Velocity map imaging of the near-threshold photodissociation of IBr: accurate determination of De(I–Br)

The photodissociation of jet-cooled IBr molecules has been investigated at excitation energies just above the threshold for forming ground state I(2P3/2)+Br(2P3/2) products, using a state-of-the-art ion imaging spectrometer operated under optimal conditions for velocity mapping. The image resolution is sufficient to enable determination of precise, isotope specific values for the ground state dissociation energies D00(I–79Br) and D00(I–81Br), and thus yield an accurate value for the ground state well-depth, De=14 798±1 cm−1. The measured angular distribution of the Br atom product flux formed at energies very close to threshold shows an anisotropy that declines with increasing velocity – an observation rationalised, qualitatively, in terms of breakdown of the axial recoil approximation.