Temperature-Dependent Concentration Quenching of Fluorescence by Cross Relaxation ofNd3+in LaF3

Abstract

The fluorescence lifetimes of the ${}_{}{}^4{}_{}{}^{}F_{\frac{3}{2}}^{}{}_{}{}^{}$ level of ${\mathrm{Nd}}^{3+}$ in La${\mathrm{F}}_3$ and ${\mathrm{Nd}}^{3+}$ in La${\mathrm{Cl}}_3$ were measured for various concentrations of ${\mathrm{Nd}}^{3+}$ at 4.2, 77, and 300°K and intermediate temperatures. It is found that the lifetime-concentration curves differ greatly for the two crystals. The optical energy levels of ${\mathrm{Nd}}^{3+}$ in La${\mathrm{F}}_3$ at various concentrations were examined and two sets of levels with precisely overlapping electric dipole transitions have been found; no such overlapping transitions were found for the La${\mathrm{Cl}}_3$ crystal, however. It is suggested that cross relaxation between these levels can account for the strong, temperature-dependent concentration quenching in the La${\mathrm{F}}_3$ crystal. Using rate equations with cross relaxation terms, a one-parameter expression for fluorescence lifetime versus temperature was found which fitted experimental points for the 4.7 and 8.3% concentration crystals reasonably well. Since the overlap of transitions greatly affects cross relaxation rates, the linewidths and lineshifts were examined. Large heterogeneous linewidths and wavelength shifts were observed for the intermediate concentration crystals. The transitions were observed to sharpen in the low-dilution crystals and in the 100%-concentration crystals (Nd${\mathrm{F}}_3$), but relative shifts in wavelengths are observed.