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Atomic data from the IRON project - LVI : Electron excitation of Be-like Fe XXIII for the n=2,3,4 configurations

Chidichimo, M.C. and Del Zanna, G. and Mason, H.E. and Badnell, N.R. and Tully, J.A. and Berrington, K.A. (2005) Atomic data from the IRON project - LVI : Electron excitation of Be-like Fe XXIII for the n=2,3,4 configurations. Experimental Astronomy, 430 (1). pp. 331-341. ISSN 0922-6435

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Abstract

Collision strengths for electron induced transitions in the beryllium-like ion Fe+22 are calculated using the intermediate coupling frame transformation (ICFT) version of the R-matrix programs. Our target has 98 fine structure states 1 s(2) nl n'l' S LJ corresponding to n = 2 and n' = 2, 3, 4. The present calculation is for electron impact energies in the range 3.15 to 380 Ry. When T exceeds about ten million degrees one needs to take account of contributions to the thermally averaged collision strength Y coming from electrons with energies in excess of 380 Ry. We discuss a way of allowing for these contributions. Values of Y for all the transitions between the ground state and the excited states 1s(2) 2l n l n'l' S' L' J', with n' = 2, 3,4 are tabulated as a function of log T. The temperature range 6.3 less than or equal to log T less than or equal to 8.1 is centred on log T = 7.1 which is approximately where Fe+22 has maximum abundance in ionization equilibrium. To the best of our knowledge these are the first R-matrix calculations for Fe+22 for excitations to the n = 3, 4 levels. Good agreement with previous distored-wave calculations is found. However, the resonance contributions have an important effect on the effective collision strengths and in turn on the level populations.