Molecular elimination of Br_2 in photodissociation of CH _2BrC(O)Br at 248 nm using cavity ring-down absorption spectroscopy

摘要

The primary elimination channel of bromine molecule in one-photon dissociation of CH_2BrC(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 CH_3CHBrC(O)Br and (CH_3)_2CBrC(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.