Electron-impact excitation of Fe II^*

Collision strengths and effective collision strengths for low-lying fine-structure forbidden transitions

Department of Applied Mathematics & Theoretical Physics, The Queen's University of Belfast, Belfast BT7 1NN, Northern Ireland e-mail: c.ramsbottom@qub.ac.uk

Received: 10 September 2007
Accepted: 19 September 2007

Abstract

Context.Absorption or emission lines of are observed in many astrophysical spectra and accurate atomic data are required to interpret these lines. The calculation of electron-impact excitation rates for transitions among even the lowest lying levels of is a formidable task for theoreticians.

Aims.In this paper, we present collision strengths and effective collision strengths for electron-impact excitation of for low-lying forbidden transitions among the lowest 16 fine-structure levels arising from the four LS states 3d⁶4s ⁶D^e, 3d^{7 4}F^e, 3d⁶4s ⁴D^e, and 3d^{7 4}P^e. The effective collision strengths are calculated for a wide range of electron temperatures of astrophysical importance from 30–100 000 K.

Methods.The parallel suite of Breit-Pauli codes are utilised to compute the collision cross sections for electron-impact excitation of and relativistic terms are included explicitly in both the target and the scattering approximation. 100 LS or 262-jj levels formed from the basis configurations 3d⁶4s, 3d⁷, and 3d⁶4p were included in the wavefunction representation of the target, including all doublet, quartet, and sextet terms. Collision strengths for a total of 34 191 individual transitions were computed.

Results.A detailed comparison is made with previous theoretical works and significant differences were found to occur in the effective collision strengths, particularly at low temperatures.