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

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 (2p2P → 1s2S) laser-induced fluorescence. H atom Doppler profiels were recorded and the absolute quantum yield for H atom formation, ΦH = (0.47 ± 0.11), was determined by means of photolytic calibration method where the Lyman-α photolysis of H2O was used as a reference source of well-defined H atom concentrations. This value will be used in combination with previous results from H2 and CH yield measurements to estimate the relative importance of the different product pathways in the CH4 photodissociation.