Anomalous HF product rotational distributions in the F+H2 reaction at threshold collision energies

Abstract

Time-of-flight spectra for the F+H₂→HF(v_f, j_f)+H reaction 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 fixed laboratory scattering angle have been transformed to the center-of-mass system and reveal sharp product rotational distributions for the v_f=1 and v_f=2 product vibrational states with most probable values of j_f11. Such large values of j_f can only be explained in terms of a substantial antiparallel orientation of the final rotational and orbital angular momenta, consistent with a long-lived collision complex or a quantum mechanical resonance. Moreover the sizeable signal at 36 meV when compared with ab initio calculations of the activation barrier at 50 meV suggests a significant tunneling effect.