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Metastable states of ions can be sufficiently populated in absorbing and emitting astrophysical media, enabling spectroscopic plasma-density diagnostics. Long-lived states appear in many isoelectronic sequences with an even number of electrons, and can be fed at large rates by various photonic and electronic mechanisms. Here, we experimentally investigate beryllium-like and carbon-like ions of neon and iron that have been predicted to exhibit detectable features in astrophysical soft X-ray absorption spectra. An ion population generated and excited by electron impact is subjected to highly monochromatic X-rays from a synchrotron beamline, allowing us to identify Kα transitions from metastable states. We compare their energies and natural line widths with state-of-the-art theory and benchmark level population calculations at electron densities of 1010.5 cm-3.

Thanks to high-resolution and non-dispersive spectrometers onboard future X-ray missions such as XRISM and Athena, we are finally poised to answer important questions about the formation and evolution of galaxies and large-scale structure. However, we currently lack an adequate understanding of many atomic processes behind the spectral features we will soon observe. Large error bars on parameters as critical as transition energies and atomic cross sections can lead to unacceptable uncertainties in the calculations of e.g., elemental abundance, velocity, and temperature. Unless we address these issues, we risk limiting the full scientific potential of these missions. Laboratory astrophysics, which comprises theoretical and experimental studies of the underlying physics behind observable astrophysical processes, is therefore central to the success of these missions.

We demonstrate a widely applicable technique to absolutely calibrate the energy scale of x-ray spectra with experimentally well-known and accurately calculable transitions of highly charged ions, allowing us to measure the K-shell Rydberg spectrum of molecular O_{2} with 8 meV uncertainty. We reveal a systematic ∼450 meV shift from previous literature values, and settle an extraordinary discrepancy between astrophysical and laboratory measurements of neutral atomic oxygen, the latter being calibrated against the aforementioned O_{2} literature values. Because of the widespread use of such, now deprecated, references, our method impacts on many branches of x-ray absorption spectroscopy. Moreover, it potentially reduces absolute uncertainties there to below the meV level.

We measured the L-shell soft X-ray fluorescence of Fe XVII ions in an electron beam ion trap following resonant photo-excitation using synchrotron radiation provided by the P04 beamline at PETRA III. Special attention is paid to two 2p-3d transitions, the 3C and 3D lines that are essential plasma diagnostics tools for astrophysics. Their resulting oscillator-strength ratio, f(3C)/f(3D) = 3.09(8)(6), is three times more accurate than previous results. The present ratio clearly departs by approximately 5-sigmas from the newest ab initio calculations but confirms previous laboratory measurements and astrophysical observations. A ten thousand-fold reduction in excitation-photon intensity and ten times higher spectral resolution allow us to exclude current explanations, reinstating a forty-year-old atomic-physics puzzle.

We study two fundamental transitions from the ground state 1S0 to 1P1 (w line) and 3P1 (y line) in heliumlike Kr34+ by resonant single-photon excitation using an electron-beam ion trap and monochromatic x rays at PETRA III. Our results for the transition energies E(w) = 13114.47(14) eV and E(y) = 13026.15(14) eV are in excellent agreement with quantum electrodynamics calculations, but disagree for w with the average of the hitherto reported experimental results ĒLit(w) = 13115.15(17) eV. Independently of any energy calibration, we obtain a value E(w)/E(y) = 1.006780(7), also in consistency with predictions.

We study two fundamental transitions from the ground state $^{1}$S$_{0}$ to $^{1}$P$_{1}$ (w line) and $^{3}$P$_{1}$ (y line) in heliumlike Kr$^{34+}$ by resonant single-photon excitation using an electron-beam ion trap and monochromatic x rays at PETRA III. Our results for the transition energies E(w)=13 114.47(14) eV and E(y)=13 026.15(14) eV are in excellent agreement with quantum electrodynamics calculations, but disagree for w with the average of the hitherto reported experimental results E$_{Lit}$(w)=13 115.15(17) eV. Independently of any energy calibration, we obtain a value E(w)/E(y)=1.006 780(7), also in consistency with predictions.

Physical review / A 92(2), 020502(2015). doi:10.1103/PhysRevA.92.020502, We study two fundamental transitions from the ground state $^{1}$S$_{0}$ to $^{1}$P$_{1}$ (w line) and $^{3}$P$_{1}$ (y line) in heliumlike Kr$^{34+}$ by resonant single-photon excitation using an electron-beam ion trap and monochromatic x rays at PETRA III. Our results for the transition energies E(w)=13 114.47(14) eV and E(y)=13 026.15(14) eV are in excellent agreement with quantum electrodynamics calculations, but disagree for w with the average of the hitherto reported experimental results E$_{Lit}$(w)=13 115.15(17) eV. Independently of any energy calibration, we obtain a value E(w)/E(y)=1.006 780(7), also in consistency with predictions., Published by APS, College Park, Md.

http://wwwxray.ess.sci.osaka-u.ac.jp/SuzakuMAXI2014/pdf/2014SM_conf_P144-P145.pdf; International audience; The diagnostic utility of the high resolution instruments flown on Chandra, XMM-Newton, and Suzaku is directly coupled to, and often limited by, our understanding of the atomic physics governing the X-ray emission. Hence, high accuracy, systematic laboratory measurements of X-ray emission from highly charged ions provide the necessary basis for interpreting spectra measured by X-ray observatories. Over the past two decades, electron beam ion traps have been used in laboratory astrophysics experiments to study X-ray emission from highly charged ions. These data provide reliable benchmarks of theoretical calculations and have addressed specific problems facing the X-ray astrophysics community, and will prove invaluable when interpreting the spectra measured using the SXS on Astro-H. Results focussed on addressing the Fe XVII problem and measuring the rest wavelengths of Kα transitions in L-shell ions of silicon are summarized.

Physical review / A 102(5), 052831 (2020). doi:10.1103/PhysRevA.102.052831, Published by APS, Woodbury, NY

Physical review / A 102(5), 052831 (1-10) (2020). doi:10.1103/PhysRevA.102.052831, 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 [(1$s$2$s$)15$p_{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 [(1$s$2$p_{3/2}$)$_1$4$s$]$_{3/2;1/2}$ of a Li-like satellite blend of the He-like $Kα$ 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., Published by Inst., Woodbury, NY

Physical review letters 123(2), 023201 (2019). doi:10.1103/PhysRevLett.123.023201, We present a comprehensive experimental and theoretical study on superfluorescence in the extreme ultraviolet wavelength regime. Focusing a free-electron laser pulse in a cell filled with Xe gas, the medium is quasi-instantaneously population inverted by 4d-shell ionization on the giant resonance followed by Auger decay. On the timescale of ∼10 ps to ∼100 ps (depending on parameters) a macroscopic polarization builds up in the medium, resulting in superfluorescent emission of several Xe lines in the forward direction. As the number of emitters in the system is increased by either raising the pressure or the pump-pulse energy, the emission yield grows exponentially over four orders of magnitude and reaches saturation. With increasing yield, we observe line broadening, a manifestation of superfluorescence in the spectral domain. Our novel theoretical approach, based on a full quantum treatment of the atomic system and the irradiated field, shows quantitative agreement with the experiment and supports our interpretation., Published by APS, College Park, Md.