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 a t the Lyman-alpha wavelength (121.6 nm) was investigated under collisi on-free conditions at room temperature. Narrow-band tunable Lyman-cr l aser radiation was generated by the resonant third-order sum-differenc e 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 fluorescen ce. H atom Doppler profiles were recorded and the absolute quantum yie ld for H atom formation, Phi(H) = (0.47 +/- 0.11), was determined by m eans of a photolytic calibration method where the Lyman-alpha photolys is of H2O was used as a reference source of well-defined H atom concen trations, This value will be used in combination with previous results from H-2 and CH yield measurements to estimate the relative importanc e of the different product pathways in the CH4 photodissociation.