Measurements of the free-bound and free-free continua of nitrogen, oxygen and air

Abstract

Measurements have been made of the equilibrium radiation behind incident and reflected shocks produced in an electric arc-driven shock tube over a temperature range of 8000–14000°K. Experiments are described which used a tungsten photoelectric gauge to make photometric measurements in the vacuum ultraviolet. This gauge was positioned at the axis of the shock tube and cored the oncoming shock wave. By the use of optical stops, this gauge was capable of resolving the shock front. A knowledge of the photoelectric yield of the tungsten provided a photometric measurement. The photoionization cross-section for the ground-state nitrogen atom was inferred from these measurements to be of the order of 10 -17 cm 2. Data were also obtained of the continua at 0·51 μ in nitrogen, oxygen and air which gave information on the component of the radiation produced by electrons being captured by ions and neutral atoms. The measurements agree well with free-bound calculations assuming a hydrogen-like model for N and O. Measurements were also obtained in the infrared at 6·1 μ in air over a range of shock conditions which infer an effective Z 2 g ( Z 2 times Gaunt factor) of 1·5 for the air-constituent free-free transitions.