Vacuum-Ultraviolet Photochemistry. II. Solid- and Gas-Phase Photolysis of Methane-Water Systems

The photolysis of the methane hydrate (CH4 center dot 6H(2)O) at - 195 degrees C at 1470 and 1236 angstrom, has been investigated. Mixtures of methane and water in the gas phase have also been photolyzed at 1470, at 1236 angstrom, and in the region 1450-1850 angstrom. In both gas- and solid-phase photolysis, CO and CO2 are formed at 1470 and 1236 angstrom. CO2 is formed from CO, probably by the reaction OH+CO-CO2+H, which is shown to occur readily in the solid at -196 degrees C. In the photolysis of CH4 center dot 6H(2)O at - 196 degrees C, the hydrogen produced is mainly from the methane. Ethane is formed as well, and the product ratio CO2/C2H6 is a linear function of the ratio of the reactants H2O/CH4. Formation of CO2 and C2H6 by competitive reactions of CH2 with H2O and CH4 is suggested. Similarity of results at 1236 angstrom to those at 1470 angstrom suggests that CH2 is present in both systems. Since photodecomposition of CH4 is negligible at 1470 angstrom, the production of CH2 at this wavelength is attributed to the occurrence of O+CH4 -> CH2+H2O with oxygen atoms being formed in photolysis of water.