Fluctuation effects in steric reaction-diffusion systems

We propose a simple model for reaction-diffusion systems with orientational constraints on the reactivity of particles, and map it onto a field theory with upper critical dimension dc52. To the two-loop level, long-time particle density N(t) is given by the same universal expression as for a nonsteric system, with N(t);t for d<2. For slow rotations of the particles we find an intermediate regime where N(t);t, with the crossover to the nonsteric asymptotics determined by the rates of rotations and reactions. Consequences for experiments are discussed. @S1063-651X~99!50104-9#