Trapping of optical excitation by two types of acceptors in ${\mathrm{La}}_{0.72}$${\mathrm{Pr}}_{0.25}$${\mathrm{Nd}}_{0.03}$${\mathrm{F}}_3$

We have investigated the process of quenching of fluorescence from the ${}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ state of ${\mathrm{Pr}}^{3+}$ in ${\mathrm{La}}_{0.72}$ ${\mathrm{Pr}}_{0.25}$ ${\mathrm{Nd}}_{0.03}$ ${\mathrm{F}}_3$ due to cross relaxation to Pr and Nd ions. The fluorescence decay of the ensemble of Pr ions after pulsed excitation is measured as a function of temperature in the (2-70)-K region. Cross relaxation occurs either directly or through a donor-donor transfer within the inhom-geneous line. Knowledge of the dynamics occurring within the inhomogeneous line derived from previous fluorescence line-narrowing experiments is used to analyze the cross-relaxation process from a microscopic viewpoint according to models for fluorescence decay in the presence of traps. In general the decay is consistent with an electric dipole-dipole transfer mechanism. At 2 K the decay can be well described by a direct transfer to Pr and Nd traps without donor-donor transfer. The temperature dependence of the decay at long times can be described for $35<T<\mathrm{}70\mathrm{}$ K by the hopping model. The agreement with theory is much better if the cross relaxation to Nd ions is assumed to occur mainly via the lowest Stark level of the ${}_{}{}^4{}_{}{}^{}I_{\frac{9}{2}}^{}{}_{}{}^{}$ manifold.