1 Introduction

X-ray emitting plasmas are a phenomenon frequently encountered in astrophysical studies, and are characteristic of various accretion-powered sources such as X-ray binaries, cataclysmic variables and active galactic nuclei (Mushotzky et al. 1993; Kahn & Liedahl 1995). In addition, X-rays are detected from the high temperature plasmas which constitute the upper transition regions and coronae of the Sun and other stars (Phillips et al. 1982). X-rays emitted from these phenomena are the result of many different atomic processes occurring within the plasma, such as electron impact excitation and photoabsorption, both of which frequently involve ions of iron at various stages of excitation. Modelling of the X-ray emission of each of the above sources thus requires detailed atomic data about the various processes which occur within the associated plasma.

Unfortunately, while electron impact of various ions of iron has been studied in sufficient detail, photoionization still lacks serious consideration. Some previous work has been performed, such as that by Verner et al. (1993) and Verner & Yakovlev (1995), but even these are lacking inner shell photoionization data explicitly on Fe XVIII. In addition, recent observations by the Advanced Satellite for Cosmology and Astrophysics (ASCA) have shown that the current atomic data are inadequate for a proper quantitative analysis of plasma emission (Fabian et al. 1994; Liedahl et al. 1995). The Opacity Project team have calculated total photoionization cross sections for the ions of iron using the R-matrix codes, and have included their results in the TOPBASE database (Seaton et al. 1992; Cunto & Mendoza 1992) for use by the astrophysical community. These calculations represent a significant improvement over the data of Verner et al. due to their consideration of the possibility of the existence of resonance structure. However, these R-matrix calculations included only 6 target states without the introduction of electron correlation effects; were performed using an LS-coupling approximation; and the partial cross sections resulting from leaving the iron ion in one of the target states after photoionization have not been extracted from the total cross section.

Each of the ions of iron has a particular significance in astrophysics. Specifically, Fe XVIII has been linked (Liedahl et al. 1990, 1992) with an emission feature at about 16 $\hbox{\AA}$ found in low mass X-ray binaries (Vrtilek et al. 1991). Investigation of the atomic processes involving this ion is thus desirable. Electron impact excitation of Fe XVIII has already been studied using the R-matrix method by Mohan et al. (1987). In this paper, we therefore investigate the L-shell photoionization of Fe XVIII using the R-matrix method to obtain both total and partial photoionization cross sections.