Shock wave thicknesses and density ratios have been measured in helium, argon, and nitrogen by means of the electron beam fluorescence method, over the range 1.5 < M < 17.4, in a low density wind tunnel. The shock thicknesses in argon and helium agreed well with Mott-Smith theory at the higher Mach numbers and were between Navier-Stokes and Mott-Smith theories at the lowest Mach number. In nitrogen the measured shock thicknesses were considerably greater than the predictions of Navier-Stokes theory. Measured density ratios across the shock wave were in good agreement with theory, in the lower density flow. Poorer agreement was found at higher flow densities, leading to estimates of upper bounds for the range of linear variation of fluorescence intensity with gas density. Density ratios for shocks produced in divergent free-jet flows were found to be in better agreement with theory after a viscous curvature correction was applied. An anomalous dip in the fluorescence intensity was found ahead of helium shock waves produced in the free-jet flows. Some preliminary measurements were made of the fluorescence lifetimes in helium, argon, and nitrogen, and the presence of a halo surrounding the electron beam was investigated briefly.

The electron beam fluorescence technique has been used to measure the rotational energy distribution through shock waves in nitrogen. At Mach 1.7 the deviation from thermodynamic equilibrium was found to be small, and the rotational temperature profile followed the density profile. Comparison with theoretical calculations yielded the rotational relaxation collision number ZR = 5, in satisfactory agreement with other determinations. At high Mach numbers the measured rotational distribution function within the shock wave could not be represented by a single temperature, but was found to be representable approximately by the merging of two rotational distribution functions corresponding to temperatures upstream and downstream of the shock wave. The profile of the average local ``temperature,'' defined by the ratio of the total measured rotational energy to the gas density, was found to precede the density profile through the shock wave by an amount which increased with Mach number.

In order to be able to compare light intensity measurements derived from the anode current of a photomultiplier with measurements derived from photoelectron pulse counting, a systematic investigation of the properties of some photomultiplier tubes has been made. This has led to a correlation of the properties of a photomultiplier based on an over-all quantum efficiency etaF, the gain G, a photoelectron noise factor S, and an effective dark rate D. In terms of these quantities the signal-to-noise ratio of an experimental measurement can be calculated, given the light flux and measurement technique. It is shown that the noise power spectral density is constant for a photomultiplier, down to about 10(-2) Hz, and that pulse counting and current measurement will give similar signal-to-noise ratios under most experimental conditions.

Rotational nonequilibrium was investigated in hypersonic free jet expansions of nitrogen using the electron beam fluorescence technique. The results confirm the conclusion of previous investigations that a dipole excitation model, with an assumed Boltzmann energy distribution, is not consistent with the measured line intensities. This discrepancy was examined quantitatively and found to be independent of density and source Knudsen number (except at number densities greater than 1016 cm−3). The effect was attributed to interactions with the ejected (ionized) electron and a new excitation model was developed and shown to be consistent with the measurements throughout the flow conditions explored. The resulting rotational temperatures were compared to a simplified relaxation model of the jet and indicate a rotational collision number of 1.9.