Abstract
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.
Highlights
In hot astrophysical plasmas, the most common elements, hydrogen and helium, are fully ionized and only those with higher nuclear charge can keep some bound electrons, appearing as highly charged ions (HCIs) [1]
Light elements such as carbon, nitrogen, and the oxygen studied here abundantly appear as HCIs over a broad range of temperatures and can serve as unique spectroscopic probes of, e.g., the warm-hot intergalactic medium (WHIM), which is critical to a complete census of baryonic matter in the universe [5,6,7]
While doubly excited states commonly relax by Auger decay (AD), our FAC [83] calculations show that this channel is only relevant for Kβ1 and Kγ and confirm a main direct decay (DD) for Kβ1 and Kγ
Summary
The most common elements, hydrogen and helium, are fully ionized and only those with higher nuclear charge can keep some bound electrons, appearing as highly charged ions (HCIs) [1]. To fully exploit the data of current and upcoming high-resolution x-ray missions such as XRISM [2] and Athena [3], more accurate laboratory tests of the atomic models used in astrophysics are needed [1,4] Light elements such as carbon, nitrogen, and the oxygen studied here abundantly appear as HCIs over a broad range of temperatures and can serve as unique spectroscopic probes of, e.g., the warm-hot intergalactic medium (WHIM), which is critical to a complete census of baryonic matter in the universe [5,6,7]. We detect their fluorescencephoton yield and energy as a function of the incident photon energy and observe surprisingly strong and sometimes dominating two-electron–one-photon (TEOP) transitions in Li-like oxygen
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
