Selective Excitation and Decay of Er 3 + Fluorescence in La F 3

A simple treatment of pulsed and steady-state selective-excitation experiments in multilevel systems is given and used to interpret the fluorescence excitation and decay properties of ${\mathrm{Er}}^{3+}$ in La${\mathrm{F}}_{3}$. Fluorescence from eight different excited states was studied. From excitation spectra and pulsed selective excitation, decay modes were established involving level-by-level nonradiative cascade, arising from multiphonon emission, and level-bypassing radiative transitions. Energy transfer due to ion-ion interactions was deliberately avoided. Using pulsed-excitation techniques, multiexponential time-dependent fluorescences were observed and used to determine the lifetimes of all excited states below 40 000 ${\mathrm{cm}}^{\ensuremath{-}1}$. In general, the smaller the energy gap to the next lower level, the shorter the lifetime, reflecting the dependence of the rate of multiphonon emission on the proximity of other levels. Fluorescence in La${\mathrm{F}}_{3}$ is usually not detected when the energy gap is 1600 ${\mathrm{cm}}^{\ensuremath{-}1}$. Fluorescence lifetimes were also measured as a function of temperature from 77 to \ensuremath{\sim}700\ifmmode^\circ\else\textdegree\fi{}K. Thermal quenching of the fluorescent properties is correlated with the increased importance of nonradiative transitions at higher temperatures.