Photolysis Of CH Research Articles

UV radiative transfer and photolysis in Jupiter's atmosphere

A complete quantitative calculation of UV radiative transfer and photolysis in a model Jovian atmosphere, including scattering by H 2 and He and absorption by H 2, He, CH 4, NH 3, and H 2S, in the region λ ⩽ 1700 A ̊ , has been carried out. This shows that the principal reactions are the photolysis of NH 3 above the NH 3 clouds and the photolysis of H 2S in the NH 4SH clouds, if there are gaps in the NH 3 clouds allowing penetration of UV radiation further into the atmosphere. The photolysis of CH 4 is considerably less important and, in particular, the production of ionized CH 4 is minimal. This implies that the production of substantial amounts of polymers in the upper atmosphere is unlikely, in agreement with the observation that there appear to be no large gaseous absorptions other than those of H 2, CH 4, and NH 3, above the NH 3 clouds. The colorations of the planet are proposed to be due to the photolysis of NH 3 and H 2S gases in cloud regions and the freezing out of photolysis products in cloud particles.

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

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 CH4*→CH2+H2.Our data disagree with this view and actually demonstrate that CH and CH2 play a minor role in the xenon-sensitized radiolysis of methane.

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

The photolysis of the methane hydrate (CH4·6H2O) at −195°C at 1470 and 1236 Å has been investigated. Mixtures of methane and water in the gas phase have also been photolyzed at 1470, at 1236 Å, and in the region 1450–1850 Å. In both gas- and solid-phase photolysis, CO and CO2 are formed at 1470 and 1236 Å. CO2 is formed from CO, probably by the reaction OH+CO→CO2+H, which is shown to occur readily in the solid at −196°C. In the photolysis of CH4·6H2O at −196°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 Å to those at 1470 Å suggests that CH2 is present in both systems. Since photodecomposition of CH4 is negligible at 1470 Å, 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.

Direct and Inert-Gas-Sensitized Radiolysis and Photolysis of Methane in the Solid Phase

The photolysis of CH4–CD4 mixtures has been briefly investigated at 20.4°K using the xenon and krypton resonance lines. From the isotopic composition of the hydrogen and ethane fractions, it could be derived that, in the solid phase, methylene and methyl radicals are produced. The methylene radicals, which are formed by the process CH4*→CH2+H2, insert into methane to form ethane. In the argon-sensitized radiolysis at 20.4° and 77°K, the hydrogen-molecule elimination process predominates, indicating that neutral excited-methane molecules are formed nearly exclusively. In the xenon-sensitized radiolysis, however, mainly hydrogen atoms and methyl radicals are observed. In all inert gas-sensitized radiolyses, there is a gradual decrease in the efficiency of the energy transfer with increasing dilution. In the direct radiolysis, hydrogen atoms play a more important role than in the photolysis at 1236 Å. Radiolysis of CD4−C2H4 (1:0.01) mixtures indicated that, at 77°K, hydrogen atoms react with ethylene and propylene to form higher saturated hydrocarbons while, at 20.4°K, hydrogen atoms disappear mainly by recombination with other free radicals. From the isotopic composition of the ethylene fraction formed in the radiolysis of Ar–CH4–CD4 mixtures, it is surmised that a fraction of the methylene recombines to form ethylene.

Vacuum Ultraviolet Photolysis of Methane

The photolysis of CH4—CD4 mixtures, CH2D2, and CHD3 with 1236 Å light shows that molecular elimination of hydrogen from methane is an important but not an exclusive primary photochemical process. The photolysis products are principally those expected from the insertion reaction of the methylene radical.