  Chemical reactions subjected to time-varying external forces cannot generally
be described through a fixed bottleneck near the transition state barrier or
dividing surface. A naive dividing surface attached to the instantaneous, but
moving, barrier top also fails to be recrossing-free. We construct a moving
dividing surface in phase space over a transition state trajectory. This
surface is recrossing-free for both Hamiltonian and dissipative dynamics. This
is confirmed even for strongly anharmonic barriers using simulation. The power
of transition state theory is thereby applicable to chemical reactions and
other activated processes even when the bottlenecks are time-dependent and move
across space.
