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

The gas-phase photodissociation dynamics of methane after excitation at the Lyman-alpha wavelength (121.6 nm) was investigated under collision-free conditions at room temperature. Narrow-band tunable Lyman-cr 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(2)P<--1s(2)S) laser-induced fluorescence. H atom Doppler profiles were recorded and the absolute quantum yield for H atom formation, Phi(H) = (0.47 +/- 0.11), was determined by means of a photolytic calibration method where the Lyman-alpha 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 H-2 and CH yield measurements to estimate the relative importance of the different product pathways in the CH4 photodissociation.