Quantum yield for H atom formation in the methane dissociation after photoexcitation at the Lyman-α (121.6 nm) wavelength

Abstract

The gas-phase photodissociation dynamics of methane after excitation at the Lyman-α wavelength (121.6 nm) was investigated under collision-free conditions at room temperature. Narrow-band tunable Lyman-α laser radiation was generated by the resonant third-order sum-difference frequency conversion of pulsed-dye laser radiation and used both to photodissociate the parent molecules and to detect the photolytically produced hydrogen atoms via (2p 2P → 1s 2S) laser-induced fluorescence. H atom Doppler profiels were recorded and the absolute quantum yield for H atom formation, Φ H = (0.47 ± 0.11), was determined by means of photolytic calibration method where the Lyman-α photolysis of H 2O was used as a reference source of well-defined H atom concentrations. This value will be used in combination with previous results from H 2 and CH yield measurements to estimate the relative importance of the different product pathways in the CH 4 photodissociation.