Dynamic bottlenecks and the extreme stability of high Rydberg states

High molecular Rydberg states, whose time evolution exhibits multiple time scales are discussed as an example of a system which can be examined in detail. Three bottlenecks to the sampling of phase space, associated with an incomplete mixing of the zeroth order quantum numbers, are identified. The physics of all three is that the number of open ionization channels is smaller than the number of quasi isoenergetic zero order discrete states and that an electron with a high orbital angular momentum, which is far from the core, cannot effectively couple to it. States trapped behind the bottlenecks have a high resilience to decay and such states are possible even high above the ionization threshold and in the presence of external perturbations. On the other hand, states that are directly coupled to the ionization channels decay promptly, with less sampling of the dense manifold of isoenergetic delayed states. We conclude that the decay of high molecular Rydberg states provides a useful analogue for unimolecular reactions from a dense set of states which typically results in a prompt and a delayed decay.