Non-adiabatic Effects in Condensed Phase Activated Rate Processes in the Near-Adiabatic Limit

Thermally activated barrier crossing with simultaneous non-adiabatic transitions between quantum states is analyzed in detail in the near-adiabatic limit (NAL). The analysis is performed in the semiclassical approach within Markovian and non-Markovian models for the classical stochastic motion, which controls the barrier crossing process. It is shown that physically meaningful behavior of the non-adiabatic transition rate in NAL can be obtained only within the non-Markovian models, and the transition rate essentially depends on the velocity correlation time of the stochastic classical motion. The obtained general expressions are applied to the description of diffusive passing over the cusp barrier resulting from avoided crossing of two coupled linear terms. Possible manifestations of non-adiabatic transitions are thoroughly discussed in the model of linear terms. The non-adiabatic transitions in NAL are shown to determine the electron-transfer reaction rate in polar liquids in some important cases.