Fluoreszenzlebensdauern und Mehrphononenprozesse in wasserhaltigen Salzen von Eu3+ und Tb3+

The lifetimes of the fluorescent levels⁵ D ₀ and⁵ D ₁ of Eu³⁺ and⁵ D ₄ of Tb³⁺ were measured in the hydrated ethylsulfate, bromate, chloride, nitrate, and sulfate at 4.2°K, 77°K and 300°K. In the case of the ethylsulfate and chloride, the lifetimes were also measured in deuterated crystals and in crystals diluted with diamagnetic Yttrium. The results show that the lifetimes of the fluorescent levels are determined by spontaneous radiationless energy transfer from the rare earth ion to the optical vibrational modes of the water of crystallisation. Assuming a Coulomb interaction between the ion and the surrounding water molecules, the probability for the radiationless transition⁵ D ₁→⁵ D ₀ of the Eu³⁺-ion in the ethylsulfate was calculated in the low temperature limit. The agreement between the theoretical and experimental value is satisfactory. For the probabilities of radiationless desactivation of the fluorescent levels⁵ D ₀ of Eu³⁺ and⁵ D ₄ of Tb³⁺, the empirical form of a product 1] of two factors, one depending only on the rare earth ion and the other only on the lattice, was derived theoretically. Both fluorescent levels are desactivated by a 5-phonon process with the stretching vibrations of the surrounding water molecules.