Optical methods for the evaluation of the thermal ionization barrier of lanthanide excited states in luminescent materials

We improve a recently proposed approach for the evaluation of the thermal ionization barrier of the lanthanide excited states in luminescent materials by taking into account the effect of traps and their decay time temperature dependence. We present two distinct methods, and we apply them to the case of Lu2Si2O7:Pr. To this purpose, wavelength resolved thermally stimulated luminescence and photoluminescence time decay measurements extending up to the ms time scale have been performed. In the frame of the first method, the thermal ionization barrier of the Pr3+ 5d(1) excited state has been evaluated by studying the progressive filling of traps during illumination by ultraviolet light within the 4f-5d(1) absorption band of Pr-3+ at different temperatures. The thermal ionization barrier turned out to be 0.54 +/- 0.05 eV. In the second approach this parameter has been calculated by a numerical reconstruction of the temperature dependence of the 5d(1)-4f delayed recombination decay integral in two different time windows ([p53.3 mu s -10.3 ms] and [53.3 mu s -600 s]) with the sum of contributions from different traps whose parameters have been investigated by thermally stimulated luminescence (TSL). The results obtained are in agreement with those found using the trap-filling method. The advantages and limits of both approaches have been critically exposed, in order to discuss the possibility of their extensive employment for the determination of the ionization barrier of a rare earth ion excited-state level in an insulating host.