Vibrational Energy Exchange between N2 and CO. The Vibrational Relaxation of Nitrogen

The vibrational energy exchange N2(v=1)+CO(v=0)=N2(v=0)+CO(v=1) should be fast, due to near resonance of the energy levels. We have studied the rate of this exchange relative to the rate of energy transfer from translational to vibrational degrees of freedom in a shock-heated mixture of 1% CO—99% N2. Vibrational relaxation of the CO is observed by its infrared emission near 2000 cm—1. The density change behind the incident shock is simultaneously observed with an interferometer and gives a measurement of the N2 relaxation time. In the 3200°—4200°K range both methods are applicable; they yield identical relaxation times within the data scatter of ±20%. In contrast, at 3600°K, the vibrational relaxation time of pure N2 is four times that of pure CO. The exchange reaction thus appears fast enough to make the vibrational temperatures of the N2 and CO equal during the vibrational relaxation of the mixture. In the 1900° to 5400°K range, our study gives vibrational relaxation times in seconds for pure N2 (p=1 atm), represented by log10 τ=102T—⅓—11.24. This agrees at high temperature with Blackman's data, but gives a somewhat different temperature dependence. Extrapolation of our data to lower temperatures indicates relaxation times an order-of-magnitude longer than given by ultrasonic experiments.