Photoluminescence characteristics of Pr3+ in ThO2: interplay of defects in a photo-induced charge transfer

Photo-luminescence studies of Pr3+ activated thorium oxide phosphor have revealed that mainly 3P→3H4 and 1D2→3H4 transitions with life-time of 30 and 600 μs are observed in this sample. An exponential reduction in the emission intensity of Pr3+ ions was observed on following continuous excitation with 275 nm corresponding to the f–d transition band of Pr3+ ions. Such a reduction in emission intensity was observed at all temperatures investigated in the range 90–330 K. The emission intensity recovered partially on dark storage only above 180 K. The recovery of emission intensity was also observed on the illumination of pre-exposed sample to light in the wavelength region 300–430 nm. Following illumination with 275 nm, Pr3+ activated thorium oxide phosphor has displayed a weak thermally stimulated luminescence. These results thus suggest that the optical excitation dynamically changes the state of the system under observation, and that changes are occurring in the valence state of Pr ions due to e/h transfer process on 275-nm exposure. On dark storage and also on 365-nm illumination of the pre-exposed sample, e/h traps recombine to cause emission signal recovery. The analysis of data on reduction in intensity obtained with exposure to 275 nm suggests the likelihood of the of Pr3+ ions existing at three different sites. The activation energies associated with the release of electrons from excited Pr3+ ions at different sites were determined from the temperature dependence of the photo-induced charge transfer process.