  In this letter we propose a model which explains ultrafast and efficient
photoisomerization reactions as driven by transitions between quasistationary
states of one dimensional (1D) double well potential of an excited electronic
state. This adiabatic potential is formed as a result of doubly crossing of a
decay diabatic potential of the ground electronic state and a bound diabatic
potential of the excited state. We calculate the eigenstates and eigenfunctions
using the semiclassical connection matrices at the turning and crossing points
and the shift matrices between these points. The transitions between the
localized in the wells below the adiabatic barrier states are realized by the
tunneling and by the double non-adiabatic transitions via the crossing points
processes. Surprisingly the behavior with the maximum transition rate keeps
going even for the states relatively far above the barrier (2 -4 times the
barrier height). Even though a specific toy model is investigated here, when
properly interpreted it yields quite reasonable values for a variety of
measured quantities, such as a reaction quantum yield, and conversion time.
