Collision Strengths for the Electron-Impact Excitation of Certain Highly-Ionized Helium-Like Ions

Abstract
Collision strengths for transitions involving singly-excited states with principal quantum numbers n ≼ 2 in the astrophysically-important helium-like ions Si XIII, Ca XIX and Fe XXV are given. The collision strengths were evaluated in both LS-coupling and intermediate-coupling using the distorted wave method. Large angular momentum contributions to collision strengths for electric dipole transitions among the n = 2 levels are evaluated in the Coulomb–Bethe approximation. Possible magnitudes of errors in the intermediate-coupling collision strengths are discussed.