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, $[2p^6{]}_{J=0}\to [(2p^5{)}_{1\mathrm{/}2}3d_{3\mathrm{/}2}{]}_{J=1}$ and $[2p^6{]}_{J=0}\to [(2p^5{)}_{3\mathrm{/}2}3d_{5\mathrm{/}2}{]}_{J=1}$, 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 $f(3C)\mathrm{/}f(3D)$ = 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.