X-ray Line Ratios of F-like Fe XVIII and Ni XX

The intensity ratios of 3 → 2 emission lines of Fe XVIII and Ni XX were measured on the Livermore electron beam ion trap (EBIT-I) with a flat-field grating spectrometer. The results were compared with distorted-wave (DW) calculations obtained with the Flexible Atomic Code and recent close-coupling calculations using the R-matrix code. The measured 3s → 2p/3d → 2p ratios are about 20–40% higher than the theoretical values. When more extended configuration interaction is included in the DW theory, the agreement with the measurements improved slightly. At the beam energies of these measurements, no significant resonance contribution is expected to be present, and the discrepancies represent the uncertainties in the direct excitation cross sections. Subject headings: atomic data — atomic processes — line: formation — X-rays: general