  Chemical selectivity, as quantified by a branching ratio, is a phenomenon
relevant for many organic chemical reactions. It may be exhibited on a
potential energy surface that features a valley-ridge inflection point in the
region between two sequential index-1 saddles, with one saddle having higher
energy than the other. Reaction occurs when a trajectory crosses the region of
the higher energy saddle (the entrance channel) and approaches the lower energy
saddle. On both sides of the lower energy saddle, there are two wells and the
question we address in this work is that, given an initial ensemble of
trajectories, what is the relative fraction of trajectories that enter each
well. For a symmetric PES this fraction is 1:1. We consider a symmetric PES
subjected to a time-periodic forcing characterized by an amplitude, frequency,
and phase. In this letter we analyse how the branching ratio depends on these
three parameters.
