| In this thesis we consider two particular cases of nonradiative decay: multiphonon relaxation in solids and vibrational energy relaxation in liquids. In Chapter 2, we develop a microscopic theory of multiphonon relaxation. Two alternative mechanisms of the relaxation process are presented and their relative importance is discussed. For each approach a closed-form analytical expression for the relaxation rate is given as a function of energy gap and temperature. In Chapter 3, we apply our theory to analyze the energy gap- and temperature-dependent experimental data for various electronic transitions of rare-earth impurities in YAlO{dollar}sb3{dollar} host. In Chapter 4, we focus on the vibrational energy relaxation of diatomics in rare gas crystals. The predictions of the theory are compared with the experimental data on the relaxation of O{dollar}sb2{dollar} in solid Ar. In Chapter 5, we develop a microscopic theory of the vibrational energy relaxation of a diatomic solute in an atomic solvent. Our treatment is based on the force-force time-correlation function approach, and involves a short-time expansion of the correlation function. Theoretical results for the relaxation rate are compared to experiments on I{dollar}sb2{dollar} in Xe over a wide range of densities and temperatures. |
