Nonexponential decays in single vibronic level fluorescence: A comparison between kinetics and quantum mechanical treatment

Abstract

We study the decay of a system resulting from the strong coupling (pv≫1) of a radiative state |s〉 with a coarse manifold of nonradiative levels {l} (``intermediate case'' molecule in the gas phase) excited by a δ-pulse (coherent) excitation. By the ``effective Hamiltonian'' formalism we show that a necessary consequence of the coherent excitation is a short quasiexponential decay very similar to the decay obtained in the ``statistical limit.'' In the absence of efficient deactivation processes, this decay is followed by a longer one (the ``anomalously long fluorescence'') due to the incoherent decay of the mixed states which result from the s-l coupling. The form of this longer decay- quasiexponential in some well-defined cases-is sensitive to the width of the zero order levels: Collision effects are accounted for by a linear dependence of these widths on the gas pressure. The statistical limit is achieved in the case of efficient deactivation processes. This model is compared to a kinetic scheme including a reversible intersystem crossing, and a good correspondence between the two models is obtained, except for some finer details that only appear in the quantum mechanical treatment. Pressure effects on the luminescence lifetimes and yields are discussed.