The Role of Nonradiative Processes in the Spectroscopy of Optically Active Centers in Solids
This article deals with the basic mechanisms responsible for the radiative and nonradiative relaxations of optically active centers in solids. In the first section the systems and the basic interactions affecting them are defined. The second section deals with the quantum theory of molecular systems: the adiabatic (Born-Oppenheimer) approximation is introduced and its implications are discussed. The subject of the third section is the “phonon” system, namely the system of vibrations of the solid; the two pictures of harmonic oscillators and, when applicable, of phonons, are considered. The fourth section deals with the Franck-Condon principle and the basic processes of absorption, induced emission, and spontaneous emission; a simple model based on a single normal coordinate is used in order to derive the Pekar formula and the Huang and Rhys formula. The fifth section introduces the basic mechanism for the nonradiative relaxation, due to deviations from the adiabatic approximation, and treats the nonradiative transitions in terms of the normal coordinates of the solid. Finally the last section deals with the phenomenon of energy transfer among optically active centers in solids, a process that may provide an additional nonradiative decay path to an excited optical center.
KeywordsVibrational State Diatomic Molecule Internuclear Distance Adiabatic Approximation Energy Transfer Process
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