Molecular elimination of Br2 in photodissociation of CH2BrC(O)Br at 248 nm using cavity ring-down absorption spectroscopy

The primary elimination channel of bromine molecule in one-photon dissociation of CH2BrC(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-2 elimination reaction is determined to be 0.24 +/- 0.08. With the aid of ab initio potential energy calculations, the obtained Br-2 fragments are anticipated to dissociate on the electronic ground state, yielding vibrationally hot Br-2 products. The temperature-dependence measurements support the proposed pathway via internal conversion. For comparison, the Br-2 yields are obtained analogously from CH3CHBrC(O) Br and (CH3)(2)CBrC(O) Br to be 0.03 and 0.06, respectively. The trend of Br-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-2 production, and its contribution might account for the underestimate of the branching ratio calculations. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4767346]