Photodissociation of dibromobenzenes at 266 nm by the velocity imaging technique.

A velocity imaging technique combined with (2+1) resonance-enhanced multiphoton ionization (REMPI) is used to detect the primary Br((2)P(3/2)) fragment in the photodissociation of o-, m-, and p-dibromobenzene at 266 nm. The obtained translational energy distributions suggest that the Br fragments are produced via two dissociation channels. For o- and m-dibromobenzene, the slow channel that yields an anisotropy parameter close to zero is proposed to stem from excitation of the lowest excited singlet (pi,pi*) state followed by predissociation along a repulsive triplet (n,sigma*) state localized on the C-Br bond. The fast channel that gives rise to an anisotropy parameter of 0.53-0.73 is attributed to a bound triplet state with smaller dissociation barrier. For p-dibromobenzene, the dissociation rates are reversed, because the barrier for the bound triplet state becomes higher than the singlet-triplet crossing energy. The fractions of translational energy release are determined to be 6-8 and 29-40 % for the slow and fast channels, respectively; the quantum yields are 0.2 and 0.8, and are insensitive to the position of the substituent. The Br fragmentation from bromobenzene and bromofluorobenzenes at the same photolyzing wavelength is also compared to understand the effect of the number of halogen atoms on the phenyl ring.

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((2)P(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 I* and I are 0.35 and 0.65 and their related photodissociation pathways are discussed in detail.