Observation of resonances in the transition state region of the F + NH3 reaction using anion photoelectron spectroscopy

The transition state of a chemical reaction is a dividing surface on the reaction potential energy surface (PES) between reactants and products and is thus of fundamental interest in understanding chemical reactivity. The transient nature of the transition state presents challenges to its experimental characterization. Transition-state spectroscopy experiments based on negative-ion photodetachment can provide a direct probe of this region of the PES, revealing the detailed vibrational structure associated with the transition state. Here we study the F + NH 3 → HF + NH 2 reaction using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled FNH 3 − anions. Reduced-dimensionality quantum dynamical simulations performed on a global PES show excellent agreement with the experimental results, enabling the assignment of spectral structure. Our combined experimental–theoretical study reveals a manifold of vibrational Feshbach resonances in the product well of the F + NH 3 PES. At higher energies, the spectra identify features attributed to resonances localized across the transition state and into the reactant complex that may impact the bimolecular reaction dynamics. The transition state, a transient species where bond transformation occurs, fundamentally controls reaction dynamics. This important species can be probed through the photodetachment of an anionic precursor, as has now been shown in the F + NH 3 reaction. A combination of theory and experiment reveals resonances that span the transition state.