1 Introduction

Collisional data for members of the Fe isonuclear sequence are important for the interpretation of astrophysical plasmas associated with the solar corona, nebulae, and stellar atmospheres (Butler et al. [1998]). As the field of X-ray astronomy continues to blossom with the increased availability of high-resolution satellite observatories, emission lines from the more highly charged Fe atomic ions will be recorded in increasingly greater detail. In this paper, we apply R-matrix scattering theory to the calculation of electron-impact excitation cross sections for Fe⁷⁺. Previous distorted-wave and close-coupling calculations for Fe⁷⁺(Pindzola et al. [1988]) included only a small number of configurations and did not attempt to map out the sometimes rich resonance structure found near excitation thresholds. The calculation reported below includes 33 LS terms of the seven configurations: 3p⁶3d, 3p⁵3d², 3p⁵3d4s, 3p⁶4s, 3p⁶4p, 3p⁶4d, and 3p⁶4f. Employing a recently developed intermediate-coupling frame transformation (ICFT) (Griffin et al. [1998]) of the K-matrices calculated from an LS R-matrix calculation, cross sections and Maxwellian-averaged effective collision strengths are obtained for 2926 transitions among the 77 LSJ levels of the original seven configurations.

In addition to the collisional data for Fe⁷⁺, we have determined dipole-allowed radiative rates between the 5 even parity levels and the 72 odd parity levels. The entire set of atomic data for Fe⁷⁺, including the effective collision strengths and radiative rates, is put in a general format for easy interface with plasma modeling codes. The formatted data set is now available via the Oak Ridge National Laboratory (ORNL) Controlled Fusion Atomic Data Center internet site (http://www.cfadc.ornl.gov). As way of example, we used the ADAS collisional-radiative modeling codes (Summers [1994]) and the Fe⁷⁺ atomic data set to calculate an emission line ratio involving a blend of the 3p⁶4f ²F $\rightarrow$ 3p⁶3d ²D transitions at around 131 A and another blend of the 3p⁵3d^{2 2}P $\rightarrow$ 3p⁶3d ²D and 3p⁵3d^{2 2}D $\rightarrow$ 3p⁶3d ²D transitions at around 168 A. This particular line ratio provides a very useful electron temperature diagnostic for a wide range of electron densities.

The remainder of this paper is organized as follows: in Sect. 2 we present our atomic structure and R-matrix scattering calculations for Fe⁷⁺, in Sect. 3 we present our ADAS line emission ratio calculation for Fe⁷⁺, and in Sect. 4 we summarize our findings.