Resonance-mediated chemical reaction: F+HD -> HF+D

Conclusive evidence is presented for the existence of a reactive resonance in the F + HD reaction. In a molecular beam experiment, the resonance appea rs in the integral cross section as a distinct steplike feature, while in t he differential cross section it is manifested as sharply varying forward-b ackward peaks in the product distribution. A detailed analysis of thr quant um dynamics establishes that a reactive resonance localized in the transiti on-state region is responsible for these remarkable observations. At collis ion energies below 1 kcal/mol, the reaction proceeds almost exclusively thr ough resonant tunneling with very little contribution from the more convent ional direct mechanism.