|Electron-excitation of the fine-structure transition|
|in fluorine-like ions|
|Astronomy & Astrophysics Supplement Series, 1994, Vol.107, No.1, pp.29-38|
Total collision strengths for the ground term fine structure transition in 17 fluorine-like ions from Ne II to Fe XVIII are obtained in the close coupling approximation. Model target ions with only two terms, 2p(52)p(o) and 2s2p(62)S are used, so that the collision strengths are perturbed by autoionizing resonances which converge on the excited S-2 term. These features are carefully delineated before averaging over a Maxwell velocity distribution to obtain effective collision strengths. The thermally averaged collision strengths should be reliable for the infrared lines excited at low temperatures in astrophysical situations where photoabsorption dominates the ionization equilibrium. The present calculation ignores resonances which converge on terms that lie above S-2. These are expected to make a significant contribution to the collision rates at higher temperatures. We give the range of temperatures where the present results should be reliable.