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

The photodissociation of jet-cooled IBr molecules has been investigated at excitation energies just above the threshold for forming ground state I(P-2 (3/2)) + Br(P-2(3/2)) products, using a state-of-the-art ion imaging spectr ometer operated under optimal conditions for velocity mapping. The image re solution is sufficient to enable determination of precise, isotope specific values for the ground state dissociation energies D-0(0)(I-Br-79) and D-0( 0)(I-Br-81), and thus yield an accurate value for the ground state well-dep th, D-e = 14798 +/- 1 cm(-1). The measured angular distribution of the Br a tom product flux formed at energies very close to threshold shows an anisot ropy that declines with increasing velocity - an observation rationalised, qualitatively, in terms of breakdown of the axial recoil approximation. (C) 2000 Elsevier Science B.V. All rights reserved.