248-NM Laser Photolysis of CHBr3/O-Atom Mixtures: Kinetic Evidence for UV CO(A)-Chemiluminescence in the Reaction of Methylidyne Radicals With Atomic Oxygen

摘要

4TH Positive and Cameron band emissions from electronically excited CO have been observed for the first time in 248-nm pulsed laser photolysis of a trace amount of CHBr3 vapor in an excess of O-atoms. O-atoms were produced by dissociation of N2O (or O2) in a cw-microwave discharge cavity in 2.0 torr of He at 298 K. The CO emission intensity in these bands showed a quadratic dependence on the laser fluence employed. Temporal profiles of the CO(A) and other excited state products that formed in the photo-produced precursor + O- atom reactions were measured by recording their time-resolved chemiluminescence in discrete vibronic bands. The CO 4th Positive transition (A1 AND no. 61520;, v'=0 AND no. 61614; AND no. 61472;X1 AND no. 61523; AND no. 61483;, v'=2) near 165.7 nm was monitored in this work to deduce the pseudo-first-order decay kinetics of the CO(A)-chemiluminescence in the presence of various added substrates (CH4, NO, N2O, H2, and O2). From this, the second-order rate coefficient values were determined for reactions of these substrates with the photo-produced precursors. The measured reactivity trends suggest that the prominent precursors responsible for the CO(A)-chemiluminescence are the methylidyne radicals, CH(X2 AND no. 61520;) and CH(a4 AND no. 61523;-), whose production requires the absorption of at least 2 laser photons by the photolysis mixture. The O- atom reactions with brominated precursors (CBr, CHBr and CBr2), which also form in the photolysis, are shown to play a minor role in the production of the CO(A or a)-chemiluminescence. However, the CBr2 + O-atom reaction was identified as a significant source for the 289.9-nm Br2-chemiluminescence that was also observed in this work. The 282.2-nm OH- and the 336.2-nm NH-chemiluminescences were also monitored to deduce the kinetics of CH(X2 AND no. 61520;) and CH(a4 AND no. 61523;-) reactions when excess O2 and NO were present.