Shock-Tube Study of Vibrational Exchange in N2–O2 Mixtures

Abstract

Vibrational excitation in gas mixtures can occur both by translation‐to‐vibration (T–V) energy conversion in simple collisions and by the exchange of vibrational energy (V–V) between components. Nitrogen–oxygen mixtures containing 5%, 10%, 21%, 33%, and 50% O₂ have been studied in a shock tube using both optical interferometry to measure the density profile through the relaxation zone and infrared emission of CO added in small quantities to serve as an indicator of the degree of vibrational excitation of N₂. The density increase has been resolved into the sum of short and long time‐constant exponentials. The former is approximately that expected from excitation of the O₂ component. The longer time constant of the increasing density is approximately that observed for the infrared emission of CO and is thus indentified with the N₂ excitation. This latter is found to be more rapid than can be accounted for by a T–V process and therefore vibrational exchange with excited O₂ must contribute signficantly. Data have been analyzed to obtain a transition probability per collision for the exchange process which varies from 10⁻⁶ to 2.3 × 10⁻⁵ over the range 1000°–3000°K.