Reactive scattering of a supersonic oxygen atom beam: O + Br2

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Abstract
Reactive scattering of O atoms with Br2 molecules has been studied at an initial translational energy E = 40 kJ mol-1 using a supersonic beam of O atoms seeded in He, and at E = 15 kJ mol-1 using O atoms seeded in Ne. Velocity distributions of OBr product were measured by cross-correlation time-of-flight analysis. Full contour maps of the differential reaction cross-section have been obtained. For O atoms seeded in Ne, there is peaking in the forward direction with a lower intensity 0·6 ± 0·1 in the backward direction. The product translational energy distribution agrees with that predicted by a long-lived collision complex model with a tight linear transition state for dissociation. For O atoms seeded in He, the scattering in the backward direction is increased to 1·2 ± 0·1 and has a higher product translational energy than the scattering in the forward direction. The reactive scattering observed at the lower initial translational energy indicates a short-lived O-Br-Br collision complex with a lifetime approximately equal to its rotational period. This is consistent with motion over a triplet potential energy surface of 3 A″ symmetry with a modest well E 0 ∼ 110 kJ mol-1 in the bent O-Br-Br configuration. However the increased scattering observed in the backward direction at the higher initial translational energy indicates a directly scattered component with product repulsion arising from collisions at small impact parameters. The forward scattered component arises from diminished reactive scattering in collisions at larger impact parameters.