Continuum state spectroscopy: A high resolution ion imaging study of IBr photolysis in the wavelength range 440–685 nm

The photodissociation of jet-cooled IBr molecules has been investigated at numerous excitation wavelengths in the range 440–685 nm using a state-of-art ion imaging spectrometer operating under optimal conditions for velocity mapping. Image analysis provides precise threshold energies for the ground, I(2P3/2)+Br(2P3/2), and first excited [I(2P3/2)+Br(2P1/2)] dissociation asymptotes, the electronic branching into these two active product channels, and the recoil anisotropy of each set of products, as a function of excitation wavelength. Such experimental data have allowed mapping of the partial cross-sections for parallel (i.e., ΔΩ=0) and perpendicular (i.e., ΔΩ=±1) absorptions and thus deconvolution of the separately measured (room temperature) parent absorption spectrum into contributions associated with excitation to the A 3Π(1), B 3Π(0+) and 1Π(1) excited states of IBr. Such analyses of the continuous absorption spectrum of IBr, taken together with previous spectroscopic data for the bound levels supported by the A and B state potentials, has allowed determination of the potential energy curves for, and (R independent) transition moments to, each of these excited states. Further wave packet calculations, which reproduce, quantitatively, the experimentally measured wavelength dependent product channel branching ratios and product recoil anisotropies, serve to confirm the accuracy of the excited state potential energy functions so derived and define the value (120 cm−1) of the strength of the coupling between the bound (B) and dissociative (Y) diabatic states of 0+ symmetry.