Excited-state dynamics and energy transfer of +4 curium in cerium tetrafluoride.

Abstract

Energy-transfer dynamics of 5f states of +4 curium in cerium tetrafluoride have been investigated for the first time. Emission from the lowest energy component of the second J=1 multiplet was studied using time- and frequency-resolved laser-induced fluorescence. The observed dynamics were complex due, in part, to the strong ion-lattice coupling and the dense f-state energy-level structure of ${\mathrm{Cm}}^{4+}$. The influence of experimental parameters such as excitation frequency, excitation intensity, sample temperature, and prior optical irradiation on the observed nonexponential fluorescence decays was determined. The observed 5f-state dynamics of 0.1 at. % ${\mathrm{Cm}}^{4+}$:${\mathrm{CeF}}_{4}$ are well fit by a cross-relaxation model, except at low temperature and high excitation intensity, where photoinduced site distortion and ground-state depletion via a two-photon process occurred. In this case, the initial fluorescence decay rate of ${\mathrm{Cm}}^{4+}$ on one of the two metal-ion sites was reduced in comparison with the rate observed at lower excitation intensity. A cross-relaxation model incorporating exciton-exciton annihilation was necessary to describe the fluorescence dynamics of 5 at. % ${\mathrm{Cm}}^{4+}$:${\mathrm{CeF}}_{4}$. The derived energy-transfer rates for 5f states of ${\mathrm{Cm}}^{4+}$ via cross relaxation and up conversion are two orders of magnitude higher, at equal ion densities, than those of lanthanide-ion 4f states and approach those of transition-metal-ion d states.