Hydrogen fine-structure effects at low electron densities

Measurements of the plasma-broadened H_α line shape in a wall-stabilized arc show considerable differences between experimental and theoretical line profiles at electron densities below 10¹⁶ cm^-3. The half-width of the experimental profile is up to three times larger than predicted theoretically. At low electron densities ({} 10¹⁵ cm^-3), part of such discrepancies can be attributed to the neglect of fine-structure splitting in current Stark-broadening calculations, especially for H_α, L_α, and hydrogenic-ion lines.