Radiative and Nonradiative Relaxation Processes in Cerium

The decay processes of excited cerium ions incorporated into a number of crystals and glasses have been systematically studied and their optical properties analyzed. The probabilities of the radiative and nonradiative decay from the lowest excited Ce^{3+} 5d state were determined by measuring the fluorescence lifetime of the 5d to 4f transition as a function of temperature. For crystals, the onset of the thermally induced nonradiative transitions occurs at temperature T* between 310 K and 920 K depending on the host. At temperatures lower than T*, there is an additional but much weaker temperature dependence of the radiative lifetime. The nonradiative decay rates exhibited an exponential dependence on the inverse of the temperature. In fitting the temperature dependences to an Arrhenius function, some insight is gained into the nonradiative relaxation mechanism. Nonradiative relaxation directly from the lowest excited 5d state to the 4f ground state via multiphonon emission is negligible because the 4f-to-5d energy gap of the cerium ions is too large for efficient multiphonon decay. The time-resolved decay dynamics of Ce ^{3+} in silicate and ZBLA glasses show similar site- and temperature-dependent behavior of their nonexponential fluorescence dacays. The nonexponential nature of the decay in glass arises from a superposition of Ce^{3+} ions with different local crystal field environment. Large systematic change in the shapes of the decay curve with emission wavelength was observed and is attributed to a wide distribution of optically active Ce^{3+} centers, each emitting with a characteristic time scale at selected transition energy. Moreover, the variation of activation energy with emission wavelength in ZBLA and silicate glasses suggests that the nonradiative relaxation is also site-dependent. There is a wide variation in fluorescence wavelengths, radiative lifetimes, and indices of refraction among the different hosts studied. Analysis of the radiative lifetime shows that the radial integrals < { it 4}f| r|{it 5 }d> among the set of the hosts are quite constant. The mean value of the radial integral is 0.30 A and is a factor of 1.5 smaller than the free -ion value. The reduction is attributed to the nephelauxetic effect when the ion is embedded in the lattice.