Photochemistry in the Photo-Ionization Region. II. Photochemistry of Methane, Ethane, and Ethylene at Wavelengths below 900 Å

Abstract

The photolysis of methane has been investigated, using light from a microwave discharge through helium which is thought to have consisted mostly of the strongly self-reversed 584-Å resonance line. A windowless fast-flow separation system was used to separate the light source from the photolysis vessel. Methane pressure was varied from 0.001 to 0.3 mm, with helium as a carrier gas making up a total pressure of 1.85 mm. Products were correlated with saturation ion-currents, ranging up to 25 μA, which were measured in the photolysis vessel. The main products were hydrogen, ethane, ethylene, and acetylene, with estimated yields of 4.0, 0.37, 0.31, and 0.11 molecules per ion-pair, respectively. Smaller amounts of propane and propylene, and traces of C4 hydrocarbons, were also observed. Formation of unsaturated polymer was inferred from the carbon deficit of the volatile products. The effects of adding ethylene and neon to the system were studied, and in some experiments an electric field was applied. The photolyses of ethane and ethylene were also briefly investigated. Hydrogen was the only product measured, with yields ranging from 1.0 to 1.9 and from 0.8 to 1.2 molecules per ion pair from ethane and ethylene, respectively. Mechanisms are presented to account for the observed results. It was concluded that both direct photolysis and helium sensitization were important primary processes in all three hydrocarbons.