In this chapter, we discuss the decay of an initially excited state into a quasi-continuum of final states. We introduce the Green’s formalism to calculate the propagator and apply it to the ladder model, which can be solved analytically. In the statistical limit which is applicable to larger, especially bio-molecules, an exponential decay results. We use the saddle point method to treat a more realistic model with a distribution of coupling matrix elements and energies. Applying the displaced oscillator model, we obtain the Marcus expression in the classical limit. Finally, we discuss the energy gap law for intramolecular radiationless transitions.
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