Photodissociation of 1,2‐Dibromoethylene at 248 nm: Br₂ Molecular Elimination Probed by Cavity Ring‐Down Absorption Spectroscopy

Abstract

The Br2 elimination channel is probed for 1,2-C2H2Br2 in the B(3)Pi(+)ou-X(1)Sigma(+)g transition upon irradiation at 248 nm by using cavity ring-down absorption spectroscopy (CRDS). The nascent vibrational population ratio of Br2(v=1)/Br2(v=0) is obtained to be 0.7+/-0.2, thus indicating that the Br2 fragment is produced in hot vibrational states. The obtained Br2 products are anticipated to result primarily from photodissociation of the ground-state cis isomer via four-center elimination or from cis/trans isomers via three-center elimination, each mechanism involving a transition state that has a Br-Br distance much larger than that of ground state Br2. According to ab initio potential energy calculations, the pathways that lead to Br2 elimination may proceed either through the electronic ground state by internal conversion or through the triplet state by intersystem crossing. Temperature-dependence measurements are examined, thereby supporting the pathway that involves internal conversion--which was excluded previously by using product translational spectroscopy (PTS). The quantum yield for the Br2 elimination reaction is determined to be 0.120.1, being substantially contributed by the ground-state Br2 product. The discrepancy of this value from that (of 0.2) obtained by PTS may rise from the lack of measurements in probing the triplet-state Br2 product.

Probing the Ignored Elimination Channel of Br2 in the 248 nm Photodissociation of 1,1‐Dibromoethylene by Cavity Ring‐Down Absorption Spectroscopy

“…[23][24][25][26][27] Their corresponding energies were controlled in the ranges of 8.8-14.4 and 0.5-1 mJ, respectively. A ring-down cell was made of a four-armed stainlesssteel reaction chamber, with the long arms sealed by two mirrors with a high reflectance of 99.98 % at 488 nm and 99.95 % at 515 nm, a diameter of 25.4 mm, and a radius of curvature of 1 m. The photolyzing laser beam was focused onto the ring-down cell at right angles to the cavity, whereas the probe laser beam was injected through the front mirror along the axis of the cavity after a time delay of 20 ns.…”

“…[23][24][25][26][27] In brief, a 20 ns-pulsed excimer laser at 248 nm was used for the photolysis of 1,1-dibromoethylene (1,1-C 2 H 2 Br 2 ), while a 5-8 ns-pulsed Nd:YAG laser-pumped dye laser was used to probe the released Br 2 fragment in the…”

“…As with the cases of 1,2-dibromoethylenes, [23] the laser energy and pressure dependence are all examined to ensure that the obtained Br 2 fragments result from the primary photodissociation channel. A single peak at 519.68 nm, contributed mainly by the Br 2 rotational line P(37) of the (35,0) band, was selected for these experiments.…”

“…[22] In contrast, we recently performed a similar experiment by using cavity ring-down absorption spectroscopy (CRDS). [23] Although CRDS lacks the capability of probing angular and translational energy distributions of fragments like the PTS technique, it can provide a corresponding absorption spectrum with high sensitivity. For the first time the optical spectra of Br 2 were observed in the primary dissociation channels of bromoethylene compounds.…”

“…[23][24][25][26][27] Such observations have an important impact on the photochemistry of atmospheric bromine, which requires updated modification of the photochemi-In the photodissociation of 1,1-C 2 H 2 Br 2 at 248 nm, the Br 2 elimination channel is probed by using cavity ring-down absorption spectroscopy (CRDS). In terms of spectral simulation, the vibrational population ratio of Br 2 A C H T U N G T R E N N U N G (v=1)/Br 2 A C H T U N G T R E N N U N G (v=0) is found to be 0.55 AE 0.05, which indicates that the Br 2 fragment is vibrationally hot.…”