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. A Walden inversion-like transformation and multiple Feshbach resonances are observed in the Cl + NH3 reaction, providing a complete mapping of the transition-state region for a chlorine-involved polyatomic reaction.