Photodissociation of tert-butyl iodide at 277 and 304 nm: Evidence for direct and indirect dissociation in A-band photolysis of alkyl iodide

The photodissociation of t-C4H9I has been studied at 277 and 304 nm in a supersonic molecular beam. The fragments (I and t-C4H9 radical) are selectively ionized by resonance-enhanced multiphoton ionization and then projected onto a two-dimensional position-sensitive detector to obtain their translational energy and angular distributions. The energy distribution is found to consist of three components: one Maxwell-Boltzmann and two Gaussian distributions. Their anisotropy parameters range from 0.7 to 1.6 and display a parallel transition characteristic, where the greater the kinetic energy of the component, the stronger its anisotropy. From present and previous work, these three components are interpreted in terms of three independent reaction paths on an excited potential energy surface: (1) the prompt dissociation along the C-I stretching mode for the high-energy component, (2) the repulsive mode along the C-I stretching, coupled with some bending motions for the medium-energy component, and (3) the indirect dissociation, probably due to large contribution of the bending motions for the low-energy component. Relative quantum yields for I(P-2(3/2)) at 277 and 304 nm have been determined and found to be 0.93 +/- 0.03 and 0.92 +/- 0.04, respectively. Experiments have shown that t-C4H9I has the highest curve-crossing probability from the (3)Q(0) to (1)Q(1) State among low-carbon alkyl iodides. The extensive vibrational coupling between two states in the proximity of a crossing point supports this interpretation.