Photodissociation dynamics of iodobenzene by state-selective photofragment translational spectroscopy

Abstract

State-selective photofragment translational spectroscopy is used to probe the detailed nature of the photodissociation dynamics of iodobenzene at 304 nm. Simultaneous determination of the recoil speed, the spatial anisotropy, and the final state of the iodine fragment reveals that three dissociation channels with different dynamical characteristics compete in the photodissociation of iodobenzene at 304 nm. Based on the observed energy partitioning between the internal and translational modes and the dissociation time t d determined from the spatial anisotropy by using a rotational depolarization model, the three dissociation channels are assigned as follows. Two fast dissociation channels, which result in formation of I *( 2 P 1/2) ( t d=0.4 ps, quantum yield Φ=0.005±0.002) and high velocity I( 2 P 3/2) ( t d=0.3 ps, Φ=0.70±0.04), are due to a parallel transition to the repulsive 3 Q 0( n,σ *) state in the C I bond, followed by dissociation along the same state or curve crossing to the 1 Q 1 state respectively. A slow dissociation channel ( t d=0.5–1.4 ps, Φ=0.30±0.04) which produces low velocity I( 2 P 3/2) is due to a parallel transition to the triplet π, π* state(s) in the phenyl ring that is predissociated by the repulsive n,σ* state(s). The dissociation times determined in the present work are in excellent agreement with those of the recent femtosecond real-time measurements by Cheng et al. at 278 nm (P.Y. Cheng, D. Zhong and A.H. Zewail, Chem. Phys. Lett., 237 (1995) 399).