ions are also reported.