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

The central regions of the plasma-broadened Balmer lines ${\mathrm{H}}_{\alpha }$, ${\mathrm{H}}_{\beta }$, and ${\mathrm{H}}_{\gamma }$ have been measured in a wall-stabilized arc over an electron density range between approximately 4×${10}^{14}$ and 2×${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$. 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 ${\mathrm{H}}_{\alpha }$ half-width as well as the central minimum of ${\mathrm{H}}_{\beta }$ deviate by about a factor of three from current hydrogen-broadening theories at low electron densities (${10}^{-5\mathrm{}}$ ${\mathrm{cm}}^{-3\mathrm{}}$). Consistent with earlier experiments, the central minimum of ${\mathrm{H}}_{\beta }$ 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 ${\mathrm{H}}_{\beta }$ minimum depends approximately linearly on the relative mean velocity between radiator and perturber. Extrapolation of the ${\mathrm{H}}_{\beta }$ results to the static case yields good agreement with the quasistatic calculations. The theoretically predicted "shoulder" in the ${\mathrm{H}}_{\gamma }$ 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 ${\mathrm{H}}_{\alpha }$, and possibly a small part in the case of ${\mathrm{H}}_{\beta }$ at low electron densities (≲${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$).