Investigation of the M-shell unresolved transition array of aluminium-like iron using monochromatic soft x-ray synchrotron radiation

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

Highly Charged Ions for High-Resolution Soft X-ray Grating Monochromator Optimisation

The energy-resolving performance of a synchrotron radiation monochromator can be characterised by measuring the fluorescence response of a gas in scans across characteristic absorption lines. Here, we describe a method using exceptionally narrow absorption features in the soft x-ray range. The features belong to helium-like ions and examples of the transition 1s → 2p in O$^{6+}$ and Ne$^{8+}$ are shown. We describe the instrument PolarX-EBIT and show typical data. A performance with ten times sharper effective feature width, when compared to neutral-gas absorption features, is demonstrated

High Resolution Photoexcitation Measurements Exacerbate the Long-Standing Fe XVII Oscillator Strength Problem

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, but uncertainties had precluded definitive statements on this much studied conundrum. Here, we resonantly excite these lines using synchrotron radiation at PETRA III, and reach, at a millionfold lower photon intensities, a 10 times higher spectral resolution, and 3 times smaller uncertainty than earlier work. Our final result of $f(3C)/f(3D) = 3.09(8)(6)$ supports many of the earlier clean astrophysical and laboratory observations, while departing by five sigmas from our own newest large-scale ab initio calculations, and excluding all proposed explanations, including those invoking nonlinear effects and population transfers.Comment: Main text (6 pages, 3 figures), Supplmentary Material (8 pages, 4 figure), Published in Physical Review Letter

High-resolution Photo-excitation Measurements Exacerbate the Long-standing Fe XVII Emission Problem

We measured the L-shell soft X-ray fluorescence of Fe XVII ions in an electron beam ion trap following resonant photo-excitation using synchrotron radiation provided by the P04 beamline at PETRA III. Special attention is paid to two 2p-3d transitions, the 3C and 3D lines that are essential plasma diagnostics tools for astrophysics. Their resulting oscillator-strength ratio, f(3C)/f(3D) = 3.09(8)(6), is three times more accurate than previous results. The present ratio clearly departs by approximately 5-sigmas from the newest ab initio calculations but confirms previous laboratory measurements and astrophysical observations. A ten thousand-fold reduction in excitation-photon intensity and ten times higher spectral resolution allow us to exclude current explanations, reinstating a forty-year-old atomic-physics puzzle