THE WAVELENGTH DEPENDENCE OF THE RATES OF INTERNAL ENERGY REDISTRIBUTION DURING THE PHOTODISSOCIATION OF 3-IODOPYRIDINE

The heterocyclic aromatic molecule 3-iodopyridine (C5H4NI) is photodissociated and studied with state-selective photofragment translational spectroscopy at 266 nm and 304 nm. Angular, velocity, and translational energy distributions are obtained for the ground state I(P-2(3/2)) and excited state I*(P-2(1/2)) photodissociated iodine atoms. For the I channel, both a fast and a slow recoil speed distribution are observed at both 266 nm and 304 nm, which result from parallel electronic transitions. The fast I and I* distributions are proposed to result from an excitation to the alkyl iodide-type (3)Q(0)(n,sigma*) repulsive state, in accordance with mechanisms proposed for the photodissociation of iodobenzene. The observed slow I distributions are suggested to arise from absorptions to pyridine pi,pi* optical doorway states which are predissociated by mixing with the alkyl iodide-type n,sigma* repulsive states. At 266 nm, the observed large increase in the relative quantum yield of slow iodine relative to that observed for iodobenzene correlates with the fact that the magnitude of the molar extinction coefficient for the (1)L(b) band of 3-iodopyridine is larger than that for the (1)L(b) band of iodobenzene. Like iodobenzene, the widths of the translational energy distributions of fast I and I* exhibit strong excitation photon energy dependences, which suggests a direct correlation between the rate of energy redistribution, which is in competition with the prompt dissociation process, and the density of pyridine-type dark vibronic states. Assuming that the observed correlation between the recoil velocity and the anisotropy parameter for the slow dissociation channel of I results from rotational depolarization, we are able to estimate dissociation times and rates of internal energy redistribution. We find that the rate of internal energy redistribution (IER) for the indirect slow dissociation process in 3-iodopyridine at 304 nm is comparable to that observed previously in iodobenzene, while the rate of IER observed for 3-iodopyridine at 266 nm is significantly larger than that observed previously in iodobenzene. Internal energy redistribution in the fast and slow dissociation channels is discussed in terms of the density of states and the coupling between the optical doorway states and nearby dark states.