1 Introduction

The international collaboration known as the IRON Project (IP, Hummer et al. 1993; Butler 1996) is concerned with a systematic treatment of the electron impact excitation of iron ions. A complete list of previous IP papers is available at http://www.am.qub.ac.uk/projects/iron/papers/papers.html. In the present report we discuss the results for Fe XVI, an important ion in solar physics. Behring et al. (1976) identified some Fe XVI emission lines in the solar spectrum taken by a rocket-borne spectrograph in 1973. Sandlin et al. (1976) and Dere (1978) report on these lines with an accuracy of 0.03 Å. More recently, Thomas & Neupert (1994) have shown that all five transitions between the n=3 configurations appear in the spectrum of an active region taken by the Solar EUV Rocket Telescope and Spectograph (SERTS) in its 1989 flight at $\lambda\lambda$ 251.067, 262.978, 265.018, 335.401 and 360.754 Å, measured with a precision of 5 mÅ. Young et al. (1998) have confirmed these identifications, and give some branching ratios and emission line ratios that are insensitive to density and temperature for these lines using the CHIANTI atomic data.

Emission-line ratios derived from transitions in Fe XVI can be used as diagnostics in high-temperature plasmas such as the corona, active regions and flares (Flower & Nussbaumer 1975; Vernazza & Reeves 1978; Dere 1978, 1982; Mason & Monsignori Fossi 1994; Dere et al. 1997; Brosius et al. 1997a). Keenan et al. (1994) have found good agreement between the observed and theoretical line ratios when they included the electron excitation rates computed by Tayal (1994) in their calculations. Furthermore, spatially resolved EUV emission-line profiles of this ion can be used to detect velocity fields in the low corona (Neupert et al. 1992); and simultaneous EUV and radio observations enable the coronal magnetic field structure to be interpreted (Brosius et al. 1993, 1997b). Fe XVI emission lines have also been observed in the EUV spectra of non-supergiant B stars (Cassinelli 1994), of the eclipsing binary Algol (Stern et al. 1995) and of the nearby K2 dwarf $\epsilon$ Eridani (Schmitt et al. 1996).

As a study object of atomic physics Na-like Fe XVI is a fairly simple ion. Hence we use this calculation as a test case to evaluate important effects that must be taken into account when studying electron impact excitation of highly ionized iron-group ions. The ultimate aim of the present work is to produce reliably evaluated collision data for astrophysical applications. With these goals in mind we compare the present results with the two most recent computations for this system: the 10 level close-coupling approximation by Tayal (1994), which takes into account the resonance structure at low energies, and a distorted wave calculation by Cornille et al. (1997). For reviews of earlier work see Badnell & Moores (1994) and Dere et al. (1997).