THEORY OF NONSTATIONARY ACTIVATED RATE-PROCESSES - NONEXPONENTIAL KINETICS

We have explored a simple microscopic model to simulate a thermally ac tivated rate process where the associated bath which comprises a set o f relaxing modes is not in an equilibrium state. The model captures so me of the essential features of non-Markovian Langevin dynamics with a fluctuating barrier. Making use of the Fokker-Planck description, we calculate the barrier dynamics in the steady-state and nonstationary r egimes. The Kramers-Grote-Hynes reactive frequency has been computed i n closed form in the steady state to illustrate the strong dependence of the dynamic coupling of the system with the relaxing modes. The inf luence of nonequilibrium excitation of the bath modes and its relaxati on on the kinetics of activation of the system mode are demonstrated. We derive the dressed time-dependent Kramers rate in the nonstationary regime in closed analytical form which exhibits strong nonexponential kinetics of the reaction coordinate. The feature can be identified as a typical non-Markovian dynamical effect. (C) 1998 American Institute of Physics. [S0021-9606(98)51337-2]