Ionization Accompanying the Acetylene—Oxygen Reaction in Shock Waves

A Langmuir probe was used to study the ionization produced during the oxidation of acetylene in shock waves. While the results were ambiguous at low applied potentials, positive probes were shown to measure approximately the free electron concentrations. These were found to be several orders of magnitude higher than those of thermodynamic equilibrium. The ionization behaves very similarly to the emission of short uv radiation from shock waves in acetylene—oxygen mixtures described by Kistiakowsky and Richards: after an induction period it rises exponentially and then decays. The values of the induction periods and of the exponential time constants agree with those obtained from uv emission studies. Emission of visible and near uv radiation has also been studied now and very similar results obtained. The temperature coefficient of the maximum ionization and maximum radiation intensities is the same, equivalent to 23 kcal/mole. The decay of ionization is second order. The mechanisms of chemiluminescence and chemi‐ionization are discussed, and it is suggested that the following reactions are involved: $${\mathrm{C}}_2\mathrm{H}+{\mathrm{O}}_2\to {\mathrm{CO}}_2+{\mathrm{CH}}^{*},$$ or $${\mathrm{C}}_2\mathrm{H}+\mathrm{O}\to \mathrm{CO}+{\mathrm{CH}}^{*};$$ $$\begin{array}{c}{\mathrm{CH}}^{*}\to \mathrm{CH}\mathrm{+hv}\\ \mathrm{CO}+\mathrm{O}\to {\mathrm{CO}}_2\mathrm{+hv}\end{array}\}\mathrm{\hspace{0.17em}\hspace{0.17em}(}\mathrm{visible\; and\; near\; uv}\mathrm{),}$$ $$\begin{array}{c}{\mathrm{CH}}^{*}+\mathrm{O}\to \mathrm{CO}{\mathrm{(A}}^1\mathrm{\pi )+}\mathrm{H},\\ \mathrm{CO}{\mathrm{(A}}^1\mathrm{\pi )\to }\mathrm{CO}{\mathrm{(X}}^1\mathrm{\Sigma )+hv\hspace{0.17em}\hspace{0.17em}(}\mathrm{vacuum\; uv}\mathrm{),}\\ {\mathrm{CH}}^{*}+\mathrm{O}\to {\mathrm{HCO}}^{+}{\mathrm{+e}}^{-}\mathrm{\hspace{0.17em}\hspace{0.17em}(}\mathrm{ionization}\mathrm{).}\end{array}$$ The effect of small additions of lead tetramethyl and acetone on the kinetics of the reaction was investigated. Both additives reduce sharply the induction periods in the reaction, without affecting strongly the rate of exponential growth of the reaction rate as evidenced by ionization and radiation.