Unimolecular dissociation of selectively excited polyatomic molecules

Selective activation processes (e.g., induced by a Franck-Condon transition) lead to an initially restricted sampling of the available phase space. The rate constant calculated by RRKM theory need then not be an upper bound to the exact value. A faster reaction rate is possible if the sampled phase space is preferentially coupled to the reaction coordinate. Such will be the case if the limited region of phase space accessed during the earlier stages of the energy redistribution is significantly spanned by "local mode" states. A computational example is provided as an illustration. It also shows how recrossings of the transition state result in an enhanced sampling of the bound phase space.