View article

A new approach is suggested for evaluation of the radiationless transition rate for the curve‐crossing problem in the presence of dissipation. The rate is evaluated by using the conventional Landau–Zener theory but for a collective system–bath coordinate, which is characterized by a maximal mean‐free path in the vicinity of the crossing point. Variational transition state theory (TST) is employed for determination of this quasiballistic mode. The resulting uniform rate expression bridges between the known nonadiabatic, solvent controlled and TST limits. The main effect of dissipation is the reduction of the slope difference of the potential of mean force along the quasiballistic mode compared to that along the original reaction coordinate. This results in an increase of the reaction adiabaticity. Application of the theory is illustrated for the symmetric normal crossing of two parabolic diabatic terms with Ohmic dissipation …