Paper XLVIII

Electron excitation of the 2s2p$^6$ and 2s$^2$2p$^43l$ fine-structre in Fe XVIII

Keith A. Berrington and J.C Pelan

Astronomy & Astrophysics Supplement Series, 2000, in press

Electron excitation collision strengths for fine-structure transitions from the 2s$^2$2p$^5$ $^2$P$^\circ$ ground levels to the 2s2p$^6$ and 2s$^2$2p$^43l$ levels in Fe XVIII, are calculated using the Breit-Pauli R-matrix as part of the IRON Project (IP). The target basis of 113 levels also include 2s2p$^53l$ and 2p$^63l$ levels. The collision strength is averaged over a Maxwellian veolcity distribution to obtain the effective collision strengths $\Upsilon$(T) as a function of electron temperature from $\log T$ = 6 to 7.4. Resonances are found to enhance $\Upsilon$ at the lower temperature by up to factor 2 for many 2s$^2$2p$^5$ $^2$P$^\circ$ - 2s$^2$2p$^43l$ transitions, giving rise to similar enhancements in level populations, though such effects diminish with increasing temperature.