Photodissociation dynamics of C₃H₅I in the near-ultraviolet region.

Abstract

The ultraviolet photodissociation dynamics of allyl iodide (C3H5I) have been studied by ion-imaging at 266 nm and 213 nm. These photolysis wavelengths are located in the two lowest absorption bands in the near-ultraviolet region. The atomic iodine products were detected by [2+1] resonantly enhanced multiphoton ionization spectroscopy. The spectra showed that the branching fraction for the spin-orbit excited ((2)P(1/2)) state was larger than that for the ground ((2)P(3/2)) state at both photolysis wavelengths. The state-resolved scattering images of iodine showed two maxima in the velocity distributions in the (2)P(3/2) state and a single peak in the (2)P(1/2) state. The spin-orbit specificity indicates that the C-I bond cleavage at both absorption bands is governed by the dissociative n(I)σ*(C-I) potential energy surfaces. The nascent internal energy distribution of the allyl radical (C3H5) counter product, which was obtained by the analysis of the state-resolved scattering distributions, showed a marked difference between the photolysis at 266 nm and 213 nm. The generation of the colder C3H5 with the higher translational energy at 266 nm implied the direct photoexcitation to the n(I)σ*(C-I) repulsive surfaces, whereas the internally hot C3H5 at 213 nm was ascribed to the local π(CC)π*(CC) photoinitiation in the allyl framework followed by predissociation to the n(I)σ*(C-I) states.