The reaction between H2 and D2 in a shock tube: Study of the atomic vs molecular mechanism by atomic resonance absorption spectrometry

Abstract

The exchange reaction between hydrogen and deuterium was studied behind reflected shocks in a single pulse shock tube. A vacuum uv monochromator and a Lyman‐α radiation source were attached to the end block of the driven section in order to determine the hydrogen atom profile during the hot phase. These atoms are the result of impurities which are present in the shock tube. Two calibration attempts of the I_t/I₀ vs [H]_t around 1250 °K, using the decomposition scheme of propane and the postexplosion conditions in H₂/O₂/Ar mixtures, were unsuccessful. A calibration method which utilizes an integrated absorption profile is described. A two parameter calibration function (modified Beer–Lambert law) was derived: I_t/I₀=exp(−α[H]^β_t), where α=2.92×10⁷ and β=0.73 in units of mole‐cm. For each test, a sample was withdrawn from the tube and was analyzed mass spectrometrically for postshock distribution of product and reactants. In addition, the absorption profile at 1215.7 Å was recorded and the extent of HD produced by hydrogen atoms (which originated from impurities) was numerically integrated using the two parameter calibration function and the atomic chain reactions H+D₂→HD+D and D+H₂→HD+H. The mass spectrometrically measured extents of exchange were always considerably higher than the ones calculated from the absorption profile, on the average by a factor of 2–3. If the difference between the two conversions is attributed to a molecular mechanism, then if calculated on the basis of the rate law H₂+D₂→2HD the following rate constant is obtained: k₆=10^14.1±0.8 exp[−(38±5) ×10³/RT] mole⁻¹ cm³ sec⁻¹.