Atomic data from the IRON Project

LXV. Electron-impact excitation of Fe⁶⁺

¹ Department of Physics, University of Strathclyde, Glasgow, G4 0NG, UK
² NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA e-mail: witthoef@milkyway.gsfc.nasa.gov

Received: 15 November 2007
Accepted: 16 January 2008

Abstract

We present R-matrix results for the electron-impact excitation of Fe⁶⁺. The intermediate-coupling frame transformation (ICFT) method has been used to obtain level-resolved collision strengths. We then calculate effective collision strengths assuming a Maxwellian distribution for the incident electron velocities. A large configuration interaction (CI) expansion of 1896 LS terms (4776 fine-structure levels) is used to obtain an accurate target. This is reduced to 89 close-coupling (CC) terms (189 levels) for the scattering calculation. To investigate the importance of CI, we also perform a second calculation with the same CC expansion but a smaller CI expansion. We discuss the difficulties of such a comparison and look at which transitions show the most sensitivity to the CI expansion. Our effective collision strengths are compared with a previous IRON Project report (Berrington et al. 2000, A&A, 142, 313) for transitions within the ground configuration and a recent distorted wave calculation (Zeng et al. 2005, MNRAS, 357, 440) for transitions to excited configurations. We find good agreement with the results of the previous R-matrix calculation and with the high-temperature distorted-wave results for most transitions, although there are some significant differences at lower temperatures in the latter case.