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

Abstract

The photolysis of CH. and of CH.-CD. mixtures has been investigated at 1236 A (10.0 eV) and at 1048-67 A (11.6-11.8 eV). The excited methane molecule dissociates to form H2, H, CHa, CH2, CH, and probably also C. The CH and CH2 radicals insert into methane to form internally excited C2H.• and C2H6* species, respectively. Below one atmosphere, all C2H. radicals decompose to form C2H., while the ethane molecules are partially stabilized. The relative quantum yield of CH increases about threefold when the wavelength is reduced from 1236 A to 1048-67 A. On the basis of an isotopic analysis of the hydrogen produced in the photolysis of CD.-H2S mixtures, it is concluded that at 1236 A, 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 CD.-H2S experiments. The production of hydrogen in the radiolysis of Xe-CH.-CD.-NO mixtures has also been reexamined 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 CH,*->CH2+Hz• Our data disagree with this view and actually demonstrate that CH and CH. play a minor role in the xenon-sensitized radiolysis of methane.

Modeling of methane photolysis in the reducing atmospheres of the outer solar system

“…In order to model the photochemistry of methane accurately, we need precise values for the absorption cross sections and quantum yields for the primary dissociations. Analyses of product yields after irradiation, using Kr or Ar emission lines [Ausloos et al, 1964;Gorden and Ausloos, 1967], have been difficult to interpret because of the complexity of possible secondary reactions. However, more recent direct measurements, using photofragment imaging techniques [Heck et al, 1996;Mordaunt et al, 1993], have given us a clearer picture of the primary photodissociation mechanisms.…”

Photochemistry of methane in the Earth's early atmosphere

“…Finally, due to our neglect of nitrogen chemistry, the present model overlooks the production of hydrogen cyanide w a compound of special interest to biochemists studying the origin of life. Hydrogen cyanide could have been formed at altitudes above 40 km by CH4 +hv-*CH+H2 +H followed by CH+N2 ~HCN+N About 7% of methane photolysis reactions yield CH radicals (Gorden and Ausloos, 1967), which react with N2 at a rate of between 7.3 × 10 -14 (Braun et al, 1967) and 1 × 10 -~2 (Bosnali and Perner, 1971). Hydrogen cyanide, like CH4, photolyzes only at very short wavelengths (Huebner and Carpenter, 1979), so most of the HCN produced at high altitudes could have survived long enough to be transported down into the lower atmosphere.…”

Photochemistry of methane and the formation of hydrocyanic acid (HCN) in the Earth's early atmosphere

“…Triplet methylene may then be oxidized, polymerized, or react with atomic nitrogen to from HCN. The branches (R68) and (R68') are equally likely [Slanger and Black, 1982], while production of CH has a quantum yield of about 0.07 [Gordon and Ausloos, 1967] The reported reaction rate with CO2 (k = 3.8 x 10 -x½ cm 3 s-• [Laufer, 1981]) is quite low and, perhaps, represents the presence of a considerable activation barrier. If so, then this reaction may be very much slower at mesospheric temperatures where it would have taken place.…”