248 nm photolysis of CH2Br2 by using cavity ring-down absorption spectroscopy: Br2 molecular elimination at room temperature

Abstract

Following photodissociation of CH2Br2 at 248 nm, Br2 molecular elimination is detected by using a tunable laser beam, as crossed perpendicular to the photolyzing laser beam in a ring-down cell, probing the Br2 fragment in the B 3Piou+ -X 1Sigmag+ transition. The nascent vibrational population is obtained, yielding a population ratio of Br2(v = 1)Br2(v = 0) to be 0.7 +/- 0.2. The quantum yield for the Br2 elimination reaction is determined to be 0.2 +/- 0.1. Nevertheless, when CH2Br2 is prepared in a supersonic molecular beam under cold temperature, photofragmentation gives no Br2 detectable in a time-of-flight mass spectrometer. With the aid of ab initio potential energy calculations, a plausible pathway is proposed. Upon excitation to the 1B1 or 3B1 state, C-Br bond elongation may change the molecular symmetry of Cs and enhance the resultant 1 1,3A'-X 1A' (or 1 1,3B1-X 1A1 as C2v is used) coupling to facilitate the process of internal conversion, followed by asynchronous concerted photodissociation. Temperature dependence measurements lend support to the proposed pathway.

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…”

“…[23,25,27] The absorption cross section s of CH 2 Br 2 is reported to be 3. 7 10 À19 cm 2 at 248 nm, [38] but that of 1,1-C 2 H 2 Br 2 is not available.…”

“…[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.…”

BrCl+ elimination from Coulomb explosion of 1,2-bromochloroethane induced by intense femtosecond laser fields

“…The photodissociation of polyhalogenalkanes induced by intense laser elds has been extensively studied and is the subject of recent reviews. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Two major reactions are possible for dissociation of polyhalogenalkanes: the halogen atom release reaction channel and the halogen molecular elimination reaction channel. Thus far, most studies have been focused on the halogen atomic product channel, and the halogen molecular elimination channel as a research topic has received less attention in earlier papers.…”