TM-YB-TM ENERGY TRANSFERS AND EFFECT OF TEMPERATURE ON THE FLUORESCENCE INTENSITIES IN OXYFLUORIDE TELLURITE COMPOUNDS

Up-conversion processes in PbF2+TeO2+WO3 glass-ceramics doped with Tm3 + and codoped with Tm3+ and Yb3+ ions were investigated under 680 nm e xcitation. Emission, excitation and time-resolved spectra and decay pr ofiles were measured at temperatures between 12 and 300 K. A strong bl ue emission centred at 478 nm originating from the (1)G(4) level of Tm 3+ was observed in a doubly doped sample via a two-step energy transfe r mechanism, (Tm) F-3(4) --> (Yb) (2)E(5/2) energy transfer and a posi tive feedback (Yb) F-2(5/2) --> (Tm) (1)G(4) process, raising the exci ted Tm3+ ion from the F-3(4) to the (1)G(4) level following the absorp tion of 680 nm excitation light. The time-resolved emission spectra pr ove the validity of these energy transfer processes. The optical prope rties of the excited levels and the energy transfer processes involved in the up-conversion were interpreted using a kinetic model describin g the interactions between rare-earth ions in terms of energy migratio n and luminescence quenching. The microscopic interaction parameter fo r Tm-Tm intersection in the absence of Yb3+ ions was determined to be 6.1 x 10(-40) cm(6) s(-1) in the sample having 1.0 mol% TmF3. The same parameter for the energy transfer from Tm to Yb was calculated to be 1.2 x 10(-38) cm(6) s(-1) in the sample having 0.15 mol% TmF3 and 15 m ol% YbF3. No effect of temperature on the emission intensity of the (T m) F-3(4) level was observed although the emission intensities of the (Tm) (1)G(4) and (Yb) F-2(5/2) levels were decreased with increasing t emperature. The former decrease accounts for the migration of energy b etween Yb3+ ions and controls the temperature dependence of the blue-u p conversion emission.