Non-equilibrium interlevel transitions in condensed phase far away from the avoided crossing region

The quantum-classical dynamics of interlevel transitions in condensed phase is considered in a special case when the transitions are localized far away from the avoided crossing region on the ground state free energy surface. A resonance case of a transition is studied when the two energy levels have the same shapes and equilibrium positions and are only shifted on the energy axis. For this simplified model the analytical expression for the non-equilibrium rate constant was derived starting from the multichannel Smoluchowski equations in the high-temperature limit. A deficiency of the existing multichannel diffusion theory is revealed and discussed. The illustrative calculations were performed for the model of proton transfer (PT) reaction in condensed phase. The results support the conclusion that the earlier proposed two-state model exaggerates the role of tunneling in PT processes. (C) 1998 Elsevier Science B.V. All rights reserved.