  The rate of a chemical reaction can often be determined by the properties of
a rank-1 saddle and the associated transition state separating reactants and
products. We have found evidence that such rates can be controlled and even
enhanced by external driving in at least one such system. Specifically, we
analyze a reactive model in two degrees of freedom that has been used earlier
to describe driven chemical reactions. Therein, changes in the external driving
can lead to a local maximum of the decay rate constant or even to bifurcations
of periodic trajectories on the normally hyperbolic invariant manifold (NHIM)
corresponding to the transition state. Inspired by these bifurcations, we show
that in this case, the dynamics on the NHIM can be connected to the geometry of
reactive trajectories and to reaction probabilities of Maxwell-Boltzmann
distributed reactant ensembles.
