KINETICS AND MECHANISM OF THE SELF-REACTION OF THE BRO RADICAL

A flash photolysis-long path absorption technique was used to measure the rate coefficients for the self-reaction of BrO at 298 and 220 K ov er a pressure range of 75-600 Torr of He, N2, and SF6. The rate coeffi cients were measured using both a conventional monochromator/photomult iplier (PMT) system and a diode array spectrometer system. The overall rate coefficient for this reaction was found to be (2.75 +/- 0.50) X 10(-12) cm3 molecule-1 s-1, independent of pressure at 298 K but depen dent on pressure at 220 K, ranging from (2.00 +/- 0.41) x 10(-12) at 1 00 Torr to (3.10 +/- 0.30) x 10(-12) cm3 molecule-1 s-1 at 400 Torr. T he relative rate coefficients for the two branches, BrO + BrO --> 2Br + O2 (1a) and BrO + BrO --> Br2 + O2 (1b), were determined at 298 and 220 K. The branching ratio, k1a/k1, was determined to be 0.84 at 298 K and 0.68 at 220 K, independent of pressure. An additional absorption feature with a peak absorption at 312 nm was observed at 220 K and was tentatively attributed to Br2O2. The additional absorption interferes with measurements of k1b using the monochromator/PMT method; however, use of the diode array spectrometer overcomes this problem. A mechani sm for reaction 1, based on the formation of a short-lived intermediat e, is proposed. An upper limit of 5 X 10(-17) cm3 molecule-1 s-1 at 29 8 K for the reaction BrO + O3 --> Br + 2 O2 was derived from the analy sis of BrO temporal profiles. The atmospheric significance of these fi ndings are discussed.