Controlling electron transfer in strong time-dependent fields:: Theory beyond the Golden Rule approximation

In this work we apply a generalized Zusman model to study the influence of an external periodic electric field on the dynamics of electron transfer (ET) reactions coupled to an overdamped reaction coordinate which is treated semiclassically. Being nonperturbative in the tunneling coupling this approach goes beyond the conventional Golden Rule description and includes both adiabatic and nonadiabatic electron transfer regimes. Explicit expressions for the ET rates are derived in the high-frequency driving regime and compared with exact numerical results. Our novel analytical findings constitute a useful approximation scheme, as long as the dynamics can be characterized by a single exponential relaxation. We further demonstrate that the Golden Rule description becomes drastically improved in the presence of strong, fast oscillating fields. Moreover, we discuss interesting phenomena such as an inversion of populations and a driving induced transition from an adiabatic to a nonadiabatic reaction dynamics. (C) 2000 American Institute of Physics. [S0021-9606(00)50248-7].