Vibrational Relaxation of the Bending Mode of Shock-Heated CO2 by Laser-Absorption Measurements

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Abstract
The vibrational relaxation characteristics of shock‐heated CO2 have been studied using a tuned CO2 laser absorption technique. Absorption‐coefficient histories were obtained for a single rotational state in each of the 10°0 and 02°0 levels over the temperature range from 500 to 2000°K, and for 21 rotational states of the 10°0 level at 1000°K. These histories have been combined with translational‐rotational temperature histories based on interferometer measurements to calculate vibrational relaxation times for the bending mode. The results verify the mutual equilibrium of the bending and symmetric‐stretch modes due to Fermi resonance. The bending mode relaxation times are approximately 10% shorter than predicted from interferometer results using the ratio of specific heats. Furthermore, relaxation times based on measurements of different rotational states at 1000°K show a variation with quantum number J, indicating a possible rotational nonequilibrium during the vibration relaxation process.