Laboratory Measurements and Modeling of the Fe XVII X-Ray Spectrum

Detailed measurements, line identifications, and modeling calculations of the Fe XVII L-shell emission spectrum between 9.8 and 17.5 Å are presented. The measurements were carried out on an electron beam ion trap under precisely controlled conditions where electron-impact excitation followed by radiative cascades is the dominant line formation process. In addition to the strong transitions emanating from the n = 3 shell, we identify and accurately determine wavelengths for transitions from higher shells up to n = 11, including two electric quadrupole transitions that have not been previously identified. Various theoretical values, including new distorted wave calculations, are compared to our measurements, which establish definitive values for testing spectral modeling predictions. We find a value of 3.04 ± 0.12 for the ratio of the intensity of the 2p-3d1P1 resonance and of the 2p-3d3D1 intercombination line situated at 15.01 and 15.26 Å, respectively. This value is higher than the values observed in solar spectra, which supports claims that the solar value is affected by resonant scattering. However, because our value is significantly lower than calculated values, the amount of scattering has probably been overestimated in past analyses. Comparisons of the measured intensity ratios of the transitions originating in levels of higher principal quantum number n with present distorted wave calculations show good agreement up to n = 6. The combined flux of all 2p-nd transitions with n ≥ 5 and all 2s-np transitions with n = 4 and 5 relative to the flux of the 15.01 Å resonance line has been measured to be 0.13+ 0.04−0.03 .

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