Variation of the Fluorescence Decay Time of a Molecule in Front of a Mirror

Abstract The out‐going wave‐front of a radiating oscillator (e. g. a Hertzian dipole) is disturbed by a reflecting surface. The resulting interference phenomenon affects the radiated power, which thus depends on the distance between oscillator and mirror. If this concept is applied to the emission of light by molecules, a dependence of the fluorescence decay time on the distance between excited molecule and mirror is to be expected. This effect has been investigated for the case of monomolecular layers of an europium dibenzoylmethane complex, which are kept in a definite distance from an evaporated metal surface (gold, silver or aluminum) by cadmiumarachidate layers. Exciting with a flash of 366 nm light the intense red fluorescence (λ = 612 nm) of the europium ion is observed. The fluorescence follows an exponential decay law over at least two decades with a decay time of roughly 1 msec. The expected variation of the decay time with distance from the mirror is verified by the experiment. The decay time is lengthened or shortened in close agreement with the above theory, except for small distances, where energy transfer processes occur. The mirror affects only the radiating process, while the probabilities for non‐radiating desactivation processes remain unaltered. Thus the quantum yield of fluorescence of the emitting state can be determined, which is an otherwise hardly accessible quantity.