A direct isothermal measurement has been made of the rate of formation of atomic hydrogen behind shock waves in hydrogen–argon mixtures. This has been accomplished by using atomic resonance absorption spectrophotometry at the Lyman‐α line of both ¹H and ²H. The observations were made in an ultrahighpurity shock tube. Absorption coefficients for the atoms were determined experimentally. The hydrogen–argon dissociation rate is given by the expression

k_{H_{2} −Ar} {= 2.23 × 10}^{12} T^{1 / 2} \exp (−92 600 / RT) {cm}^{3} {mole}^{−1} \cdot \sec^{−1}

over the temperature range 2290°–3790°K. The hydrogen molecule was found to be five times more effective than argon as a collision partner. Because of the sensitivity of the technique, it was also possible to determine quantitatively the catalytic effect of the addition of small amounts of oxygen. From these observations an expression

k_{{H−O}_{2}} {= 6.0 × 10}^{12} T^{1 / 2} \exp (−17 750/RT) {cm}^{3} {mole}^{−1} \cdot \sec^{−1}

was obtained for the reaction H + O₂ = OH + O over the temperature range 1700°–2700°K.

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