Photodissociation of molecular beams of aryl halides: Translational energy distribution of the fragments

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Abstract
Molecular beams of the aryl halides, 1,5‐diiodonaphthalene, 4,4′‐diiododiphenyl, 4,4′‐dibromobiphenyl, and 9‐bromofluorene were dissociated by pulsed laser light at 265, 280, 300, and 321 nm. The measured time of flight distribution of the fragments was converted to a translational energy distribution. This distribution is independent of the wavelength of the light absorbed. The average translational energy of the fragments of the above four molecules is 16.3, 17.4, 11, and 12 kcal/mol, respectively. The I atoms are produced mainly in their ground electronic state. These results are consistent with a model in which the light energy is absorbed by the aromatic system, and the electronic but not the vibrational energy is subsequently transferred to the carbon–halogen bond. Knowing the bond energy one can transform the translational energy distribution into an internal energy distribution of the fragments, which has a Poissonian form. In a typical dissociation 5–10 vibrational quanta are excited in the aromatic rings by the repulsive force of the departing halogen atom. The measured angular distributions of the fragments of 1,4‐diiodobenzene, 4,4′‐diiodobiphenyl, and 1,5‐diiodonaphthalene were fitted with anisotropy parameters β from which lifetimes were extracted. Uncertainties in the β values are such that all we definitely know is that dissociation lifetimes of these and similar iodocompounds are ?2 ps.