The resolution time of the apparatus for rapid measurements of gas densities in shock and detonation waves by the absorption of x-rays was evaluated by studies of shock waves and found to be largely determined by the slit width. The rotational relaxation of N2, O2, H2, and CH4 in mixtures with xenon and the vibrational relaxation of O2, H2, and CH4 were too fast for observation. The dissociation rate constants were determined for hydrogen : kD = 2.5 and 4.2×10—16 cc/sec at 2850 (?) and 3250°K and for oxygen : kD = 2.3 and 6.8×10—16 cc/sec at 3100 and 3500°K. The initial (shock front) densities in 2H2+O2+½Xe detonating mixtures were found to be consistent with calculations based on full rotational but no vibrational relaxation, when it was assumed that the rates of exothermic reactions rise to a maximum within less than one μsec and then decrease somewhat faster than in a second order reaction. The pre-exposure of the cold gases to x-rays for a few seconds results in faster initial reaction rates, while a pre-exposure of only a few milliseconds has no measurable effect. No effect of pre-exposure on the rates after about 50% completion of the reaction is evident. These findings indicate a highly branched mechanism of chain carrier generation. The effects of total pressure and gas composition on the duration of the reaction zone were investigated with several mixtures. Substitution of deuterium for hydrogen reduces the rate by about 30%. The retarding effect of water vapor on the rate of the hydrogen-oxygen reaction is not pronounced.
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Chesick et al. (1958) studied this question.