Non-monotonic dependence of electronic relaxation rate on solvent viscosity

Electronic relaxation processes in solution in which intramolecular potential barriers to reactive motion are absent are modelled by brownian motion of a solute particle on a harmonic surface with a position-dependent sink. The resulting Fokker-Planck equation with the sink term is solved by generating stochastic trajectories. We find that the rate constant for population decay is a non-monotonic, function of solvent friction. The Smoluchowski equation is found to overestimate the rate significantly in the low-friction regime