Studies on Photoionization and Electron Trapping Process in Rare Earth Codoped Alkaline Earth Fluoride Crystals.

In crystals with fluorite structure, such as CaF _2, SrF_2, and BaF_2, the process of photoionization of divalent rare earth ion (RE^{2+ }) impurities along with the trapping of electrons by different trivalent RE^{3+} ions have been investigated. It was found that the direct transition from a localized ground state of a rare earth to the conduction band states of the host has a negligible photoionization rate; and photoionization occurs through a localized excited state of the RE ion by thermal ionization or quantum tunneling, depending on the relative energy between the excited state and conduction band. The low energy of the exciton state of Eu ^{2+} in BaF_2 accounts for the lack of photoionization of Eu ^{2+} from the E_ {rm g} band. The Sm^ {3+} and Tm^{3+} demonstrated much higher trapping ability than Ce^{3+}. Electrons can be trapped not only by uncompensated RE^ {3+} but also by the charge compensated one. Relative trapping efficiency of Sm^ {3+} in both sites was measured in the BaF_2 crystal. Further investigation on the charge compensated Sm^{2+} ion, which is stable at low temperature, reveals that the interstitial fluoride compensator can substantially reduce the photoionization threshold, and quench the luminescence of Sm^ {2+} at lower temperature. It is also confirmed from Sm^{2+} absorption and emission spectra that Sm^ {2+}:F^{-} complex has C_{rm 3v} local symmetry in BaF_2. Ce^{3+} ions in cubic sites were reduced by X-ray irradiation at 77K in both SrF_2 and BaF_2 crystals for the first time ever. Ce^ {2+} will return to trivalent state after warming sample to room temperature. When a Ce:Na:CaF _2 crystal was irradiated with x-ray, the absorption spectra of both Ce^{3+} and Ce^{2+} showed no evidence of trapping of electron by Na^ {+} compensated Ce^{3+ }. In general, this work has shown that the ionization efficiency of RE^{2+} in fluorite lattice is affected by the energy of the self -trapped exciton state relative to the localized excited state of impurity and the proximity of charge compensator. It also demonstrates that the trapping cross section of RE ^{3+} strongly depends on the position of the charge compensator and the lattice size of host as well as the chemistry of the impurity itself.