Atomic data from the IRON project^*

LXVI. Electron impact excitation of Fe¹⁸⁺

¹ Institut für Astronomie und Astrophysik, Scheinerstr. 1, 81679 München, Germany e-mail: butler@usm.uni-muenchen.de
² Department of Physics, University of Strathclyde, Glasgow G4 0NG, UK e-mail: badnell@phys.strath.ac.uk

Received: 14 May 2008
Accepted: 15 July 2008

Abstract

Context. Accurate electron collisional data are required for the analysis of the Fe xix astrophysical spectrum, in particular in the sun. Such an analysis can provide information on the physical characteristics of the coronal plasma.

Aims. An extensive target is used in an R-matrix scattering calculation to provide the necessary data for Fe¹⁸⁺. The use of the R-matrix method includes the resonance contribution lacking in the distorted wave approach and the large target improves the accuracy of the close-coupling approximation.

Methods. The R-Matrix package described by Berrington et al. (1995, Comput. Phys. Commun., 92, 290) as provided by the UK RmaX project has been used to calculate electron collisional data among 342 levels of Fe¹⁸⁺. We have used the intermediate-coupling frame-transformation (ICFT) method (Griffin et al. 1998, J. Phys. B: At. Mol. Opt. Phys., 31, 3713) to transform data obtained in a 166 term LS-coupling calculation. Contributions from the mass and Darwin interactions have also been included in the Hamiltonian.

Results. Collision stengths for all transitions between the 342 levels of Fe¹⁸⁺ are presented. They are tabulated over a wide range of electron temperatures of astrophysical interest. The results are compared with the earlier Iron Project work of Butler & Zeippen (2001, A&A, 372, 1083) and also with that of McLaughlin et al. (2001, J. Phys. B: At. Mol. Opt. Phys., 34, 4521) and Landi & Gu (2006, ApJ, 640, 1171). The agreement is reasonable for the low-lying transitions. Larger differences are found for the more highly excited states.