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 d(c)=2. To the two-loop level, long-time par ticle density N(t) is given by the same universal expression as for a nonst eric system, with N(t)similar to t(-d/2) for d less than or equal to 2. For slow rotations of the particles we find an intermediate regime where N(t)s imilar to t(-d/4), with the crossover to the nonsteric asymptotics determin ed by the rates of rotations and reactions. Consequences for experiments ar e discussed.