Hydrogen fine-structure effects at low electron densities

Abstract

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