Laser-induced fluorescence detection of singlet CH₂ in low-pressure methane/oxygen flames

Abstract

Methylene, CH₂, is a chemically important intermediate in hydrocarbon combustion but has previously eluded optical detection in a combustion environment. The CH₂ signal as a function of height above the burner surface in a premixed, laminar, methane/oxygen flame (5.6 Torr and fuel equivalence ratio ∼1.05) is measured by laser-induced fluorescence (LIF) in the$\tilde b^1$ B ₁ − ã¹ A ₁ electronic system. The ã state which lies ∼3165 cm⁻¹ above the ground state is populated at the high temperatures of the flame (800–1800 K). Although less than one photon for each laser pulse is detected, we can unambiguously attribute the LIF features in the region 450 to 650 nm to CH₂ by both scanning the excitation laser and dispersing fluorescence. LIF temperatures and CH and OH LIF concentration profiles are also obtained for the flame. The CH₂ radical concentration maximum occurs closer to the burner than that of either OH or CH, as expected from models of methane combustion chemistry.