Gas-Phase Photolysis and Radiolysis of Methane. Formation of Hydrogen and Ethylene

Abstract

The photolysis of CH₄ and of CH₄—CD₄ mixtures has been investigated at 1236 Å (10.0 eV) and at 1048–67 Å (11.6–11.8 eV). The excited methane molecule dissociates to form H₂, H, CH₃, CH₂, CH, and probably also C. The CH and CH₂ radicals insert into methane to form internally excited C₂H₅^* and C₂H₆^* species, respectively. Below one atmosphere, all C₂H₅ radicals decompose to form C₂H₄, while the ethane molecules are partially stabilized. The relative quantum yield of CH increases about threefold when the wavelength is reduced from 1236 Å to 1048–67 Å. On the basis of an isotopic analysis of the hydrogen produced in the photolysis of CD₄—H₂S mixtures, it is concluded that at 1236 Å, D atoms constitute at least 65% of the ``molecular'' deuterium yield. In the radiolysis, ethylene is largely, although not exclusively formed by the insertion of CH into methane. It is demonstrated that addition of small concentrations of an unsaturated hydrocarbon to methane profoundly affects the ion‐molecule reaction mechanism and, therefore, does not lead to a dependable value of the ``initial'' ethylene yield as suggested in earlier studies. Upon application of an electrical field, the production of CH and CH₂ is augmented in the saturation current region. The importance of the latter two radicals in the direct and rare‐gas‐sensitized radiolysis is examined briefly. The formation of hydrogen in the radiolysis will be discussed on the basis of new information derived from CD₄—H₂S experiments. The production of hydrogen in the radiolysis of Xe–CH₄–CD₄–NO mixtures has also been re‐examined in view of a recent study in which it was asserted that all of the hydrogen in such a mixture is due to the unimolecular decomposition process $${\mathrm{CH}}_4^{*}\to {\mathrm{CH}}_2+{\mathrm{H}}_2.$$ Our data disagree with this view and actually demonstrate that CH and CH₂ play a minor role in the xenon‐sensitized radiolysis of methane.