Plasma screening effects on proton-impact excitation of positive ions

We investigate the role of plasma screening in fine-structure excitation of excited (n=2) hydrogenic ions, He⁺, Ne⁹⁺, Ar¹⁷⁺, and Fe²⁵⁺ in collisions with protons. Within the context of the semiclassical, impact-parameter formalism, both ion-sphere and Debye-H\"uckel static screening models have been used. For certain plasma conditions, the former model is easily justified by arguments based on the ordering of various time scales; however, the latter model is more appropriate for many plasmas presently subject to spectroscopic analysis. Substantial (e.g., factor-of-10) reductions from unscreened cross-section values are readily achievable. We discuss numerous qualitative trends exhibited by the screened cross sections, and obtain empirical relations that roughly scale the cross sections for different charges of the target ion.