Fluorescence quenching by cross relaxation in LaF3:Pr3+

Abstract

We have investigated the process of quenching of fluorescence from the ${}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ excited state of ${\mathrm{Pr}}^{3+}$ in La${\mathrm{F}}_3$ due to cross relaxation among the Pr ions. The fluorescence decay of the ensemble of Pr ions after pulsed excitation is measured as a function of temperature for Pr concentration of 20 mol%. About 12% of the ions are in nonquenching sites and contribute an exponential component to the decay. The remainder of the decay originates from the bulk of the ions in quenching sites. Cross relaxation occurs directly in steps after transfer within the inhomogeneous line. Knowledge of the dynamics occurring within the inhomogeneous line derived from fluorescence line-narrowing experiments is used to analyze the cross-relaxation process from a microscopic viewpoint based on a recent model for fluorescence in the presence of traps. At 2 K the decay is consistent with an electric dipole-dipole transfer mechanism with a nearest-neighbor quenching rate of 8.9 × ${10}^4$ ${\mathrm{s}}^{-1\mathrm{}}$ which is independent of temperature for $T<\mathrm{}32\mathrm{}$ K and which successfully predicts the observed decay rate in Pr${\mathrm{F}}_3$. The temperature dependence of the decay for $8\le T\le 32$ K is well described by a hopping model for excitation trapping. For $T>\mathrm{}35\mathrm{}$ K an activation process becomes dominant consistent with the onset of cross relaxation involving the ${}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}\mathrm{}\left(5\right)\mathrm{}$ and ${}_{}{}^3{}_{}{}^{}H_6^{}{}_{}{}^{}\mathrm{}\left(1\right)\mathrm{}$ states in which a 190-${\mathrm{cm}}^{-1\mathrm{}}$ phonon is created.