PHOTODISSOCIATION DYNAMICS OF IODOBENZENE BY STATE-SELECTIVE PHOTOFRAGMENT TRANSLATIONAL SPECTROSCOPY

State-selective photofragment translational spectroscopy is used to pr obe the detailed nature of the photodissociation dynamics of iodobenze ne at 304 nm. Simultaneous determination of the recoil speed, the spat ial anisotropy, and the final state of the iodine fragment reveals tha t three dissociation channels with different dynamical characteristics compete in the photodissociation of iodobenzene at 304 nm. Based on t he observed energy partitioning between the internal and translational modes and the dissociation time t(d) determined from the spatial anis otropy by using a rotational depolarization model, the three dissociat ion channels are assigned as follows. Two fast dissociation channels, which result in formation of I(P-2(1/2)) (t(d) = 0.4 ps, quantum yiel d Phi = 0.005 +/- 0.002) and high velocity I(P-2(3/2)) (t(d) = 0.3 ps, Phi = 0.70 +/- 0.04), are due to a parallel transition to the repulsi ve (3)Q(0)(n,sigma) state in the C-I bond, followed by dissociation a long the same state or curve crossing to the (1)Q(1) state respectivel y. A slow dissociation channel (t(d) = 0.5 - 1.4 ps, Phi = 0.30 +/- 0. 04) which produces low velocity I(P-2(3/2)) is due to a parallel trans ition to the triplet pi,pi state(s) in the phenyl ring that is predis sociated by the repulsive n,sigma state(s). The dissociation times de termined in the present work are in excellent agreement with those of the recent femtosecond real-time measurements by Cheng et al. at 278 n m (P.Y. Cheng, D. Zhong and A.H. Zewail, Chem. Phys. Lett., 237 (1995) 399).