Conclusive evidence is presented for the existence of a reactive resonance in the $\mathrm{F}+\mathrm{HD}$ reaction. In a molecular beam experiment, the resonance appears in the integral cross section as a distinct steplike feature, while in the differential cross section it is manifested as sharply varying forward-backward peaks in the product distribution. A detailed analysis of the quantum dynamics establishes that a reactive resonance localized in the transition-state region is responsible for these remarkable observations. At collision energies below $1\mathrm{kcal}/\mathrm{mol}$, the reaction proceeds almost exclusively through resonant tunneling with very little contribution from the more conventional direct mechanism.

• Received 27 March 2000

DOI:https://doi.org/10.1103/PhysRevLett.85.1206