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

Abstract

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