ENERGY MIGRATION IN RARE EARTH COMPLEXES

Abstract

Thesis submitted to Univ. of New Mexico, Albuquerque. Complexes of trivalent samarium, europium, terbium, and dysprosium were prepared for investigations of the luminescence properties of complexed rare earth ions. Rare earth chelates derived from benzoylacetone, dibenzoylmethane, and acetylacetone; rare earth chlorides solvated with water and deuterium oxide; and rare earth oxides dissolved in sodium metaphosphate glasses were synthesized for these studies. Initial investigations were conducted on the BETA -diketone chelates dissolved in organic glasses at 77 deg K. The luminescence spectra of the samples, arising from excitation by ultraviolet light, are reported, and groups of spectral lines are given J-J assignments. The luminescence decays of the rare earth ion emissions, arising from radiative transitions originating at the lowest resonance level of the ions, were measured. The decay curves were analyzed statistically, and a lower limit of 5 x 10/sup 5/ sec/sup -1/ is reported for the rate constant associated with intramolecular energy transfer. From a consideration of the rate of energy transfer and the magnitude of the molecular phosphorescence decay times of the chelates, it is concluded that intramolecular energy transfer cannot originate at the same level from which phosphorescence occurs. Intramolecular energy transfer rate constants for transfer of energy to the ions from higher levels of the triplet energy systems in these compounds are discussed. The visible absorption spectra of solutions of rare earth chelates and rare earth chlorides are presented and analyzed. Rare earth ion radiative transition probabilities, calculated from the weak ion absorption bands, are reported. It is shown that the increased luminescence decay times of the chelated rare earth ions as compared to the solvated rare earth ions must be due to a decrease in quenching of the ion states, not to a decrease in the radiative transition probabilities. The luminescence investigations were also carried out on microcrystalline chelates, crystalline solvated chlorides, and metaphosphate glasses. The emission spectra are presented and J-J assignments are made. Measurements of spectral lines arising from vibronic coupling of chelated rare earth ions are given. The luminescence decay curves of the complexes are reported, and decay times calculated from the curves are tabulated. For a few of the samples, the temperature dependencies of the luminescences are also reported. Quite significant differences were found in the luminescence intensities and decay times of rare earth ions in different environments. Possible mechanisms for quenching of rare eahh ion states are presented, and it is concluded that the variations of the luminescence properties of the ions are due to variations in vibronic quenching efficiency. The nature of the vibronic quenching in each of the different rare earth complex systems is discussed. Secondary environmental eifects on the rare earth ion luminescences are considered, and it is shown that factors such as the nature of the solvent and the method of sample preparation mugt be considered in any quantitative discussion of the luminescence properties. The origins of the discrepancies among the luminescence decay times reported by other authors are also considered. (auth) pumping under isothermal conditions was designed. The