Abstract

It is well known that the probabilities of radiative transitions in a medium differ from those in vacuum. Excitation of a fluorescent molecule and its radiative decay are examples of radiative transitions. The rates of these processes in solution depend on the optical characteristics of the solvent. In this lecture the radiative decay rate and the extinction coefficient of a fluorescent molecule in solution will be expressed in terms of the intrinsic properties of the fluorescent molecule (electronic transition moments) and the optical characteristics of the solvent (refractive index, group velocity of light). It will be shown that the group velocity does not enter in the final expressions for the radiative decay rate and the extinction coefficient; this means that the dispersion of the refractive index has no effect on these quantities. The expressions for both the radiative decay rate and the extinction coefficient contain the refractive index of the solvent and the local field correction factor. The latter depends on the cavity model, and, for some cavity models, on the shape of the cavity. Four types of cavity models will be discussed; for each model the limits of applicability will be examined. Experimental evidence in support of specific cavity models will be reviewed.