A post-transition-state bifurcation is a fork in the reaction path that lies beyond the transition state. The molecule has already crossed the energy barrier — the hard part is done, the reaction will proceed. But which product forms is still undecided. The trajectory rolls downhill from the saddle point and can land in one of two product basins. The branching ratio — the proportion of trajectories going left versus right — is determined by the curvature and coupling of the potential energy surface beyond the barrier. It is a post-commitment decision: the reactant committed to reacting, but not to which product.
Mondal, Kumar, and Keshavamurthy (arXiv:2603.09713, March 2026) show that this post-commitment decision can be steered by an optical cavity. When a molecular vibration couples strongly to a cavity photon mode — vibrational strong coupling, or VSC — the molecule and the cavity form a hybrid light-matter state. The reaction still crosses the same transition state. But the dynamics beyond the barrier are modified: the cavity mode participates in the energy redistribution that determines which product basin the trajectory enters. The branching ratio changes by nearly a factor of two.
The mechanism is not that the cavity changes the barrier. It does not alter which reactions are possible or how fast they proceed. What it changes is the post-barrier dynamics — the steering after commitment. The hybrid light-matter modes redistribute energy between the reactive coordinate and the cavity in a way that favors one product basin over the other. The selectivity is modified by the cavity's participation in a process that, without the cavity, was governed entirely by intramolecular energy flow.
The result extends the conceptual scope of cavity chemistry. Previous work focused on whether cavities can change reaction rates — whether the barrier height or barrier crossing dynamics are modified by coupling to photons. This paper shows that even if the rate is unchanged, the products can differ. The cavity does not accelerate the reaction. It redirects it. The fork exists with or without the cavity. But with the cavity, one road is wider.
Mondal, Kumar, and Keshavamurthy, "Vibrational strong coupling influences product selectivity in a model for post transition state bifurcation reactions," arXiv:2603.09713 (March 2026).