Optical observation of the kinetics of thermalization between the 3P0 and 3P1 excited states of Pr3+ in symmetrical Pr3+-Gd3+ pairs in CsCdBr3

The time dependence of thermalization between the P-3(0) and P-3(1) electronic states of Pr3+ in symmetrical Pr3+-Gd3+ pairs in CsCdBr3, following pulsed laser excitation into either state, is reflected in the time dependence of the luminescence from both states. The P-3(0) and P-3(1) states achieve thermal equilibrium in the microsecond time domain over the temperature range of study (215-340 K). Because the P-3(0)-P-3(1) energy gap is larger than the phonon cutoff in CsCdBr3, thermalization occurs via multiphonon processes. A rate-equation model for the thermalization process is presented and the temperature dependence of the rate constants for P-3(1) <-- P-3(0) multiphonon absorption and P-3(1) --> P-3(0) multiphonon emission is reported from 215-340K. In contrast to CsCdBr3, the analogous thermalization kinetics in Pr+-Gd3+ pairs in isostructural CSMgCl3 is not discernable in the P-3(0) and P-3(1) luminescence, because thermalization is instantaneous within the time resolution of our experiments (similar to20 ns). The difference in the thermalization kinetics in the two lattices is attributed to the difference in the number of phonons required to bridge the P-3(0)-P-3(1) I energy gap. (C) 2004 Elsevier B.V. All rights reserved.