A shock tube study of line broadening in a temperature range of 6100 to 8300 K

Line widths of the Ca(II) 3968 Å and Na(I) 5890 Å resonance lines broadened by electric microfields in a plasma were measured. A pressure-driven shock tube was used as the light source. Radiation from the equilibrium region behind the reflected shock wave was studied using a rapid scanning Fabry-Perot interferometer. Electron and argon atom densities of about 8 x 10¹⁶ and 1 x 10¹⁹ per cm³, respectively, were achieved at the relatively low temperature of around 7500 K by vaporizing a cesium powder that had been added to the shock tube as a solid aerosol in argon gas. The measured line widths were predominantly Stark broadened by the electrons and ions in the plasma, although Doppler, van der Waals, instrument function and optical depth effects had to be taken into account. The electron broadened line width of Na(I) 5890 Å was lower by about 16% than the predicted value from a semiclassical calculation, in agreement with a recently reported measurement, while for Ca(II) 3968 Å the results agree with previous reliable experiments and are lower than two calculated values by 20 and 36% respectively. The van der Waals broadening by argon was also determined and showed qualitative agreement with other results.