STUDIES ON TRANSITION FROM QUASI-STATIC TO IMPACT ELECTRON CONTRIBUTIONS OF LINEAR STARK BROADENING

The ratio of halfwidth to critical value for the transition between quasi-static and impact domain of electron contributions to linear Stark broadening (according to Unsöld) is expected to vary approximately with n4, where n is the upper principal quantum number of the line considered. Low-density plasmas generated by a radio frequency arrangement permit the observation of many non-overlapping Balmer lines and thus a drastic variation of the ratio; for identical plasma parameters, access deep into the quasi-static as well as the impact regime of electron broadening is realized to a degree not achieved previously. As recently suggested, the observed broadening can be approximated in terms of an effective quasi-static perturber density, which is a universal function of the displacement from the line center expressed in units of the above mentioned critical transition value. Normalization of the resultant profiles is maintained. Doppler and apparatus broadening contributions are quantitatively taken into account in the analysis of the observations. The limitations of the approach are discussed. The greatly simplified description of the complex broadening phenomena summarizes all low-density observations, in good approximation, and agrees in its essential qualitative features with the generalized impact theory as recently corrected by Griem for the asymptotic regime, although these recent calculations favor stronger electron impact contributions. © 1968