Unimolecular dissociation from a dense set of states

The intramolecular dynamics in a dense set of levels at energies above the dissociation threshold are discussed. Special reference is made to the typical case where there is a high-energy barrier to dissociation so that the number of channels (=number of accessible states of the transition state) is small compared to the number of bound states to which they can couple. The bifurcation into prompt and delayed decay is discussed with special reference to computational examples. The results suggest that it is possible to access the (far fewer) promptly decaying states, which do not sample the accessible isoenergetic bound space prior to their dissociation. The role of external perturbations (e.g., due to collisions) is also discussed as a mechanism for breaking down selection rules and thereby increasing the effective density of states of the energy rich molecule.