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

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 Vigue 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.