Fluorescence decays and phonon processes in the excited states of fluorescent ions

Abstract

Consider a three-level fluoresent system with levels 1, 2 and 3 indicating the ground state and two excited (metastable) states, respectively. Assume that the system can be optically excited by pumping into an absorption band above level 3 and that the excited centers decay from this band to level 3 by fast radiationless processes. When excited by a fast pulse of light levels 2 and 3 will decay with patterns which depend on the populations at the end of the pulse and the various transition probabilities. These probabilities include phonon processes and are dependent on the temperature of the crystal and the coupling between fluorescent system and host lattice. The conditions under which the excited levels may decay with the same pattern or with such patterns as pure exponentials, fluorescence rise followed by decay, double exponential, are examined. The decays of Cr3+ and Sm2+ in several host lattices and in the (4÷700) °K region are presented and the variations of thetype of pattern are related to the responsible phonon processes. The evidence of upward, phonon annihilation processes in the decay patterns of the5D0 and5D1 states of BaClF:Sm2+ is also presented.