Nonradiative Processes in Molecular Systems

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The term nonradiative or radiationless transitions has been in common use for many decades to describe radiation-induced processes in which no energy is exchanged with the radiation field. We shall not deal with (radiationless) Auger transitions in atoms or with (radiationless) multiphonon transitions in solids but shall focus on radiationless transitions in molecules, especially those transitions that apparently involve only discrete (as opposed to continuous) energy levels.(1–5) Beginning in the early 1960s,(6–11) this subject has developed from a relatively obscure branch of molecular spectroscopy into a broad and vigorously expanding discipline. A great stimulus has been the recognition that the notion of a nonradiative process provides a convenient common denominator for a large number of experimental observations. Typically, these observations are associated with small terms in the molecular Hamiltonian, and many of them have recently become more accessible due to improvements in instrumentation. This experimental development has been accompanied and-sometimes preceded by the development of a unifying theoretical description.(12–14) it has also been realized that a quantitative interpretation of radiationless parameters is potentially a valuable source of information on molecules. A separate section will be devoted to each of these three aspects. The first section of this chapter is introductory. In it we shall define the field and aim at removing a few potential ambiguities right from the start.