Spectral energy transfer in PrF3, PrCl3

Time-resolved fluorescence line narrowing is used to study spectral energy transfer in Pr${\mathrm{F}}_3$ and Pr${\mathrm{Cl}}_3$. The excitation spectrum of Pr${\mathrm{F}}_3$ in the region of the ${\left({}_{}{}^3{}_{}{}^{}H_4^{}{}_{}{}^{}\right)}_1\to {}_{}{}^3{}_{}{}^{}P_0^{}{}_{}{}^{}$ absorption indicates that several strong satellite lines accompany the main transition, their origin being ascribed to different ${\mathrm{Pr}}^{3+}$ ions lying in strongly perturbed lattice sites. The spectral transfer within the inhomogeneously broadened main transiton is measured, where the energy mismatch is about 2 ${\mathrm{cm}}^{-1\mathrm{}}$. This process is characterized by a strong ($\sim T^{4.3}$) dependence of the transfer probability on sample temperature. The growth of the full inhomogeneous emission profile accompanies a uniform decay of the donor line shape. A transfer process between the lowest-energy satellite ions is also measured. Here the energy mismatch is 32 ${\mathrm{cm}}^{-1\mathrm{}}$ resulting in a weak temperature dependence for the transfer rate. Both the intraline and intersatellite transfer are characterized by simple exponential rate equations. Recent calculations of phonon-assisted transfer probabilities are compared to our measurements, with the intersatellite transfer ascribed to a "one-phonon direct" process whereas the transfer within the main transition occurs by a "one-phonon second-order" mechanism. In Pr${\mathrm{Cl}}_3$ the transfer times are faster than the 15-nsec response time of our detection system indicating a much stronger ion-ion coupling.