Hydrogen fine-structure effects at low electron densities

Measurements of the plasma-broadened $H_{\alpha }$ line shape in a wall-stabilized arc show considerable differences between experimental and theoretical line profiles at electron densities below ${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The half-width of the experimental profile is up to three times larger than predicted theoretically. At low electron densities (≲ ${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$), part of such discrepancies can be attributed to the neglect of fine-structure splitting in current Stark-broadening calculations, especially for $H_{\alpha }$, $L_{\alpha }$, and hydrogenic-ion lines.