Anomalous HF product rotational distributions in the F+H-2 reaction at threshold collision energies

Time-of-flight spectra for the F + H-2 --> HF(upsilon(f), j(f)) + H reactio n have been measured for the first time at collision energies less than 80 meV (1.84 kcal mol(-1)). The results at 70 meV, 55 meV and 36 meV at a fixe d laboratory scattering angle have been transformed to the center-of-mass s ystem and reveal sharp product rotational distributions for the upsilon(f)= 1 and upsilon(f) = 2 product vibrational states with most probable values o f j(f) greater than or equal to 11. Such large values of j(f) can only be e xplained in terms of a substantial antiparallel orientation of the final ro tational and orbital angular momenta, consistent with a long-lived collisio n complex or a quantum mechanical resonance. Moreover the sizeable signal a t 36 meV when compared with ab initio calculations of the activation barrie r at 50 meV suggests a significant tunneling effect.