Reactions of Photochemically Produced Hot Hydrogen Atoms. II. Reaction and Moderation Probabilities for 3-eV H Atoms in D2, CD4, and C2D6 and for 3-eV D Atoms in H2, CH4, and C2H6

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Abstract
Three‐electron‐volt H atoms produced in D2, CD4, or C2D6 by the photolysis of HBr may produce HD by hot‐atom reaction with the deuterated species, or H2 by hot reaction with HBr, or may be thermalized by moderating collisions and scavenged by thermal reaction with HBr. Plots of the product ratios, H2/HD, vs the reactant ratios, HBr/RD, give straight lines. The values of the intercepts are equal to the ratio of the atoms moderated to those which undergo hot reaction with RD in pure RD. From the slopes and intercepts it is possible, with plausible assumptions, to deduce approximate values for the average probability of reaction per collision in the hot range and for the average fractional energy loss per moderating collision with the hydrocarbon. The latter value when compared with the calculated loss for elastic collisions gives an indication of the inelasticity of the collisions in the experimental system. These determinations have been made for the systems stated and for the isotopically reversed systems of DBr in H2, CH4, and C2H6. They indicate that the average probabilities of reaction per collision for the H atoms colliding with D2, CD4, and C2D6 are 0.18, 0.0034, and 0.019, respectively, while those for the reactions of the D atoms with the corresponding hydrogenated compounds are 0.35, 0.062, and 0.14. The ratio of actual energy loss per collision in this range to loss assuming elastic hard‐sphere collisions is estimated to be about 1 for H+CD4, 4 for H+C2D6, 2 for D+CH4, and 4 for D+C2H6, based on the assumption that H+D2 and D+H2 moderating collisions are completely elastic. If these latter collisions are partially inelastic, both the estimated reaction probabilities and moderating efficiencies given above should be higher.