Continuum Radiation from Ionized Rare Gases in Reflected Shock Waves

Abstract

Observations have been made of the temperature, pressure, and frequency dependences of the visible and ultraviolet continuum radiation emitted by the thermally ionized rare gases Ar, Kr, and Xe. The plasmas, whose continua generally are attributed to the radiative recombination of free electrons and ions, have been formed in reflected shock waves in which the equilibrium temperatures range from 7500° to 12 000°K and the electron densities range from 10¹⁵ to 10¹⁸ cm^—3. Some distance behind the reflected shock wave a region is found which possesses the predicted shock properties and in which the intensity of the continuum radiation attains a steady‐state value. This steady‐state intensity has been examined at seven wavelengths from 2451 to 4800 Å; in addition, the ratio of the Kr continuum to the 4319.6 Å Kr line intensity was observed. The continuum intensity is shown to be proportional to the electron density squared and essentially independent of the ion involved. The observed e^—hv/kT decay of the intensity in the ultraviolet, together with the total temperature dependence and the measured absolute intensity of the continuum, demonstrates that the continuum originates from radiative recombination. The expected temperature and pressure behavior also is found for the Kr line emission.