Kinetics of CH radicals with O2: Evidence for CO chemiluminescence in the gas phase reaction

Abstract

The production of CO vis-uv-chemiluminescence has been observed for the first time when a trace amount of CHBr3 vapor was photodecomposed in a pulsed-photolysis reactor using a 248-nm laser under multiphoton-dissociation conditions in an excess of O2 in diluent helium or argon carrier gas at 298 K and in the pressure range 2–95 Torr. The time-resolved chemiluminescent traces due to vibronic emissions at several band positions in the CO(A–X), CO(a–X), and CO(d–a) systems, and in the (1–0) and (0–0) bands of the OH(A–X) system were observed to decay nonexponentially. Their integrated intensities all showed quadratic dependence on the photolysis fluence employed. The OH and CO chemiluminescence data can be best interpreted by postulating the occurrence of O2 reactions with the methylidyne radicals in two different electronic states, CH(X 2Π) and CH(a 4Σ−). The production of CO(A) and CO(d) in these reactions require the methylidyne radicals to be internally (vibrationally) excited. The decay kinetics of the chemiluminescence was investigated in the presence of various added substrates, the buffer gas pressure, and in conditions of excess added CH4, which allowed us to isolate and study the CH(a 4Σ−)+O2 reaction, since the CH(X 2Π) could rapidly be removed from the system. The reactions of highly internally excited brominated radical species such as CBr♯, CHBr♯, and CBr2♯ with O2, in principle, can also produce CO chemiluminescence, but are argued to be of negligible importance in the present CHBr3/O2 photolysis system.