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

Abstract

Photodissociation dynamics of organic halides has been extensively investigated due to its potential to the stratosphere ozone depletion and relevant environmental problems. For example, alkyl bromide as the simplest organic halides has been used for model of photodissociation dynamics. The A-band of alkyl bromide arises from the C–Br bond localized transition and consists of three overlapping transitions to repulsive states (Q1, Q0, and Q1: on ascending order of excitation energy). The dominant transition in the A-band is to the Q0+ state which correlates to the Br(P1/2) products, suggesting the spin-orbit ground Br(P3/2) atom formation arises as a results of nonadiabatic coupling between the Q0+ and Q1 PESs via a conical intersection along the C–Br bond coordinate. Compared to the photodissociation of alkyl bromide, the photodissociation dynamics of aryl halides 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. It has been unraveled by numerous studies that dissociation pathways are substantially dependent on the type of halogen, atomic substituents, and excitation wavelengths. In the early work on the bromobenzene (C6H5Br) photodissociation near 266 nm, 7-12,14 the main dissociation channel is an indirect dissociation involving the bound (π, π) and repulsive (π, σ) states. In the short wavelength, significant UV absorption of bromobenzene is caused by and transition; the mixing between these states becomes more probable and additional routes can be observed as in the case of photodissociation of iodobezene. With the purpose of elucidation of dissociation dynamics of bromobenzene at short-wavelengths, we have previously investigated 234 nm dissociation dynamics of bromobenzene using velocity ion map imaging. Observed trimodal translational distributions for Br/Br formation channels have been attributed to the direct and indirect dissociation mechanisms originating from the initially excited (π, π) state. This is indicative that dissociation mechanisms of bromobenzene at 234 nm are more complicated than those near 270 nm. In this regards, more systematic studies focused on effects of atomic substituents such as fluorine, bromine and chlorine on phenyl group are required to have a comprehensive understanding of the photodissociation dynamics of bromobenzene near 234 nm. The objective of the present work is to investigate the fluorine substitution effects in bromobenzene near 234 nm further. In this note, as continuing efforts for this purpose, we present ion-imaging study for photodissociation dynamics of pentafluorobromobenzen (C6F5Br) at 234 nm using velocity map imaging coupled with [2+1] resonance enhanced multiphoton ionization (REMPI) scheme. With aid of ab initio calculations, detailed photodissocation pathways are discussed. The velocity map imaging apparatus, similar to that used previously, consists of molecular beam chamber and main interaction chamber with TOF spectrometer. Liquid samples of C6F5Br were used without further purification. Vapors of liquid C6F5Br (10-20 torr, 98.5%) were carried by helium gas at 1.5 atm through the pulsed value operating synchronously with the pulse laser at 10 Hz. Output laser pulses from the Nd:YAG pumped dye laser (HD500, Lumonics) were frequency-doubled to emit at 234 nm with an energy range of 50-150 μJ and subsequently focused onto the skimmed molecular beam with a 150 mm focal lens. C6F5Br molecules were dissociated and Br fragments were simultaneously ionized using [2+1] REMPI schemes. With a set of ionoptics for the velocity map ion-imaging, the Br were expanded, accelerated by nonhomogeneous electric field and projected onto a 40 mm Chevron-type dual MCP coupled to phosphor screen. The transient image from the phosphor screen were taken by a charge-coupled device (CCD) camera. All timings of the pulsed value, Nd:YAG laser, and the gating pulse for MCP were manipulated by a multichannel delay generator. (SRS, DG535). Upon irradiation at 234 nm, C6F5Br dissociates to release the Br/Br photofragments. The resultant Br and Br images resulting from 234 nm photolysis of C6F5Br are presented in Figures 1(a) and 1(b), respectively. Because each image is a 2D projection of 3D distribution of photofragments with cylindrical symmetry around laser polarization axis, the shape of an image is dependent on the speed (energy) and angular distributions of fragments. From the measured images for Br and Br channels, the speed distributions (P(ν)) were extracted by integrating the 3D speed distribution over all angles at each speed, and subsequently the total translation σ n ←