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

Abstract

At low temperatures the vibrational relaxation time τ_v is much shorter than the dissociation time τ_d. The O₂–Ar results of Camac [J. Chem. Phys. 34, 448 (1961)] and Camac and Vaughan [J. Chem. Phys. 34, 460 (1961)] yield τ_d/τ_v=60 at 5000°K and, upon extrapolation, τ_d/τ_v=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 k_d=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° k_d 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/τ_v=0.4, at 8000° Δt/τ_v=1, and at 5500° we estimate that Δt/τ_v≈2.