Radiationless transitions and molecular quantum beats

Radiationless transitions in isolated molecules are examined from the point of view of the breakdown of the Born-Oppenheimer approximation and the nature of the decay of coherently excited states. The problem of intramolecular vibronic coupling between zero-order Born-Oppenheimer states in an isolated molecule seems amenable now to a new approach both in terms of interpretation and of experimental investigation. We have classified molecules exhibiting vibronic mixing into two extreme cases, which we call the resonant case and the statistical case, characterized by low and high densities of molecular eigenstates, respectively. We have then considered the coherent excitation of these states and derived general expressions for photon counting rates and for the integrated intensity. Quantum beat signals can be investigated to analyze close-lying molecular eigenstates in the resonance limit. In the statistical limit the interference effects within a manifold of close-lying states are manifested by a radiationless decay process. An analysis of the radiationless decay tunes and the quantum yields is given.