Formation of Triplet CO in Atomic Oxygen Flames of Acetylene and Carbon Suboxide

Chemiluminescence spectra have been recorded for low-pressure, highly dilute, fuel-lean atomic oxygen flames of C 2 H 2 and C 3 O 2; under typical flame conditions the total pressure was 0.83 Torr with 1.1 mTorr of O atoms, 0.44 mTorr of O 2, 0.20 mTorr of fuel, and the balance Ar. Spectral coverage was complete from 183 to 1500 nm for C 2 H 2, and from 183 to 885 nm for C 3 O 2 . Strong CO a → X Cameron band emission was found to be the dominant feature of the chemiluminescence signature from 190 to 260 nm for C 2 H 2 oxidation under these conditions, with distinct but relatively minor CO A → X fourth positive emission between 185 and 195 nm. There is strong emission from higher triplet states of CO throughout the visible and near-IR, with CO e → a, d → a, and a‘ → a emission features observed. Nearly identical CO emission spectra were obtained for both C 2 H 2 and C 3 O 2 fuels, which indicates that for both fuels the source of electronically excited CO is the C 2 O + O reaction. Quantification of emission yields for the four triplet electronic states of CO observed here indicates that little or no CO(a) is formed directly, rather its population results from radiative emission from higher triplet states. Approximately 70% of the triplet CO formed directly from the C 2 O + O reaction is in the a‘ state, which had not been previously identified in this system.