Role of migration in the dynamics of bromine atoms reacting with iodine molecules Original Research Article

Differential cross sections have been measured for the reactive scattering of Br(2P32) atoms with I2 molecules at initial translational energies E ≈ 87 and 52 kJ mol−1 using a supersonic beam of Br atoms seeded in He and Ne buffer gases generated from a high pressure microwave discharge source. The centre-of-mass angular distributions of IBr scattering peak sharply in the forward direction with subsidiary backward peaks of relative height ∼ 0.55 and 0.60. The product translational energy distributions are broad, peaking at a fraction ƒ′ ≈ 0.30 ± 0.05 of the total available energy but with the forward scattering being shifted to lower translational energy than the wide angle scattering. An extended phase space theory is proposed to model the observed reactive scattering without prior assumptions as to the geometry of the collision complex or its mode of dissociation. This provides a good description of the observed reactive scattering only when the effects of a potential energy barrier in the exit valley of the potential energy surface are absent from the calculation. This is attributed to the contribution of migratory trajectories at large impact parameters sampling the less stable IBrI configuration and yielding scattering which is sharply peaked in the forward direction, while smaller impact parameter collisions sample the more stable BrII configuration and yield scattering over a wide angular range. The photoinitiated reaction samples only the BrII configuration due to its constrained geometry and low initial translational energy.