The reaction H2+D2⇄2HD. A long history of erroneous interpretation of shock tube results

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Abstract
An ultraclean 2 in. i.d. single pulse shock tube coupled to an atomic resonance absorption system was constructed in order to determine hydrogen atom concentration by Lyman‐α absorption. The tube was baked to 300 °C and pumped down to ∼10−7 Torr. Ultrapure argon could be shock heated to ∼2500 K with no spurious H atom absorption. The system was constructed in order to study the kinetics of chemical reactions which are strongly catalyzed by H atoms, under the conditions where no such atoms are present. Specifically, the role of H atoms in the H2+D2→2HD exchange reaction was studied. Mixtures of hydrogen and deuterium diluted in argon were shock heated to 1375–1760 K; samples were then taken from the tube and analyzed mass spectrometrically for the ratio [HD]/[D2]. 1400 K was the highest temperature at which no spurious H atom absorption was observed in a shocked mixture of 1% H2–1% D2. At 1400 K, under the conditions of no absorption, no, or ≤1%HD conversion was obtained. At higher temperatures Lyman‐α absorption was detected and more HD conversion was observed. A comparison between these results and results obtained previously in conventional systems suggests that the high HD conversion observed in the past was caused by hydrogen atoms generated from impurities. The existence of a molecular mechanism in the H2–D2 exchange reaction is highly doubtful.