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Abstract

Highly charged oxygen ions exist in astrophysical plasmas in a variety of charge states. The experimental
investigation of the transitions of these ions in the soft X-ray region is therefore of great importance
for the validation and improvement of astrophysical models. One of the strongest transitions is the Kα
transition 1s²2s² → 1s2s²2p_3/2 of Be-like oxygen O⁴⁺.
The aim of this work was to investigate this transition at 554 eV by resonant excitation with soft X-ray
synchrotron radiation. The ions were produced and trapped in an electron beam ion trap and excited with
synchrotron radiation from the Elettra synchrotron in Trieste, Italy. With the EBIT, both radiative decay
channels and autoionizing decay channels can be measured in parallel. The charge state distribution
is determined via a time-of-flight measurement, so that ions with different charges and masses can be
resolved separately. This makes it possible to search for an isotope shift. In addition to searching for an
isotope shift of O₁₆/O₁₈ in the photoionization from the K_α transition of Be-like oxygen, the energy for
this transition is determined absolutely in this work and compared with other sources.