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 U⁴⁺, with the pertinent constants determined as F₀=13175, F₂=200, F₄=40, F₆=8.05, ζ5f=1660, and α=26. Also calculated crystal-field-split levels of U⁴⁺ in C₃ᵥ symmetry are compared to experimental values, and the various "best fit" values of the crystal-field constants determined. ESR results on trigonal CaF₂: U⁴⁺ are correlated with predicted values derived from optical measurements. Experimental studies of the U²⁺ ion in calcium fluoride are related to the probable U²⁺ configuration, f³s. Crystal-field splittings of the lower levels of CaF₂: U²⁺ are determined from measurements of absorption and fluorescence at 20^∘{}K. The calculated value of ζ5f for U²⁺, based on approximate L-S coupling conditions for the lowest levels, is 1560 cm^-1, and is compared with a similarly calculated value for U³⁺ of 1520 cm^-1. Experimental crystal-field-split levels of tetragonal CaF₂: U³⁺ are shown to fit symmetry requirements, and various laser wavelengths of CaF₂: U³⁺ 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.