PROGRESS ON THE MODELING OF THE COLLISIONAL ENERGY-TRANSFER MECHANISMIN UNIMOLECULAR REACTIONS

The RRKM theory of unimolecular reaction rates is a statistical mechan ical theory based on an assumption of microcanonical equilibrium in th e reactant phase space. The energy transfer in reactant medium collisi ons was originally described by a canonical strong collision assumptio n, i.e., an assumption of full thermal equilibration in each collision . In our work we first introduce a microcanonical strong collision ass umption which gives the RRKM theory a consistent form. We then introdu ce parametrizations of the degree of weakness (nonergodicity) of the c ollisions. A concept of collision efficiency is defined. The weakness of the collision is expressed in terms of reduced subsets of active re actant and medium degrees of freedom. The corresponding partially ergo dic collision theory (PECT) yields physical functional forms of the co llisional energy transfer kernel P(E',E). In order to resolve the ener gy and temperature dependence and the dependence on interaction streng th a multiple encounter theory is introduced (PEMET). Initially each e ncounter may be described by a semiempirical PECT model. Eventually th e encounters may be resolved by quantum dynamical calculations of the semiclassical or CAQE (classical approach/quantum encounter) type. Sim ple statistical collision models only distinguish between ''hits and m isses''. In reality the energy transfer efficiency exhibits characteri stic fall off with increasing impact parameter b. This b-dependence ca n be explicitly accounted for in the master equation for the reaction rate coefficient.