Photodissociation of bromobenzene at 266 nm

Abstract

The photodissociation of C 6 H 5 Br at 266 nm has been investigated on the universal crossed molecular beam machine, and time-of-flight spectra as well as the angular distribution of Br atom have been measured. Photofragment translational energy distribution P(E t ) reveals that about 47% of the available energy is partitioned into translational energy. The anisotropy parameter ␤ at this wavelength is Ϫ0.7Ϯ0.2. From P(E t ) and ␤, we deduce that C 6 H 5 Br photodissociation is a fast process and the transition dipole moment is almost perpendicular to the C-Br bond. Ab initio calculations have been performed, and the calculated results show that the geometry of the first excited state of bromobenzene has changed apparently compared with that of the ground state. Two kinds of possible fast dissociation mechanism have also been proposed.

Dynamics of Br(2Pj) Formation in the Photodissociation of Bromobenzene

“…The curve crossing from the initially pumped B2 1 (π,π*) state to the repulsive B1 3 (n,σ*) state may be the origin of these middle components. In experimental studies of the photodissociation of C 6 H 5 Br at 266 nm, 11 it was suggested that this predissociation process is the only Br atom formation channel. The time constant of this predissociation has been measured to be 28 ps.…”

“…In particular, the photodissociation dynamics of bromobenzene has been investigated using photofragment translational spectroscopy, photofragment ion-imaging, and femtosecond spectroscopy. 11,12,16 Recently, the effects of the halogen substituents on the dissociation of bromobenzene have also been studied. 14,[25][26][27] However, all the studies have concentrated on the dynamics near 270 nm; at these dynamics the π*←π transition dominates the absorption spectrum.…”

“…For example, in the case of aryl halides (C6H5X), the dynamics of dissociation become more complicated because of another chromophore on the benzene ring. 6,[11][12][13][14][15][16][17][18][19] By far-UV excitation, aryl halides are excited to the lowest singlet state (π,π*), which is delocalized on the benzene ring, and X atoms are produced via predissociation to the triplet (n,σ*) localized on the C-X bond. 16,20 The dynamics of the predissociation depend on the coupling strength between these electronic states, which is influenced by the type of halogen, 13 the substituents on the benzene ring, [21][22][23][24] and the excitation wavelength.…”

“…11,13 The two-dimensional photofragment ion-imaging technique was used to obtain the spatial distributions of the nascent Br atoms. The [2+1] resonance-enhanced multiphoton ionization (REMPI) scheme was used for state-selective detection of Br( 2 P3/2) and Br*( 2 P1/2) that were generated after photolysis of C 6 H 5 Br.…”

Photodissociation Dynamics of C6F5Br at 234 nm: Fluorination Effects on Br/Br*Formation Pathways

“…The A-band of alkyl bromide arises from the C-Br bond localized transition and consists of three overlapping transitions to repulsive states ( Q 1 PESs via a conical intersection along the C-Br bond coordinate. [4][5][6] Compared to the photodissociation of alkyl bromide, the photodissociation dynamics of aryl halides [7][8][9][10][11][12][13][14][15][16] show more complicated because more electronic states are involved and thus making multiple dissociation pathways probable. Multiple dissociation pathways include the predissociation between excited electronic states, internal conversion processes from vibrational excited states, and dissociation processes from repulsive states.…”