Paper XXVIII

Electron excitation of the $^2$P $^\circ_{3/2} \rightarrow ^2$P$^\circ_{1/2}$ fine structure transition in fluorine-like ions at higher temperatures

K.A. Berrington, H.E. Saraph and J.A. Tully

Astronomy & Astrophysics Supplement Series, 1998, Vol.129, pp.161-172

The collision strength $\Omega$($^2$P$^\circ_{3/2}$,$^2$P$^\circ_{1/2}$) for the fine structure transition within the ground term of 17 fluorine-like ions from Mg IV to Ni XX is calculated using the R-matrix method. We include target terms with $n=3$ in order to take account of the resonance structure in $\Omega$ at energies above 2s2p$^6$ $^2$S. These resonances are delineated using a fine mesh of several thousand energy points to scan the region between the first and 27th excited terms. Results from an earlier calculation (IRON Project Paper IV) are used to cover the region below 2s2p$^6$ $^2$S. We make use of the fact that $\Omega$ tends to the Born limit as the collision energy $E\rightarrow\infty$. The effective collision strength $\Upsilon$ is tabulated as a function of $\log T_s$, where $T_s = T/(Z-8)^2$ is a $Z$-scaled temperature. We compare results for Si$^{+5}$, Fe$^{+17}$ and Ni$^{+19}$ with those obtained by Mohan and co-workers. An explanation is given for the large discrepancy in the case of $\Upsilon$(Si$^{+5}$).