アイテム詳細

要旨

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 f3C/ f3D 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 f3C/ f3D = 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.