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

Yb3+ ions with various site symmetries have been observed in the absorption and emission spectra of Yb3+ :CaF2 crystals, both gamma-irradiated and annealed in hydrogen. The absorption intensity value is found to be much higher for the gamma-iffadiated crystal and strongly dependent on the gamma dose. The UV absorption spectra of gamma-iffadiated and H-2-annealed CaF2:5 at.% Yb3+ crystals are quite 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. For gamma-irradiated CaF2:30 at.% Yb3+, an additional band at 234 nm can be seen. It is suggested that only a negligible amount of Yb3+ ions are converted into Yb2+ under the gamma-irradiation. The presence of Yb2+ is confirmed by the 565 and 540 nm luminescence under 357 nm excitation. It is also suggested that the excitation in the A, C, D and F absorption bands of Yb2+ gives rise to photoionization of Yb2+ ions and electrons in the conduction band to form the excited Yb3+ ions which emit IR Yb3+ luminescence. The UV absorption and emission spectra obtained for gamma-irradiated and H-2-annealed crystals have different structures. This suggests that different mechanisms are responsible for the creation of Yb 2, ions. gamma-irradiation favours Yb2+ isolated centres by reduction of Yb3+ ions located at Ca2+ lattice sites, whereas annealing in hydrogen favours Yb 2, centres neighbouring Yb 3, ions when a Yb3+ ion pair captures a Compton electron. Also, gamma-irradiation does not change the position of Yb 3, ions converted into Yb 2, in the CaF2 lattice. In the case of H-2 annealing, a Yb3+ ion converted to Yb2+ is shifted to the Ca2+ position in the lattice.