Experiments on the measurement of air emission intensity behind the front of incident shock wave were carried out in a shock tube at an initial pressure of 0.25 Torr and shock wave velocities of 6.3–8.4 km/s. The emission intensity was measured in absolute units both in the form of an integral spectral distribution in a wavelength range of 120−400 nm (panoramic spectra) and as the time evolution of emission at the individual atomic lines of nitrogen and oxygen atoms. The results of the measurements demonstrated that the emission in air behind a shock wave in the vacuum ultraviolet region of 120–200 nm had a much higher radiation flux level than the emission in a range of 200–900 nm.

We have measured the absorption cross sections of oxygen molecules in oxygen and in an oxygen-argon mixture heated by a shock wave, in the wavelength range 190–250 nm at temperatures of 1500–7000 K, for thermal equilibrium conditions behind the shock wave front. Analysis of the absorption cross sections obtained allowed us to select a data set that adequately describes the absorption characteristics of the electronic transition X3Σ g − → B3Σ u − for the oxygen molecule.

1 Institute of Mechanics, Moscow State University, 119192, Moscow, Russia 2 P.I. Baranov Central Institute of Aviation Motors, 111116, Moscow, Russia 3 IFPHT Ukrainian Academy of Sciences, 03028 Kiev, Ukraine 4 A.V. Luikov Heat Mass Transfer Institute, Belarus Academy of Sciences, 220072, Minsk, Belarus 5 Physikalisch-Chemisches Institut, Ruprecht-Karls-Universitat, Heidelberg 69120, Germany 6 Laboratoire d’Aerothermique CNRS, Orleans 45071, France

Expressions are derived for the dissociation rate constants of molecules of carbon dioxide in the temperature range from 300 to 40 000 K under both thermally equilibrium and nonequilibrium conditions. Under nonequilibrium conditions, the rate constants are represented as a two-temperature dependence (on the gas temperature T and on the unified temperature T-V of all vibrational modes of CO2) and as a one-temperature dependence in which averaged vibrational nonequilibrium is included.

By means of an analysis of currently available experimental data and theoretical models, expressions for the dissociation rate constantk 0 are obtained for thermal equilibrium conditions. These expressions are necessary for describing the molecular dissociation process under both thermal equilibrium and non-equilibrium conditions in the gas.k 0 values are presented for the O2, N2, NO, CO, CN, and C2 molecules at temperatures from 300 to 40,000 K, and their errors are estimated.

A calculation was made of the gain as a result of interaction of light with S2 molecules formed by recombination of sulfur atoms during cooling of a mixture of gases containing sulfur, first heated in a forechamber, by supersonic expansion. The optimal parameters of the gas flow and nozzle (initial composition of the mixture, pressure, temperature, nozzle profile and coordinates) corresponding to the maximum possible values of the optical gain were found. Gas mixtures CS2–Ar and S2–Ar were considered. A calculation of steady-state flow of a gas in a nozzle was carried out allowing for chemical reactions in the one-dimensional approximation. A search for the gain maximum (for a given initial gas pressure in the forechamber) was made by the method of configurations. At gas pressures of 10–100 atm in the forechamber, calculations yielded maximum values of the gain in the range 10−4–2 × 10−3 cm−1(corresponding to the initial relative concentrations 13–5% and 17–5% of the CS2 and S2 molecules, respectively).

The shock tube consisted of a cylinderical channel made of stainless steel (with inner diameter 50 ram), separated into low and high pressure chambers. At the time the membrane is ruptured, a shock wave, which at some time reaches the measuring sections, propagates with supersonic velocity V along the low-pressure chamber, filled with the gas being studied at initial pressure Pl. The radiation (either characteristic radiation of the gas in the shock tube or probing radiation from a pulsed source), passing through the optical windows of the measuring sections, was incident on the monoehrometers, which separate out the narrow parts of the spectrum in the violet system of CN (the transition B2Z + ~ X2E+). From the monoehrometers, the radiation was incident on the photomultipliers , operating in the linear region of light characteristics. The electrical signals from the photomultipliers were recorded by a pulsed oscillograph. The frequency characteristics of the amplification channel photomultiplier + oscillograph taking into account the integrating action of the optical slits (situated on the windows of the measuring sections) provided, in recording the radiation, a time resolution not worse than 0.25 t~sec. The cyanogen radiation was observed in the (0, 1) band of the violet system. The instrumental function of this optical channel had a trapezoidal form with a base A; h = 420.2-422.2 rim. In this interval of wavelengths, there are approximately 60 rotational doublet lines of CN. A typical oscillogram of the radiation of cyanogen is shown in Fig. la, where it is evident that the intensity of the radiation immediately behind the shock wave front increases, passes through a maximum, and then drops to a stationary level. The beginning of the radiation coincides with the arrival of the shock wave front at the slit in the window of the measuring section. The time of arrival of the front was recorded with the help of a piezoelectric sensor. Special experiments showed that the oscillograms obtained represent emission precisely by CN molecules, and not by other molecules, impurities, or the background. With the help of control experiments, it was established that in the wavelength interval separated at the temperatures, pressures, and initial compositions of the gases studied, there was no self-absorption of radiation of cyanogen for an optical path length l = 5 cm (internal diameter of the shock tube). This means that the intensity of the cyanogen radiation at any point in the osciUogram is proportional to the concentration of CN molecules in the excited electronic state B2Z + (v' = 0).

Mixtures of CO (or CO2) gases and N2 behind strong shock fronts at temperatures 4000–10 000 ° K have been investigated with a view to elucidating the mechanism of the physicochemical processes in the unsteady region of the gas flow behind a shock front leading to the behavior of strongly radiating CN and C2 molecules and C atoms and also determining the quantitative characteristics of the chemical reactions. A shock tube was used in the investigations, which made it possible to obtain the intensity distribution of the radiation of several components — CN, C2, and C — behind shock fronts.

On the example of CN and C2 we consider the mechanism of efficient excitation of electronic states of diatomic molecules behind a strong shock front at temperatures of 4000–8000°K.