Experiments were conducted in the free-piston shock tube and shock tunnel with dissociating nitrogen and carbon dioxide, ionizing argon and frozen argon to measure the transition condition in pseudosteady and steady flow. The transition condition in the steady flow, in which the wall was eliminated by symmetry, agrees with the calculated von Neumann condition. I n the real gases this calculation assumed thermodynamic equilibrium after the reflected shock. I n the pseudosteady flow of reflexion from a wedge the measured transition angle lies on the Mach-reflexion side of the calculated detachment condition by an amount which may be explained in terms of the displacement effect of the boundary layer on the wedge surface. A single criterion based on the availability of a length scale a t the reflexion point explains the difference between the pseudosteady and steady flow transition condition and predicts a hysteresis effect in the transition angle when the shock angle is varied during steady flow. No significant effects on the transition condition due to finite relaxation length could be detected. However, new experiments in which interesting relaxation effects should be evident are suggested.

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The methods and techniques currently used to measure oscillator strengths (or, equivalently, f-values and transition probabilities) are reviewed. Both linear and non-linear optical methods are discussed.Following references to critical compilations and bibliographies, the definitions concerning the interaction between an electromagnetic radiation field and a medium (consisting of free atoms, for example) are given: linear and non-linear susceptibilities and radiative constants are introduced. Next, the basic principles of linear methods (dispersion, absorption and emission) and non-linear methods (interaction effects near and off resonance) are explained.Measurement techniques are described in detail. The subjects covered are as follows: introductory remarks on equilibrium, number densities, and relative and absolute oscillator-strength scales; methods based on anomalous dispersion (hook and fringe-shift methods, further interferometric techniques and magneto-rotation); methods based on emission and absorption (the curve of growth, absorption methods yielding absolute and relative $values, and emission methods). We introduce the designation 'Ladenburg method' for a combined method that permits oscillator strengths to be determined without assumptions on plasma state. Subsequently, we discuss the methods based on non-linear interactions (phase-matching in non-linear wave mixing, stimulated Raman scattering, dynamic Stark effect) as well as further methods involving lasers.We emphasise the progress toward more accurate measurements in quantitative spectroscopy and the concomitant new applications. We also point out areas where technological advances concerning light sources, spectrometers and standards, open new opportunities for further, more refined studies in experimental and theoretical atomic and molecular physics.

The stability of strong shock waves in argon and carbon dioxide has been studied in a free-piston shock tube using time-resolved interferometry. Unstable shock fronts have been found to occur close to velocities where (dP/dV)H (where P and V are the pressure and the specific volume behind the shock) on the Rankine-Hugoniot curve is positive due to completion of first ionization or dissociation. Such an instability has been described theoretically by D'iakov (1954). However, his stability limits have not been reached in this case, and this suggests that the linearized perturbation analysis is not accurate for the case of the real shock tube flow.

The classical methods for measuring transition probabilities, viz. the determination of lifetimes and branching ratios, and the absorption, hook, and emission techniques are briefly reviewed. The main advantages and difficulties of each method, as well as the accuracies reached, are described. Improved evaluation methods for hook spectrograms are summarized and the advances made with combinations of classical methods for determining and assessing oscillator strengths are described. The hitherto rarely exploited potential of magneto-rotation measurements for deriving accurate oscillator strengths and the promise of optogalvanic spectroscopy are mentioned. Applications of non-linear optical methods for f-value determinations are also discussed.