Unimolecular reactions and phase space bottlenecks

We discuss the phase space bottlenecks for the unimolecular decomposition of HeI2. An intermolecular bottleneck can be defined in phase space which is a ''point of no return'' or critical configuration for the unimolecular reaction, even though there is no barrier to dissociation for the simple case studied here, HeI2 restricted to T-shaped geometry (two degrees of freedom). In addition, one can find intramolecular bottlenecks and the combination of the two types of bottlenecks yields theoretical reaction rates which are in excellent agreement with classical numerical simulations. Rates, derived from an approach using only intermolecular bottlenecks or approaches using both intermolecular and intramolecular bottlenecks, are two orders of magnitude better than simple RRKM rates. The paper further demonstrates how the phase space bottlenecks can be employed to discern probabilities of complex formation and discusses the extension of the present work to other types of reactive systems