Halogen effect on the photodissociation mechanism for gas-phase bromobenzene and iodobenzene

The velocity imaging technique combined with (2+1) resonance-enhanced multiphoton ionization (REMPI) is used to detect the halogen fragments in the photodissociation of bromobenzene and iodobenzene at 266 nm. With the aid of potential energy curve calculations by Lunell (Y. J. Liu, P. Persson, S. Lunell, J. Phys. Chem. A 2004, 108, 2339-2345.), the Br fragmentation is proposed to stem from excitation of the lowest excited singlet (pi - pi*) state followed by predissociation along a repulsive triplet (n - sigma*) state. The slowed dissociation rate leads to production of the isotropic Br fragments and 93% internal energy deposition. Only the ground state Br(P-2(3/2)) is detectable. In contrast, when iodine is substituted, the iodine effect stabilizes the repulsive states associated with the I-C6H5 bond rupture and the subsequent dissociation channels become more complicated. 84% of the iodobenzene molecules obtained follow a direct dissociation channel, while the remaining undergo a predissociative process. Both routes result in rapid dissociation with anisotropy parameters of 0.7 +/- 0.2 and 0.9 +/- 0.2 as well as 70% and 26% in the fractions of translational energy deposition, respectively. The relative quantum yields of 1* and 1 are 0.35 and 0.65 and their related photodissociation pathways are discussed in detail.