Rare-Earth Infrared Lifetimes and Exciton Migration Rates in Trichloride Crystals

Abstract

Lifetimes of trivalent rare‐earth ion levels below 14 000 cm⁻¹ have been measured in the LaCl₃, GdCl₃, and YCl₃ hosts, using the infrared quantum counter and ir fluorescence techniques. Because of their low radiative and multiphonon relaxation rates, these levels are well suited to concentration‐quenching studies. Temperature dependences provide evidence that the ion‐pair decays of ${}^3{H}_6$ and ${}^3{F}_3$ in Pr³⁺ and ${}^4{I}_{\text{13/2}}$ and ${}^4{I}_{\text{15/2}}$ in Nd³⁺ occur resonantly (i.e., without phonon assistance) between excited Stark components. This enables the measured decay rates for these four levels to be used in estimating the rate for a single resonant transfer of all of the ion's energy to a neighboring ion. This latter process is the basic step of the excitation–migration process. From the lifetimes of a number of levels which are quenched by migration to “sinks,” an estimate of 4 × 10⁵ is then obtained for the number of transfers that occur before a sink is reached. Levels quenched by migration to sinks are found to decay exponentially, with a lifetime which increases with increasing crystal purity.