A shock-tube program was carried out in which the NO concentration was followed as a function of time behind the shock front by absorption of 1270 A radiation, where ground vibrational state O2 and N2 are essentially transparent. The absorption coefficients of the species NO, O2, and N2 as functions of the respective vibrational temperatures were determined by measuring the absorption by the shock-heated gas at a point in the time history corresponding to complete vibrational relaxation but before the onset of dissociation. Time history analyses were made on a total of 42 shock-tube runs covering a temperature range of 3000°—8000°K on the following six mixtures: ½% NO, ½% NO+¼% O2, 10% NO, 50% NO, 20% air, and 100% air—the diluent in all cases being argon. An IBM 704 computer was programmed to integrate the vibrational and chemical rate equations as a function of time behind the shock front, subject to the constraints of the conservation equations. The pertinent rate constants were varied in a systematic trial-and-error manner in order to get the best fit to all the data.

The problem of spontaneous ionization (i.e., no externally applied electromagnetic fields, nor hard radiation) in the reaction zone behind strong normal shock waves in air has been treated concurrently with the problem of dissociation and vibrational relaxation. Through a comparison of specific ionization rates, one may conclude that up to a shock velocity of 9 km/sec (about 27 times the speed of sound at room temperature), the predominant electron production process would be atom—atom ionizing collisions. This would be followed in an approximately decreasing order of importance by photoionization, electron impact, atom—molecule collisions, and molecule—molecule collisions. The charge exchange reactions, while not contributing directly to the electron production process, were found to have a small but noticeable indirect effect on the resultant electron density distribution at some distance behind the shock due to their continuous shifting of the relative population between atomic and molecular ions (which recombine with the electrons at different rates). The specific rate constants for the atom—atom processes required to interpret all existing experimental results appear to be consistent with a simple extrapolation of the low-temperature rate constants according to the crossing-point model of Bates and Massey for atom—atom ionizing collisions.

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 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 P10 = 0.01 to 0.3 for T = 1400–2800°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.

The physical properties behind a normal shock in nitrogen are calculated as a function of time. These include the variation of temperature, composition, ionization, and the intensity of radiation from the N2+ first negative band system. This calculation incorporates a rate equation for the dissociation of nitrogen, the conservation laws, an equation describing vibrational relaxation, and a method of coupling the vibrational relaxation with the dissociation rate. The N2+ radiation is computed assuming excitation of the radiating state by collision with vibrationally excited nitrogen molecules. A particular case is considered for which experimental data are available, and regions sensitive to particular rates are indicated.

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