Characterization of Energy Transfer Interactions between Rare Earth Ions

Energy transfer studies employing the sodium rare earth tungstates (scheelite structure) indicate a number of modes of, and requirements for, nonradiative interaction. Direct dipole‐dipole interactions are prevalent. However, dipole‐quadrupole interactions are observed for the states of the larger rare earth ions that lie high in energy. The relation of lifetime to intensity changes with the mode of transfer. Phonon assisted transfer is enhanced by exchange coupling between rare earth ions in nearest or next nearest neighbor positions. Excitation may migrate between Tb ions in such positions when thermally activated and provided a suitable quenching center (e.g., Nd, Eu) is present. Such migration appears to be directional in comparison to transfer by radiation reabsorption. The latter is essentially random. Methods for analyzing lifetime data for transfer interactions are also discussed.