Asymmetries in spectral lines due to plasma-ion broadening: Some unusual cases and a possible test for plasma homogeneity.

Abstract

We have measured the profiles of plasma-broadened and slightly red- or blue-shifted spectral lines of neutral argon and nitrogen in a wall-stabilized arc and have 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 mostly broadened by electron impact resulting in symmetrical Lorentzian profiles, and are also broadened by the plasma ions, which produce a small additional contribution to the width as well as asymmetries in the line profile. As in two earlier experimental studies, we have utilized this difference in symmetries to study the effects of ion broadening. In this paper we have extended our work to some unusual cases where lines exhibit relatively rare ``blue'' shifts and where we study spectral transitions which are appreciably broadened because they originate from high-lying atomic levels. Our results are again in close agreement with the quasistatic theory of ion broadening which predicts that the asymmetry pattern has a characteristic minimum and maximum near the central part of the lines, but this pattern is reversed for the blue-shifted lines. We have also utilized the asymmetries for a check on plasma homogene- ity. By deliberately including nonhomogeneous areas of emission into the line of sight, we found drastic, readily observable distortions in the asymmetry patterns.