Influence of a dielectric interface on fluorescence decay time

When a mirror is placed near a fluorescing molecule, not only the angular distribution of the fluorescence is affected, but also the decay time. This effect is considered for the case of the reflecting interface between two dielectrics of different refractive indices. The fluorescence decay time varies markedly due to the presence of such an interface. In case of fluorescing molecules placed directly at the interface between a medium of refractive index 1.54 and air, the decay time is lengthened by a factor of 1.08 if their transition moment is oriented parallel to the interface, and by a factor of 3.61 if the transition moment is oriented perpendicular to the interface. These considerations are compared with measurements on monomolecular layers of an europium dibenzoylmethane complex which were held at a variable, but well defined distance from the interface.