High-precision soft X-ray transition measurements of neon-like mid-Z ions using ultra monochromatic synchrotron radiation

For more than 40 years, most astrophysical observations and laboratory studies of two key soft X-ray diagnostic 2p-3d transitions, 3C and 3D, in Fe XVII ions found oscillator-strength ratios $f_{3C}/ f_{3D}$ disagreeing with theory. This discrepancy hampered the proposed plasma diagnostic utility based on the observed intensity ratios ever since. In previous laboratory measurements, the ratio was determined by scrutinizing the photon emission of an electron-impact excited plasma. First X-ray laser spectroscopy experiments at a free-electron laser (FEL) confirmed the discrepancy between experiments and theory independently of electron-excitation cross sections that could have falsified previous laboratory measurements. In this thesis, the 3C/3D oscillator-strength ratio was measured by resonantly exciting the transitions utilizing highly brilliant and monochromatic X-ray synchrotron light. Thereby, two proposed systematical effects that could have lowered the results of the FEL measurements were excluded. The final result $f_{3C}/ f_{3D}$ = 3.51(7) of this work is in excellent agreement with the newest state-of-the-art theoretical predictions and appears to finally resolve the long-lasting conundrum. Additionally, for the first time, the individual natural linewidths of lines 3C and 3D, among others, were determined with an uncertainty of ~15%. The findings of this thesis now allow the observed intensity ratios of 3C and 3D to enable their usefulness in the diagnostics of astrophysical spectra