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Abstract

We determined relative X-ray photon emission cross sections in Fe XVII ions
that were mono-energetically excited in an electron beam ion trap. Line
formation for the 3s (3s-2p) and 3d (3d-2p) transitions of interest proceeds
through dielectronic recombination (DR), direct electron-impact excitation
(DE), resonant excitation (RE), and radiative cascades. By reducing the
electron-energy spread to a sixth of that of previous works and increasing
counting statistics by three orders of magnitude, we account for hitherto
unresolved contributions from DR and the little-studied RE process to the 3d
transitions, and also for cascade population of the 3s line manifold through
forbidden states. We found good agreement with state-of-the-art many-body
perturbation theory (MBPT) and distorted-wave (DW) method for the 3s
transition, while in the 3d transitions known discrepancies were confirmed. Our
results show that DW calculations overestimate the 3d line emission due to DE
by ~20%. Inclusion of electron-electron correlation effects through the MBPT
method in the DE cross section calculations reduces this disagreement by ~11%.
The remaining ~9% in 3d and ~11% in 3s/3d discrepancies are consistent with
those found in previous laboratory measurements, solar, and astrophysical
observations. Meanwhile, spectral models of opacity, temperature, and
turbulence velocity should be adjusted to these experimental cross sections to
optimize the accuracy of plasma diagnostics based on these bright soft X-ray
lines of Fe XVII.