Single Molecule Chemical Reaction: Kramers Approach Revisited

Single molecule chemical reactions yield new insight into fluctuation phenomena which are obscured in measurement of ensemble of molecules. Kramers escape problem is investigated here in a framework suitable for single molecule reactions. In particular we obtain distributions of escape times in simple limiting cases, rather than their mean, and investigate their sensitivity on initial conditions. Rich physical behaviors are observed: sub-Poissonian statistics when the dynamics is only slightly deviating from Newtonian, super-Poissonian behavior when diffusion is dominating, and Poissonian behavior when Kramers original conditions hold. By varying initial conditions escape time distributions can follow a (usual) exponential or a $\tau^{-3/2}$ decay, due to regular diffusion. We briefly address experimental results which yield the $\tau^{-3/2}$ behavior (with cutoffs) and propose that this behavior is universal.