Energy Transfer between the Low-Lying Energy Levels of ${\mathrm{Pr}}^{3+}$ and ${\mathrm{Nd}}^{3+}$ in La${\mathrm{Cl}}_3$

The energy-transfer mechanisms acting between the low-lying energy levels of ${\mathrm{Pr}}^{3+}$ and ${\mathrm{Nd}}^{3+}$ in La${\mathrm{Cl}}_3$ have been investigated. In all cases studied the ${\mathrm{Pr}}^{3+}$ ions acted as donors and the ${\mathrm{Nd}}^{3+}$ ions acted as acceptors. Fluorescence-intensity measurements have been made on the ${\mathrm{Pr}}^{3+}{}_{}{}^3{}_{}{}^{}F_3^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}H_6^{}{}_{}{}^{}$ donor systems. The shape of the time decay of the ${\mathrm{Pr}}^{3+}{}_{}{}^3{}_{}{}^{}F_3^{}{}_{}{}^{}$ level has been investigated. For low Pr concentrations (<2%) the excitation is transferred directly to the ${\mathrm{Nd}}^{3+}$ ions without significant migration between the ${\mathrm{Pr}}^{3+}$ ions. The experimental results are consistent with a dipole-dipole interaction mechanism with donor-acceptor energy-transfer constants of $C=6\mathrm{}\times {10}^{-38\mathrm{}}$ ${\mathrm{cm}}^6$ ${\sec }^{-1\mathrm{}}$ for the ${\mathrm{Pr}}^{3+}{}_{}{}^3{}_{}{}^{}F_3^{}{}_{}{}^{}$ donor system and $C=2\mathrm{}\times {10}^{-38\mathrm{}}$ ${\mathrm{cm}}^6$ ${\sec }^{-1\mathrm{}}$ for the ${\mathrm{Pr}}^{3+}{}_{}{}^3{}_{}{}^{}H_6^{}{}_{}{}^{}$ donor system. For Pr concentrations ≥ 2 at.% the effects of donor-donor transfer between ${\mathrm{Pr}}^{3+}$ ions are evident from both the fluorescence-intensity measurements and the shape of the decay curves; diffusion constants were obtained from these decays. The diffusion rates indicate a donor-donor transfer constant of $C\sim 3\times {10}^{-38\mathrm{}}$ ${\mathrm{cm}}^6$ ${\sec }^{-1\mathrm{}}$ for...