Quantum yields and energy partitioning in the ultraviolet photodissociation of 1,2 dibromo-tetrafluoroethane (Halon-2402)

The photodissociation of 1,2 dibromo-tetrafluoroethane (Halon-2402) has bee n investigated at 193 nm using photofragment translational spectroscopy wit h vacuum ultraviolet ionization and at 193, 233, and 266 nm using state-sel ected translational spectroscopy with resonance-enhanced multiphoton ioniza tion. The product branching ratios, angular distributions, and translationa l energy distributions were measured at these wavelengths, providing insigh t into the ultraviolet photodissociation dynamics of CF2BrCF2Br. The total bromine atom quantum yields were found to be 1.9 +/-0.1 at both 193 and 233 nm and 1.4 +/-0.1 at 266 nm. The first C-Br bond dissociation energy was d etermined to be 69.3 kcal/mol from ab initio calculations. The second C-Br bond dissociation energy was determined to be 16 +/-2 kcal/mol by modeling of the bromine quantum yield. In addition, variational Rice-Ramsperger-Kass el-Marcus theory was used to calculate the secondary dissociation rates for a range of dissociation energies above threshold. These results suggest th at CF2CF2Br photofragments with sufficient internal energies will undergo s econdary dissociation prior to collisional stabilization under atmospheric conditions. Based on the measured translational energy distributions and pr oduct branching ratios, a model is proposed to describe the wavelength-depe ndent bromine quantum yield and the implications of these results to atmosp heric chemistry are discussed. (C) 2000 American Institute of Physics. [S00 21-9606(00)01941-3].