Multiphonon Relaxation in Glasses

Abstract

Experimental data of multiphonon relaxation between electronic levels of rare earth ions in glasses are presented. These are based on measurements of the decay times or quantum efficiencies of the specific levels and subtracting the radiative transition probabilities in oxide, chalcogenide and fluoride glasses. It is shown that the nonradiative transfer in all glasses depends mainly on the energy gap between the emitting and next lower level of the ion incorporated in the glass and that the high energy phonons of the network formers are responsible for the relaxation permitting the lowest order process. The phenomenological parameters a, 3 and e appearing in the formulae of multiphonon transfer are computed and compared to these values in crystals. It is also shown that the Huang-Rhys number S being a measure of the electron-phonon coupling strengths is smaller than 0.1 for the oxide glasses as predicted by the theory of multiphonon relaxation. However it achieves a large value of about 2 in the chalcogenide glasses. The discrepancy in this high value is explained by the covalency of the rare earths incorporated in the chalcogenide glasses with the surrounding sulfur ions, thus the rare earth ions cannot be treated as isolated centers in these glasses. Experimental findings of phonon-assisted energy transfer between uranyl and rare earth ions in phosphate glasses are presented. It is shown that the experimental dependence between multiphonon relaxtion rates and the energy gap is that obtained for similar to the multiphonon relaxation in a single ion. The coupling constant is higher in the case of the energy transfer due to the stronger coupling of uranyl to the glass.