Energy Transfer in Rare Earth Ion Clusters and Fluorescence from Rare Earth Doped LANTHANUM(1.85)STRONTIUM(0.15)COPPER -OXYGEN(4) Superconductors.

Laser spectroscopy of rare earth ions in solids was used to study mechanisms of non-resonant energy transfer within rare earth clusters, and to detect insulating, impurity phases in rare earth doped La_{1.85 }Sr_{0.15}CuO _4 superconductors. The mechanisms of phonon-assisted, non-resonant energy transfer were studied in well-defined dimer sites in Er^{3+ }:SrF_2 and Pr ^{3+}:CaF_2. Application of a magnetic field to Er^{3+} :SrF_2 greatly increased the energy transfer rate. The magnetic field dependence in Er^{3+}:SrF _2 indicates that the mechanism of non-resonant energy transfer is a two-phonon, resonant process (Orbach process). Application of a magnetic field to Pr ^{3+}:CaF_2 had no effect on the energy transfer rate because no significant Zeeman splittings occurred. The temperature dependence of the energy transfer rate in Pr^{3+ }:CaF_2 showed the mechanism to be a one-phonon-assisted process at low temperatures and predominantly an Orbach process above 10 K. In the second part of this thesis, laser spectroscopy of a Eu ^{3+} probe ion is developed to detect impurity phases in La_{1.85 }Sr_{0.15}CuO _4 superconductors. Two impurity phases were found in polycrystalline La_ {1.85}Sr_{0.15} CuO_4: unreacted La _2O_3 starting material, and a La-silicate phase, which formed from contamination during sintering. The spectroscopic technique was found to be more than 100 times more sensitive than powder x -ray diffraction to detect minor impurity phases. In preparing the superconductors, several studies were made on the effect of Pr^{3+}, Eu ^{3+}, Bi^{3+ }, and fluorine dopants on the superconducting properties of La_{1.85}Sr _{0.15}CuO_4 and La_2Cuo_4 . Pr^{3+}, Eu ^{3+}, Bi^ {3+}, and F_2 doping all decreased the superconductivity in La_ {1.85}Sr^{0.15} CuO_4. Treating semi-conducting La_2CuO_4 in F_2 gas converted it to a superconductor with an onset T_{rm c} of 30-35 K.