Experimental investigation of plasma-broadened hydrogen Balmer lines at low electron densities

The central regions of the plasma-broadened Balmer lines H_α, H_β, and H_γ have been measured in a wall-stabilized arc over an electron density range between approximately 4×{}10¹⁴ and 2×{}10¹⁶ cm^-3. The experimental profiles exhibit much less structure in the line core than predicted by theories based on the static-ion approximations. These discrepancies increase towards lower electron densities and lower Balmerseries members, e.g., the H_α half-width as well as the central minimum of H_β deviate by about a factor of three from current hydrogen-broadening theories at low electron densities (10^-5 cm^-3). Consistent with earlier experiments, the central minimum of H_β was found to depend on the reduced mass of the radiatorion perturber system. Also, the temperature dependence of the experimental data suggests that the central H_β minimum depends approximately linearly on the relative mean velocity between radiator and perturber. Extrapolation of the H_β results to the static case yields good agreement with the quasistatic calculations. The theoretically predicted "shoulder" in the H_γ line shape is nearly absent in the experimental profile. Besides ion dynamics, fine-structure (spin) effects can account for a considerable portion of the observed discrepancies in the case of H_α, and possibly a small part in the case of H_β at low electron densities ({}10¹⁵ cm^-3).