The role of Eu++ aggregation on optical absorption, emission, luminescence time decay, and ITC plots in NaCl:Eu++

The NaCl: Eu system is investigated by using optical techniques such as absorption, luminescence, and luminescence decay times complemented by a dielectric technique such as ionic thermocurrents (ITC). The behaviour of samples as-received (where large amount of aggregates are present) is compared with that of quenched samples (where in principle only IV dipoles are present). Furthermore by annealing quenched samples, at a well defined temperature (333 K), a controlled clustering is obtained: the early clusters turn out to be dimers from the analysis of the clustering kinetics. At remarkable steps of the clustering process emission spectra and luminescence time decay scheme are measured. Regardless of the complexity of the Eu++ energy level scheme and of defects in which Eu is involved the luminescence time decay is characterized by a unique time decay constant, which, as a rule, becomes shorter and shorter as the aggregation process is going on. The possible models and mechanisms responsible for the aggregation induced shortening of the luminescence decay time, its temperature, and emission wavelenght dependence are discussed. Le systeme NaCl:Eu a ete etudie avec des techniques optiques comme absorption, luminescence et temps de decroissance de la luminescence. Elles sont completees par une technique dielectrique comme les courants thermo-ioniques. Le comportement des echantillons “as received” (dans lesquelles il y a la presence de grandes quantites d'agregats) a ete rapproche avec ce des echantillons “quenched”, dans lesquelles theoriquement il y a seulement des dipǒles I.L. Encore en ayant traite des echantillons apres un “quenching” a une temperature de 333 K on a obtenu une agregation contrǒlee. Les premiers agregats semblent etre des dimeres de l'analyse des cinetiques d'agregation. Aux quelques points importants du procede d'agregation on a mesure des spectres de luminescence et le scheme de la decroissance de la luminescence měme. Malgre la complexite du scheme des niveaux du ion Eu++ et des defauts qui le contiennent, la decroissance de la luminescence est caracterisee par une vie moyenne unique qui devient toujours plus breve lorsque augmente le proces d'agregation. Les modeles possibles et les mecanismes qui sont responsables de la reduction de la vie moyenne, de sa dependence de la temperature et de la longueur d'ondes sont discutes.