Molecular elimination of Br{sub 2} in photodissociation of CH{sub 2}BrC(O)Br at 248 nm using cavity ring-down absorption spectroscopy

• Published in: The Journal of chemical physics 2012-12, Vol.137 (21)
• Main Authors: Fan He, Tsai, Po-Yu, Lin, King-Chuen, Lin, Cheng-Wei, Yan, Chi-Yu, Yang, Shu-Wei, Chang, A. H. H.
• Format: Article
• Language: English
• Subjects: ABSORPTION SPECTROSCOPY; ATOMIC AND MOLECULAR PHYSICS; BRANCHING RATIO; BROMINE; DISSOCIATION; GROUND STATES; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; INTERNAL CONVERSION; ORGANIC BROMINE COMPOUNDS; PHOTOLYSIS; PHOTONS; POTENTIAL ENERGY; SIMULATION; TEMPERATURE DEPENDENCE; TEMPERATURE RANGE 0400-1000 K; VIBRATIONAL STATES
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.4767346

The primary elimination channel of bromine molecule in one-photon dissociation of CH{sub 2}BrC(O)Br at 248 nm is investigated using cavity ring-down absorption spectroscopy. By means of spectral simulation, the ratio of nascent vibrational population in v = 0, 1, and 2 levels is evaluated to be 1:(0.5 {+-} 0.1):(0.2 {+-} 0.1), corresponding to a Boltzmann vibrational temperature of 581 {+-} 45 K. The quantum yield of the ground state Br{sub 2} elimination reaction is determined to be 0.24 {+-} 0.08. With the aid of ab initio potential energy calculations, the obtained Br{sub 2} fragments are anticipated to dissociate on the electronic ground state, yielding vibrationally hot Br{sub 2} products. The temperature-dependence measurements support the proposed pathway via internal conversion. For comparison, the Br{sub 2} yields are obtained analogously from CH{sub 3}CHBrC(O)Br and (CH{sub 3}){sub 2}CBrC(O)Br to be 0.03 and 0.06, respectively. The trend of Br{sub 2} yields among the three compounds is consistent with the branching ratio evaluation by Rice-Ramsperger-Kassel-Marcus method. However, the latter result for each molecule is smaller by an order of magnitude than the yield findings. A non-statistical pathway so-called roaming process might be an alternative to the Br{sub 2} production, and its contribution might account for the underestimate of the branching ratio calculations.