Probing resonances in the double-well entry valley of the Cl + NH3 reaction using high-resolution anion photoelectron spectroscopy.

The Cl + NH3 → HCl + NH2 reaction is a prototypical system featuring multiple potential wells and transition states along the reaction pathway, presenting significant challenges for achieving a fully quantum-state-resolved understanding of its dynamics. By photodetaching ClNH3‾ anions, we probe the transition-state region in the reaction using high-resolution photoelectron spectroscopy combined with exact quantum dynamics calculations. Two prominent electronic bands are observed experimentally. High-level multi-reference calculations indicate that they arise from chlorine atom spin-orbit coupling. Several Feshbach resonances are identified in the lower spin-orbit state and assigned by excitations of the N-Cl translational, NH3 umbrella, and NH3-Cl rocking modes of the pre-reaction complexes RC1 and RC2. Impressively, a Walden inversion-like transformation between RC1 and RC2 is identified, driven by NH3 hydrogen reorientation via the umbrella mode excitation. These findings reveal that transition-state spectroscopy offers unprecedented opportunities to deepen our physical understanding of polyatomic reactions by probing their intricate mechanisms.