Impurity and γ-Stimulated Effect of Increasing the Transparency of CaF2 Crystals in the Fundamental Edge Region

Abstract

A comparison of absorption spectra revealed activator rare-earth ions at which the effect of impurity and radiation-induced decrease in the optical density at a wavelength of the short-wavelength edge of CaF₂ was observed; this effect is explained by the shift of the interband absorption of the crystal to the spectral vacuum UV region. Experiments showed that a multiple increase in the transparency of CaF₂ in the wavelength range of 190–200 nm occurs when doping crystals with ions of europium and ytterbium—elements having the lowest chemical potentials in the lanthanide series. This feature determines the ability of Eu³⁺ and Yb³⁺ to charge conversion of the oxidation state (+3 → +2). The absorption spectra of these ions were compared with the spectra of Nd³⁺ and Er³⁺ ions, which do not exhibit any changes in charge upon γ irradiation of CaF₂ crystals and have chemical potentials comparable in magnitude with those of other elements of the series, for which this parameter is 2–5 times larger than for Eu and Yb. The spectral features of the absorption of gadolinium ions were additionally investigated in a simple CaF₂ : 2 mol % GdF₃ crystal and a solid solution based on CaF₂ with addition of 10 mol % GdF₃ with the empirical formula Ca_0.9Gd_0.1F_2.1. A peculiarity of Gd is that, in contrast to other rare-earth elements, its ground energy 4f levels in the tri- and divalent states are located, respectively, on the top of the valence band and above the bottom of the conduction band. CaF₂ : 0.1 mol % Eu²⁺ crystals are proposed as an optical medium almost completely transparent in the wavelength range of 190–200 nm. The revealed effect is of interest for designing optical materials for systems of near-Earth and space monitoring for environmental and military purposes, as well as for the production of lenses applied in UV microphotolithography systems.