A Novel Flash Photolysis/UV Absorption System Employing Charge-Coupled Device (CCD) Detection:  A Study of the BrO + BrO Reaction at 298 K

A novel flash photolysis/kinetic absorption spectroscopy system has been constructed for the study of gas phase reactions. The experiment incorporates a charge-coupled device (CCD) detector and represents the first application of such devices to the study of gas phase kinetics. The CCD enables the recording of rapid sequential time-resolved UV/visible absorption spectra before, during, and after photolysis of a gas mixture. The unequivocal identification and monitoring of several absorbing components in the reacting mixture is therefore possible, thereby maximizing the amount of information gathered from a single flash photolysis experiment. The experimental system is described in full here. Results from a preliminary kinetic study of the BrO self-reaction at 298 K are also described: BrO + BrO → 2Br + O 2 (1a); BrO + BrO → Br 2 + O 2 (1b). Experiments were performed to independently determine the rates of the overall reaction, k 1, defined by (−d[BrO]/d t = 2 k 1 [BrO] 2 ) and both individual reaction channels (1a) and (1b), giving k 1 = (2.98 ± 0.42) × 10 -12 cm 3 molecule -1 s -1; k 1a = (2.49 ± 0.42) × 10 -12 cm 3 molecule -1 s -1; and k 1b = (4.69 ± 0.68) × 10 -13 cm 3 molecule -1 s -1 at 298 K and 760 Torr total pressure in oxygen bath gas. Errors are 2σ. Deviations from the expected second-order kinetic scheme were observed in the experimental system wherein bromine was photolyzed in the presence of excess ozone as a source of the BrO radicals. At low ozone concentrations, the formation of Br 2 O is proposed, and at very high ozone concentrations the formation of symmetric bromine dioxide, OBrO, is observed. Kinetic schemes for these side reactions have been deduced, and the mechanistic implications of these results are discussed.