Using the mathematical model thus far described the total photoionization cross sections are calculated for both the ground state, 2s²2p⁵ ²P $_{\frac32}^\circ$ , and first excited state, 2s²2p⁵ ²P $_{\frac12}^\circ$ , of Fe XVIII and the results are presented in Fig. 1 along with threshold positions. In order to compare with the only previous calculation, namely the Opacity Project data obtained for the photoionization of the LS-coupled 2s²2p⁵ ²P $^\circ$ ground state (Butler & Zeippen, unpublished), a weighted average of the present cross sections for photoionization from the 2s²2p⁴ ²P $_{\frac32}^\circ$ and ²P $_{\frac12}^\circ$ levels was obtained. Comparison is made in Fig. 2 and we note that the value of the ionization energy obtained by Butler & Zeippen is too low, and so for comparison purposes their data has been shifted in energy by 3.55 Ryd.

$\begin{figure} \vspace{0.3in} \resizebox {\hsize}{!}{\includegraphics{h0926f1.ps}}\end{figure}$

Figure 1: Total photoionization cross sections for the photoionization of Fe XVIII in the 95 to 165 Ryd photon energy range: a) photoionization of the ground state, 2s²2p⁵ ²P $_{\frac32}$ . Threshold are marked using Table 2 as a reference, b) photoionization of the first excited state, 2s²2p⁵ ²P $_{\frac12}$

$\begin{figure} \vspace{0.1in} \resizebox {\hsize}{!}{\includegraphics{h0926f2.ps}} \vspace{-0.3in}\end{figure}$

Figure 2: Weighted average of the total photoionization cross sections for the photoionization of the ground state, 2s²2p⁵ ²P $_{\frac32}$ and first excited state, 2s²2p⁵ ²P $_{\frac12}$ , of Fe XVIII (solid curve) compared with the Opacity project data (dashed curve) and the present calculation performed in LS coupling (dotted curve), in the 95 to 165 Ryd photon energy range

The present calculations demonstrate a general shape of the photoabsorption spectrum that is in excellent accord with the Opacity project data with the exception that the present work resolves a single shape resonance at 128 Ryd. Examination of Fig. 1 shows that this is a result of photoionization of the ${\frac12}^\circ$ state only. Figures 1 and 2 illustrate extensive resonance structure in the 99 to 113 Ryd photon energy range. Figure 3 thus presents a more detailed examination of this range with the full height of the resolved resonances presented.

$\begin{figure} \vspace{0.3in} \resizebox {\hsize}{!}{\includegraphics{h0926f3.ps}}\end{figure}$

Figure 3: Total photoionization cross sections for the photoionization of Fe XVIII (solid curve) compared with the Opacity project data (dashed curve) in the 99 to 113 Ryd. photon energy range: a) photoionization of the ground state, 2s²2p⁵ ²P $_{\frac32}$ , b) photoionization of the first excited state, 2s²2p⁵ ²P $_{\frac12}$

$\begin{figure} \vspace{0.3in} \resizebox {\hsize}{!}{\includegraphics{h0926f4.ps}}\end{figure}$

Figure 4: Partial photoionization cross sections resulting from 2p photoionization of the ground state and first excited state of Fe XVIII where the residual ion is left in the 2s²2p⁴ ³P_2,1,0 states: a) photoionization of the ground state, 2s²2p⁵ ²P $_{\frac32}$ , b) photoionization of the first excited state, 2s²2p⁵ ²P $_{\frac12}$

$\begin{figure} \vspace{0.3in} \resizebox {\hsize}{!}{\includegraphics{h0926f5.ps}}\end{figure}$

Figure 5: Partial photoionization cross sections resulting from 2p photoionization of the ground state and first excited state of Fe XVIII where the residual ion is left in the 2s²2p⁴ ¹D₂ and ¹S₀ states: a) photoionization of the ground state, 2s²2p⁵ ²P $_{\frac32}$ , b) photoionization of the first excited state, 2s²2p⁵ ²P $_{\frac12}$

$\begin{figure} \vspace{0.3in} \resizebox {\hsize}{!}{\includegraphics{h0926f6.ps}}\end{figure}$

Figure 6: Partial photoionization cross sections resulting from 2s photoionization of the ground state and first excited state of Fe XVIII where the residual ion is left in the 2s2p⁵ ³P $_{2,1,0}^\circ$ states: a) photoionization of the ground state, 2s²2p⁵ ²P $_{\frac32}$ , b) photoionization of the first excited state, 2s²2p⁵ ²P $_{\frac12}$

$\begin{figure} \vspace{0.3in} \resizebox {\hsize}{!}{\includegraphics{h0926f7.ps}}\end{figure}$

Figure 7: Partial photoionization cross sections resulting from 2s photoionization of the ground state and first excited state of Fe XVIII where the residual ion is left in the 2s2p⁵ ¹P₁ and 2p⁶ ¹S₀ states: a) photoionization of the ground state, 2s²2p⁵ ²P $_{\frac32}$ , b) photoionization of the first excited state, 2s²2p⁵ ²P $_{\frac12}$

Partial photoionization cross sections (for both photoionization cases) corresponding to the residual Fe XIX ion being left in one of the fine-structure levels corresponding to the 6 energetically lowest LS target states are presented in Figs. 4 to 7. Photoionization of the Fe XVIII ground state is clearly dominated by the mechanism which leaves Fe XIX in its ground state with the contribution of this process to the total cross section being more than double that of any other photoionization mechanism. We note that all the partial cross sections corresponding to 2p photoionization leaving the Fe XIX ion in the 2s²2p⁴ states make significant contributions to the total cross section as do those corresponding to 2s photoionization resulting in 2s2p⁵. However, of these two possiblities the one involving 2p photoionization is clearly the dominant process while the effect that double electron excitation of 2s or 2p (resulting in 2p⁶ and 2s²2p³3s respectively) has in this energy range is negligible. Partial cross sections for these target states have thus been omitted except for those of the 2p⁶ ¹S₀ cross section which exhibit a small amount of resonance structure. In general partial cross sections for photoionization from the ${\frac12}^\circ$ state follow the same pattern as those from the ground state with the exception that the mechanism resulting in the residual ion existing in the Fe XIX 2s²2p⁴ ¹D₂ state now dominates despite the partial cross sections for the Fe XIX ground state being of the same magnitude as in the ground state photoionization case. Thus 2p photoionization makes up a much higher percentage of 2s²2p⁵ ²P $_{\frac12}^\circ$ photoionization than in the ground state case and is responsible for the greater magnitude of the background cross section of the former compared with the latter.

The extensive resonance structure in the 99 to 113 Ryd photon energy range is due primarily to photoionization of Fe XVIII resulting in the 2s²2p⁴ ³P₂ state in both the photoionization of $J=\frac32$ and $J=\frac12$ cases. The photoionization spectrum of the first excited state also demonstrates resonance structure in the 125 to 130 photon energy range where a shape resonance is also apparent. Both features are due primarily to 2p photoionization but no individual partial cross sections dominate these structures. (Figures which illustrate the full height of the resonances in the energy range required for each partial cross section were used in the development of these conclusions. It was not felt worthwhile to include these in the present publication. However partial cross sections to all 19 target levels listed in Table 2 for both photoionization calculations are available from the authors on request).

3 Results and discussion