Natural-linewidth measurements of the 3C and 3D soft-x-ray transitions in Ni XIX

We used the monochromatic soft-x-ray beamline P04 at the synchrotron-radiation facility PETRA III to resonantly excite the strongest $2p-3d$ transitions in neon-like Ni XIX ions, [2p⁶]J=0 → [(2p⁵)1/2\,3d3/2]J=1 and [2p⁶]J=0 → [(2p⁵)3/2\,3d5/2]J=1, respectively dubbed 3C and 3D, achieving a resolving power of 15\,000 and signal-to-background ratio of 30. We obtain their natural linewidths, with an accuracy of better than 10%, as well as the oscillator-strength ratio $f(3C)/f(3D)$ = 2.51(11) from analysis of the resonant fluorescence spectra. These results agree with those of previous experiments, earlier predictions, and our own advanced calculations.