Impact excitation of the n = 2 fine-structure levels in hydrogenlike ions by protons and electrons

Quantal close-coupling calculations of the impact excitation of the n = 2 fine-structure levels in the hydrogenlike ions Ar/sup 17+/, S/sup 15+/, Mg/sup 11+/, C/sup 5+/, and He+ by protons and electrons are presented, and the resulting cross sections and rate coefficients are compared with previous values. The effects of relativity on the proton-impact excitation cross sections for ions with high nuclear charge are investigated. Plasma-density effects on the transition cross sections are explored using a Debye-screening model, and it is shown that the screening of the ion alters the excitation cross sections substantially and brings the ratio of the cross sections towards the statistical value. The calculated transition rates are used to model the ratio of the intensities of the two components of Ly-α radiation emitted by impurity hydrogenlike ions in an optically thin, hydrogen plasma. The results confirm that at intermediate plasma densities proton-impact-induced transitions redistribute the n = 2 fine-structure-level population ratio away from the statistical value