title: 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.
authors.affiliation: Department of Chemistry, National Taiwan University, Taipei 106, Taiwan.
authors.forename: Pei-Ying
authors.initials: PY
authors.lastname: Wei
authors.forename: Yuan-Ping
authors.initials: YP
authors.lastname: Chang
authors.forename: Wei-Bin
authors.initials: WB
authors.lastname: Lee
authors.forename: Zhengfa
authors.initials: Z
authors.lastname: Hu
authors.forename: Hong-Yi
authors.initials: HY
authors.lastname: Huang
authors.forename: King-Chuen
authors.initials: KC
authors.lastname: Lin
authors.forename: K T
authors.initials: KT
authors.lastname: Chen
authors.forename: A H H
authors.initials: AH
authors.lastname: Chang