Photofragmentation dynamics of n-C3H7I and i-C3H7I at 248 nm

Translational photofragment spectroscopy has been used to study the photodissociation dynamics of n-C 3 H 7 I and i-C 3 H 7 I at 248 nm. The measured anisotropies of the photofragments show that the initial absorption is the (parallel polarized) 3 Q 0 ← N transition with a dissociation lifetimes of 3·7 ± 2 × 10 -13 s and 2·5 ± 2 × 10 -13 s respectively. The of flight measurements yield the energy disposal for channels leading to I( 2 P 3/2 ) and I*( 2 P 1/2 ) for both molecules, although with greater uncertainty for i-C 3 H 7 I. The average energy disposed into translation is 43 per cent (I) and 52 per cent (I*) for n-C 3 H 7 I, and 39 per cent (I) and 57 per cent (I*) for i-C 3 H 7 I. 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 n-iodoalkanes 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 β 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.