Vibrational predissociation (VP) of van der Waals complexes occurs via an isolated resonance. An isolated resonance possesses no classical counterpart. And yet, classical calculations of the decay yield the rates that are sometimes not too different from the quantum rates. We resolve this puzzle by addressing the following points: i) Quantum theory of VP: accurate and perturbative approaches; ii) Quasiclassical theory of VP: Landau method and recovery of VP transition probabilities from the correspondence principle transition probabilities; iii) Classical theory of VP: diffusional description of long-time chaotic dynamics.
We calculate the ratio of the classical to quantum VP rates, determine the conditions when the rates of quantum dynamical tunneling is close to the classical diffusional rates across the chaotic sea and establish a classical counterpart of quantum perturbation approach.