Density Gradient Measurements of O2 Dissociation in Shock Waves

Abstract

The dissociation of O₂ in mixtures with He, Ar, Kr, and Xe has been studied behind incident shock waves in the temperature range 4000–8500°K. Using a laser‐beam deflection technique, postshock density gradients were measured on a time scale such that vibrational relaxation and the onset of dissociation are resolvable. The density gradient profiles exhibit a highly characteristic local minimum immediately following the period of vibrational relaxation, at times which correlate well with the induction periods observed by others. The magnitude of the density gradient at this point is interpreted as a measure of the initial dissociation rate, i.e., the rate corresponding to conditions of complete vibrational equilibrium with no dissociation. Rate coefficients thus obtained exhibit a degree of precision unmatched in previous dissociation studies, and agree well in magnitude and temperature dependence with other reported results. Values of $k_{{\mathrm{O}}_2‐\mathrm{M}}$ , the rate coefficient for O₂ dissociation due to ${\mathrm{O}}_2–\mathrm{M}$ collisions, were determined for M=He, Ar, Kr, and Xe from measurements with mixtures containing 2.5%, 5%, and 10% initial O₂. For ${\mathrm{O}}_2‐\mathrm{Kr}$ collisions the value $$k_{{\mathrm{O}}_2‐\mathrm{Kr}}{\text{=7.87×10}}^{13}\hspace{0.17em}\exp \text{(− 104\hspace{0.17em}800/RT)}\mathrm{cc}/\mathrm{mole}·\sec ,$$ with R in calories per mole·degree, is obtained, with $k_{{\mathrm{O}}_2‐\mathrm{Xe}}{\text{≅1.6 k}}_{{\mathrm{O}}_2‐\mathrm{Kr}}{\mathrm{,\; k}}_{{\mathrm{O}}_2‐\mathrm{Ar}}{\mathrm{\cong k}}_{{\mathrm{O}}_2‐\mathrm{Kr}}$ and $k_{{\mathrm{O}}_2‐\mathrm{Xe}}{\mathrm{〉k}}_{{\mathrm{O}}_2‐\mathrm{He}}{\mathrm{〉k}}_{{\mathrm{O}}_2‐\mathrm{Kr}}$ . The value $k_{{\mathrm{O}}_2‐{\mathrm{O}}_2}{\text{≅9 k}}_{{\mathrm{O}}_2‐\mathrm{Kr}}$ was obtained independently from data for four different inert diluents, supporting the validity of the linear mixture formula for collision partners in O₂ dissociation. The character of the density gradient profiles at later times suggests a higher rate coefficient for collisions with oxygen atoms.