Hot hydrogen atom reactions moderated by H2 and He.

Photolysis experiments were performed on the H/sub 2/-CD/sub 4/-NH/sub 3/ and He-CD/sub 4/NH/sub 3/ systems. The photolysis (1849 A) involved only NH/sub 3/. Mixtures of H/sub 2/:CD/sub 4/:NH/sub 3/ included all combinations of the ratios (200,400,800):(10,20,40):4. Two He:CD/sub 4/:NH/sub 3/ mixtures were examined where the ratios equalled the combinations 100:(10,20):4. Abstraction of a D from CD/sub 4/ by the photolytically produced hot hydrogen from ammonia was monitored by mass spectrometric determination of HD. Both experiment and semiempirical hot-atom theory show that H/sub 2/ is a very poor thermalizer of hot hydrogens with excess kinetic energy of about 2 eV. Applications of the hard-sphere collision model to the H/sub 2/-CD/sub 4/-NH/sub 3/ system resulted in predicted ratios of net HD production to NH/sub 3/ decomposition that were two orders of magnitude smaller than the experimental ratios. On the other hand, helium is found to be a very efficient thermalizer; here, the classical model yields reasonable agreement with experiments. Application of a semiempirical hot-atom program gave quantitative agreement with experiment for either system. 15 references, 2 figures, 4 tables.