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We resonantly excite the $K$ series of O$^{5+}$ and O$^{6+}$ up to principal quantum number $n=11$ with monochromatic x rays, producing $K$-shell holes, and observe their relaxation by soft-x-ray emission. Some photoabsorption resonances of O$^{5+}$ reveal strong two-electron--one-photon (TEOP) transitions. We find that for the $[(1s\,2s)_1\,5p_{3/2}]_{3/2;1/2}$ states, TEOP relaxation is by far stronger than the radiative decay and competes with the usually much faster Auger decay path. This enhanced TEOP decay arises from a strong correlation with the near-degenerate upper states $[(1s\,2p_{3/2})_1\,4s]_{3/2;1/2}$ of a Li-like satellite blend of the He-like $K\alpha$ transition. Even in three-electron systems, TEOP transitions can play a dominant role, and the present results should guide further research on the ubiquitous and abundant many-electron ions where electronic energy degeneracies are far more common and configuration mixing is stronger., Comment: Published in PRA

Calculations that achieve spectroscopic accuracy, i.e., at the level of ${10}^{\ensuremath{-}4}$ or better, are rare and typically restricted to few-electron systems. Here we present high-resolution grating spectrometer measurements of 11 $2{s}_{1/2}\text{\ensuremath{-}}2{p}_{3/2}$ transitions in neonlike ${\mathrm{Ge}}^{22+}$ that distinguish between two different implementations of the many-body perturbation theory (MBPT) method, including our own. The measurements show that our implementation of the MBPT method consistently achieves spectroscopic accuracy at the ${10}^{\ensuremath{-}4}$ level for all of the measured lines. Moreover, our calculations improve the ${10}^{\ensuremath{-}3}$ accuracy of prior MBPT calculations by an order of magnitude and allow us to make greatly improved predictions of the location of potential photopumped lines in ${\mathrm{Ge}}^{22+}$. The measurements also improve on previous measurements of $2{s}_{1/2}\text{\ensuremath{-}}2{p}_{3/2}$ transitions in fluorinelike ${\mathrm{Ge}}^{23+}$ and oxygenlike ${\mathrm{Ge}}^{24+}$. Measurements of $n=2\phantom{\rule{4.pt}{0ex}}\text{to}\phantom{\rule{4.pt}{0ex}}n=3$ transitions in ${\mathrm{Al}}^{7+}$, ${\mathrm{Al}}^{8+}$, and ${\mathrm{Al}}^{9+}$ are used to support our measurement accuracy at the ${10}^{\ensuremath{-}5}$ level.

We report a systematic measurement of the ${(2{p}_{1/2}^{\ensuremath{-}1}3{d}_{3/2})}_{J=1}$ and ${(2{s}_{1/2}^{\ensuremath{-}1}3{p}_{1/2})}_{J=1}$ levels in 14 neonlike ions between ${\mathrm{Ba}}^{46+}$ and ${\mathrm{Pb}}^{72+}$ and document the effects of their avoided crossing near $Z=68$. Strong mixing affects the oscillator strengths over a surprisingly wide range of atomic numbers and leads to the vanishing of one transition two atomic numbers below the crossing. The crossing voids the otherwise correct expectation that the ${(2{p}_{1/2}^{\ensuremath{-}1}3{d}_{3/2})}_{J=1}$ level energy is only weakly affected by quantum electrodynamics (QED). For about 10 atomic numbers surrounding the crossing, its QED contributions are anomalously large, attaining almost equality to those affecting the ${(2{s}_{1/2}^{\ensuremath{-}1}3{p}_{1/2})}_{J=1}$ level. As a result, the accuracy of energy level calculations appears compromised near the crossing.

We have measured the 3s1/2-3p3/2 transition in sodiumlike Eu52+ situated at 41.232 A with an uncertainty of 73 ppm. Our measurement extends previous high-precision measurements into the 56< Z< 78 range of atomic numbers. We also present measurements of 3s1/2-3p3/2 and 3p1/2-3d3/2 transitions in the neighboring magnesiumlike, aluminumlike, and siliconlike europium ions.