Stark broadening of the hydrogen H_α line at low densities: fine structure and spontaneous emission effects

Taking the H_alpha line as a test case, the authors give a proper treatment of the broadening including energy splitting and spontaneous emission effects. At low perturber densities (from 10¹⁰ to 10¹⁴ cm^-3) the profile is calculated in the line centre within the impact approximation. Each collision is well described by a semiclassical perturbational approach. The energy separation of the sublevels for n=2 and n=3 produces a splitting of the line into seven components and modifies the collisional cross sections. These components, well resolved at perturber densities lower than 10¹² cm^-3, are gradually mixed for increasing densities.