Journal of Physical Chemistry, Vol.99, No.19, 7395-7406, 1995
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.
Keywords:MAGNETIC CIRCULAR-DICHROISM;MOLECULAR-BEAMS;PYRIDINE VAPOR;RADIATIONLESS TRANSITIONS;ARYL HALIDES;248 NM;A-BAND;STATE;PHOTOFRAGMENTATION;EXCITATION