Vibrational Relaxation of CO in Nonequilibrium Nozzle Flow, and the Effect of Hydrogen Atoms on CO Relaxation

The vibrational relaxation of carbon monoxide was studied under conditions of rapid nonequilibrium expansion by using a shock tunnel to generate a nozzle flow with stagnation temperatures and pressures of 2000–4500°K and 5–15 atm., respectively. The vibrational temperature of the CO in the supersonic region of the nozzle was obtained from measurements of the first overtone emission at $\text{2.3 μ}$ by using a calibrated infrared detection system. From these data it was determined that the relaxation time of the CO inferred from the expansion experiment is, at most, 5 times smaller than the relaxation time measured behind incident shock waves. This factor of 5 is in sharp disagreement with previously published measurements in CO and N₂, which quote factors from 70 to 1000, implying anomalously fast de‐excitation of vibration in expanding flows, but is in agreement with other subsequently obtained measurements. Impurities were found to be more important in this type of experiment than in measurements of relaxation time behind incident shock waves mainly because of additional ways of their getting into the flow. Hydrogen atoms were found to have a probability per collision for the de‐excitation of CO of $P_{10}\text{\hspace{0.17em}=\hspace{0.17em}0.01}$ to 0.3 for $\text{T\hspace{0.17em}=\hspace{0.17em}1400–2800°}\mathrm{K}$ . It is not certain whether the observed factor of 5 for pure CO is due to residual impurity effects or due to effects of anharmonicity as discussed in recent theories.