Resonant energy transfer in stoichiometric rare earth compounds

A clear distinction is made between reversible and irreversible energy transfer. It is argued that low loss materials (stoichiometric compounds of rare earths) call for resonant, non-dissipative energy transfer describable in terms of spreading of excitonic wave packets rather than by Förster-Dexter theory. Difficulties concerning phonon collisions are recognized and suggestions for further theoretical and experimental work are made.