KINETIC TRANSITIONS IN DIFFUSION-REACTION SPACE

In this paper we study how the strength and range of the potential fun ction upsilon(r) governing the down-range motion of a diffusing coreac tant A with respect to a stationary target molecule B influence the ef ficiency of the irreversible diffusion-reaction process: A+B-->C. This problem is translated into the lattice-statistical one of determining the mean walklength before trapping of the species A on a lattice of N sites of coordination nu and dimension d. Factors affecting the reac tion efficiency are explored and quantified using a combination of ana lytical methods and numerical techniques rooted in the theory of finit e Markov processes. Our results show that there exists a transition be tween two qualitatively different types of behavior in diffusion-react ion space, viz., a regime where the coreactant's motion is totally cor related with respect to the target species, and a regime where the cor eactant's motion is totally uncorrelated. The transition between these two regimes is (relatively) abrupt, and we find that significant chan ges in the reaction efficiency can be induced by small changes in the strength and range of the correlations between coreactants, the temper ature and/or the dielectric constant of the medium. This ''order-disor der'' behavior is characterized as a kinetic transition in diffusion-r eaction space, and is explored as a function of system size and spatia l dimension.