Electron Impact Excitation Rates for Transitions from the Ground State to the<i>n</i>= 2 and 3 levels in Helium-Like Ions

Abstract

Electron excitation rates for the ground state to n = 2 and 3 S and P levels in He-like ions between F VIII and Mn XXIV have been determined by interpolating recent R-matrix and distorted wave calculations for O VII, Mg XI, Ca XIX and Fe XXV. Results are presented in the form of polynomial fits in log Te to the effective collision strengths (γ) for each transition. The fits may be used to estimate values of γ accurate to between 10 and 20% over a temperature range approximately equal to log Tmax ± 0.8 dex, where Tmax is the temperature of maximum fractional abundance in ionisation equilibrium of the relevant species.