Translational photofragment spectroscopy has been used to study the photodissociation dynamics of C2HsI and CF3CH2I at 248nm. The measured anisotropies of the photofragments show that the initial absorption is the (parallel polarized) 3Q0 ,--N transition with a dissociation lifetime of 4 + 2 x 10-13s in both cases. The time of flight measurements yield the energy disposal for channels leading to I (2P3/2) and I* (2p1/2) for both molecules. The average energy disposed into translation is 62 per cent (I) and 72 per cent (I*) for C2HsI, and 36 per cent (I) and 39 per cent (I*) for CFaCH2I. The quantum yield for I* production is 0-67 + 0"03 for C2HsI and 0"82 + 0-07 for CFaCH2I. The results, combined with those of other workers, are compared to an impulsive description of the dissociation dynamics for a variety of substituted alkyl iodides. Finally a simple LandauZener curve crossing model is used to highlight the substantial change in curve crossing probability caused by introduction of fluorine atoms at the fl position.

Translational photofragment spectroscopy has been used to study the photodissociation dynamics of n-C3HTI and i-C3HTI at 248 nm. The measured anisotropies of the photofragments show that the initial absorption is the (parallel polarized) 3Qo*--N transition with a dissociation lifetimes of 3.7 + 2 x 10 13s and 2.5 + 2 x 10-13s respectively. The time of flight measurements yield the energy disposal for channels leading to I(2P3/2) and I*(2P1/2) for both molecules, although with greater uncertainty for i-C3H7I. The average energy disposed into translation is 43 per cent (I) and 52 per cent (I*) for n-C3H7I , and 39 per cent (I) and 57 per cent (I*) for i-C3H7I.The energy disposal for a variety of alkyl iodides are found to be well described by an impulsive model of the dissociation dynamics with the niodoalkanes showing a near constant width for the translational energy distribution regardless of its absolute value. A simple Landau-Zener curve crossing description is used to highlight the substantial change in curve crossing probability caused by substitution at the fl carbon or change to a secondary structure for the radical. Finally a model which combines the soft-radical impulse model for energy disposal with the Landau-Zener curve crossing probability is found to provide an excellent description of the I* quantum yields for the n-iodoalkanes.

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