Onset of migration in the reaction of fluorine atoms with iodine molecules

Abstract

Reactive scattering of F atoms with I₂ molecules has been studied at an initial translational energy E≈ 40 kJ mol^–1, using a supersonic beam of F atoms seeded in He buffer gas generated by a high-pressure microwave discharge source. The source gives essentially complete dissociation of F₂ precursor, so that the measured laboratory angular and velocity distributions of IF scattering arise only from the F + I₂ reaction. The centre-of-mass differential reaction cross-section peaks in the backward direction with a higher product translational energy than the forward scattering which shows a broad subsidiary maximum of relative height ≈ 0.6. This is in sharp contrast to the stripping dynamics exhibited by the F + Br₂ reaction under similar experimental conditions, and is attributed to the onset of migration in the F + I₂ reaction dynamics. A forward-scattered component with very low product translational energy is attributed to reaction at large impact parameters of I₂ molecules strongly inclined to the plane of collision. This component corresponds to highly rotationally and vibrationally excited IF molecules observed in the laser-induced fluorescence measurements of Vigué and co-workers. The backward scattering is attributed to large-impact-parameter collisions with the I₂ molecules lying close to the plane of collision, in which the F atom orbits around the I atom to which it is initially bonded and then migrates to the other I atom by intervening in the extended I₂ bond. The onset of migratory dynamics is attributed to the preferred bent geometry of the FI₂ intermediate and the existence of an attractive well in the exit valley of the potential-energy surface for this L + HH (light + heavy–heavy) reaction.