Shock-Tube Study of the Coupling of the O2–Ar Rates of Dissociation and Vibrational Relaxation

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Abstract
At low temperatures the vibrational relaxation time τv is much shorter than the dissociation time τd. The O2–Ar results of Camac [J. Chem. Phys. 34, 448 (1961)] and Camac and Vaughan [J. Chem. Phys. 34, 460 (1961)] yield τd/τv=60 at 5000°K and, upon extrapolation, τd/τv=1.4 at 18 000°. According to these extrapolations, dissociation at high temperatures would proceed significantly before vibrational equilibration would occur. The purpose of this investigation was to determine how the dissociation rate will be affected by a lack of vibrational equilibrium. Studies of the dissociation rate of dilute O2–Ar mixtures were made in a 24‐in. diam shock tube from 5000°—18 000°K. The O2 concentration was monitored by its absorption of 1470 Å radiation. An Arrhenius plot of the data yielded a straight line from 5000°—11 000°K, the rate constant being given by kd=2.9(±12%)×1014 exp (—D/RT)cc/mole‐sec. Above 11 000° the data deviate from the line given by this equation—at 18 000° kd being 0.45 times the calculated value. An incubation time Δt was observed during which dissociation does not proceed to a significant extent. The ratio of this incubation time to the vibrational relaxation time (obtained by extrapolating Camac's low‐temperature results) when plotted against translational temperature displays a slight negative temperature dependence. At 18 000° Δt/τv=0.4, at 8000° Δt/τv=1, and at 5500° we estimate that Δt/τv≈2.

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