Intracavity laser absorption spectroscopy of sooting acetylene/air flames

Intracavity laser absorption spectroscopy (ICLAS) is used to measure the absolute concentration profiles of HCO and C₂ in low-pressure acetylene/oxygen/nitrogen flames with equivalence ratios ℓ=0.8, 1.0, 1.5, 2.0 and 2.5. The flames with ℓ=2.0 and 2.5 are soot-producing, with light extinction reaching 0.1% per pass in the flame with ℓ=2.5. This strong broadband extinction does not affect the sensitivity of ICLAS, however. The temperature profiles of the flames were measured using laser-induced fluorescence of the OH radicals. For C₂ concentration measurements, the (0-2) vibronic transition of the $d³ Π_g gets a³Π_u Swan band is used. The lines of this transition are located close to the HCO lines, making it possible to measure the two radical concentrations simultaneously. The C₂ concentration is highest in the ℓ=1.5 flame, and lower in the lean and heavily sooted ℓ=2.5 flames.