Ion broadening of Ar i lines in a plasma

We have measured the profiles of plasma-broadened, slightly red-shifted spectral lines of neutral argon with a wall-stabilized arc and performed a detailed line-shape analysis with a computerized data acquisition and processing system. According to Stark broadening theory, isolated lines of neutral atoms in dense plasmas are broadened mostly by electron impact, which results in symmetric Lorentzian profiles. An additional small broadening contribution, which is asymmetric in nature, is due to the plasma ions. This difference in symmetries provides the possibility of separating ion broadening effects from the electron broadening. In full agreement with the quasistatic theory of ion broadening, our experiment shows asymmetry patterns with characteristic minima and maxima near the central part of the lines. While the positions of these extrema---as well as the zero crossing point---stay essentially constant for all lines when compared on a reduced wavelength scale, the amplitudes of the extrema vary from line to line. Measurements of the amplitudes thus allow, by comparison with theoretical asymmetry patterns, the determination of ion broadening parameters.