Effect of Coadsorbed Oxygen on the Photochemistry of Methane Embedded in Amorphous Solid Water

The photochemistry of methane caged within amorphous solid water (ASW) is interesting as a model for studying interstellar and high-altitude atmospheric pathways for the formation of more complex hydrocarbons. Here, we report on the photoreactivity of clean methane and in the presence of oxygen molecules, known as electron capture species, within two 50 monolayer-thick D₂O-ASW films adsorbed on Ru(0001) substrate under ultrahigh vacuum conditions. Irradiation by 248 nm UV photons (5.0 eV), where none of the involved molecules absorb these photons in the gas phase, leads to the formation of diverse hydrocarbons. In all cases, the presence of oxygen results in significantly enhanced reactivity. The dissociative electron attachment mechanism with electrons generated within the metal substrate is thought to largely govern the photoreactivity in this system. Methyl radicals as the primary photoproducts subsequently react with the surrounding water and neighboring methane as well as with the stable O₂ ^- anion. Postirradiation temperature-programmed desorption measurements revealed cross sections for hydrocarbon formation in the range of 10^-20 to 10^-21 cm². Possible mechanisms underlying the formation of various hydrocarbons and carbon dioxide as the final oxidation product are discussed.