Optical study of Yb3+/Yb2+ conversion in CaF2 crystals

Abstract

Yb3+ ions with various site symmetries have been observed in the absorption and emission spectra ofYb3+:CaF2 crystals,both γ-irradiated and annealed in hydrogen. The absorption intensity value is found to be much higher forthe γ-irradiated crystal and strongly dependent on the gamma dose. The UV absorption spectra ofγ-irradiatedand H2-annealedCaF2:5 at.%Yb3+ crystals arequite similar. Yb2+ absorption bands are observed at 360, 315, 271, 260, 227 and 214 nm,which are called A, B, C, D, F and G bands, respectively. Forγ-irradiatedCaF2:30at.% Yb3+, an additional band at 234 nm can be seen. It is suggested that only a negligible amount ofYb3+ ions are convertedinto Yb2+ under theγ-irradiation.The presence of Yb2+ is confirmed by the 565 and 540 nm luminescence under 357 nm excitation. Itis also suggested that the excitation in the A, C, D and F absorption bands ofYb2+ gives rise tophoto-ionization of Yb2+ ions and electrons in the conduction band to form the excitedYb3+ ions whichemit IR Yb3+ luminescence.The UV absorption and emission spectra obtained forγ-irradiatedand H2-annealedcrystals have different structures. This suggests that different mechanisms are responsible for the creationof Yb2+ ions.γ-irradiation favoursYb2+ isolated centresby reduction of Yb3+ ions located at Ca2+ lattice sites, whereas annealing in hydrogen favoursYb2+ centresneighbouring Yb3+ ions when a Yb3+ ion pair captures a Compton electron. Also,γ-irradiation does notchange the position of Yb3+ ions converted into Yb2+ in the CaF2 lattice.In the case of H2 annealing, a Yb3+ ion converted to Yb2+ is shifted to the Ca2+ position in the lattice.