Item

Abstract

In various astrophysical observations, the n = 2 → 3 transitions of highly charged iron
appear in the soft x-ray region as an unresolved transition array (UTA). The structure
of the UTA is directly related to the ionization balance of the plasma and is therefore
of high astrophysical interest. The models used to analyse the astrophysical spectra are
highly sensitive to the input atomic data, which is mainly based on theoretical calculations.
Therefore high precision laboratory measurements are needed for benchmarking
theory.
Within this thesis, a systematic measurement over the whole UTA energy range has
been conducted to determine the transition energies and rates for the thirteen-fold ionized
iron (Fe¹³⁺), an important constituent of the UTA. The ions of interest were produced
by an electron beam ion trap and resonantly excited by the synchrotron radiation
of PETRA III. By utilizing an ion-extraction beamline, the radiative as well as the autoionization
decay channels have been observed in parallel. 31 hitherto unexplored
transitions of the UTA have been resolved with a relative accuracy on the level of 40
parts-per million, serveral orders of magnitude higher than the accuracy obtained in
the astrophysical observations. An additional high resolution measurement lead to the
extraction of the natural linewidth, which has been used to determine the absolute radiative
and autoionization rates of two prominent lines of the UTA. A comparison with
state-of-the-art theory revealed a significant 80(7)meV offset in transition energies as
well as a three to four-fold smaller natural linewidth, leading to the question how reliable
the astrophysical models are.