Trapped hydrogen atoms, deuterium atoms, and methyl radicals in methane and methane-d4 at 5-50 K. Yields from photolysis of HX and from radiolysis; decay mechanisms; reactions with oxygen and carbon monoxide

Abstract

This paper reports an investigation of the yields, decay properties, chemical reactions, and ESR saturation properties of trapped hydrogen atoms (H/sub t/ and D/sub t/) and methyl radicals (CH/sub 3/ and CD/sub 3/) formed by the radiolysis of CH/sub 4/ and CD/sub 4/ at 5 K and by the 254-nm photolysis of HI and DI and the 185-nm photolysis of HBr in CH/sub 4/ and CD/sub 4/ at 5 K. The quantum yields depend on the isotopic nature of the atoms and the matrix, and on the HX source. The observed products from the photolysis of HBr or HI in CD/sub 4/ are H/sub t/ and CD/sub 3/, formed by hot abstraction, and D/sub t/, formed in part by the H + CD/sub 4/ ..-->.. CD/sub 3/H + D displacement and in part by the photodecomposition of CD/sub 3/. The decay rates at higher temperatures are slower in CD/sub 4/ than in CH/sub 4/ and depend on the isotope and its photolytic source. The decay rates of H/sub t/ and D/sub t/ from HX are composite first order. The atoms remaining after approx. 50% have decayed are able to react with dissolved CO forming HCO, O/sub 2/ forming HO/sub 2/,more » and C/sub 2/H/sub 6/ forming C/sub 2/H/sub 5/. More than 90% of the H/sub t/ produced by X radiolysis of neat CH/sub 4/ decays by H + H ..-->.. H/sub 2/. Ca. 38% of the CH/sub 3/ produced when HI is photolyzed in CH/sub 4/ decays concurrent with H/sub t/ decay at 45 K, for which comparative rates are reported. The threshold powersfor ESR saturation of H/sub t/, D/sub t/, CH/sub 3/, and CD/sub 3/ range from 1 mW. Tests for the production of H/sub t/ and D/sub t/ by the photolysis of HI in several polycrystalline hydrocarbons have given negative results. Evaluation of absorbed X-ray dose rates by samples of different average atomic number is described.« less