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  1. Non-thermal electron distributions, such as beams of electrons, are found in many laboratory and astrophysical plasma sources and can produce anisotropic and polarized emission. Theories used to model the emission require sublevel specific analysis, which can be difficult to verify experimentally. Using two polarization-sensitive Johann-type crystal spectrometers at the National Institute of Standards and Technology (NIST) electron beam ion trap facility, we measured the linear polarization of well-known dielectronic recombination satellite transitions from Li-like Ar ions and two blended features from Be-like ions. The spectrometers observed the plasma at 90◦ relative to the electron beam propagation direction, and the crystalmore »dispersion planes were oriented perpendicular relative to each other to allow for differing polarization sensitivities. Measurements were taken near the resonance energies of each line and compared with theoretical predictions based on relativistic magnetic sublevel atomic kinetics using the density-matrix theory. Most of the predictions are in excellent agreement with measured values.« less
  2. High-resolution x-ray spectra were recorded at the National Institute of Standards and Technology electron beam ion trap (EBIT) using two Johann-type crystal spectrometers, with their dispersion planes oriented parallel and perpendicular to the beam direction. The linear polarizations of the 1s2−1s2l transitions in He-like argon ions were determined from the measured spectra at electron beam energies of 3.87 and 7.91 keV. The theoretical analysis was performed using detailed collisional-radiative modeling of the non-Maxwellian EBIT plasma with the NOMAD code modified to account for magnetic sublevel atomic kinetics. Effects influencing the polarizations of the observed 1s2−1s2l lines were investigated, including radiativemore »cascades, the 1s2 1S0−1s2s 1S0 two-photon transition, and the charge exchange recombination of H-like argon ions. With these included, the measured polarizations of the resonance (1s2 1S0−1s2p 1P1), intercombination (1s2 1S0−1s2p 3P1), and forbidden lines (1s2 1S0−1s2s 3S1, 1s2 1S0−1s2p 3P2 ) were found to be in good agreement with the calculations.« less
  3. The electron-beam ion trap (EBIT) at the National Institute of Standards and Technology (NIST) was employed for the measurement and detailed analysis of the δλ(124Xe, 136Xe) isotopic shifts of the Al-like 3s23p 2P1/2-3s23p 2P3/2, Al-like 3s23p 2P1/2-3s23d 2D3/2, Mg-like 3s2 1S0-3s3p 1P1, Mg-like 3s2 1S0-3s3p 3P1, Na-like 3s 2S1/2-3p 2P1/2 (D1), and Na-like 3s 2S1/2-3p 2P3/2 (D2) transitions. Systematic analysis revealed possible line blends and contributing experimental uncertainties. Highly accurate atomic-structure calculations were conducted and used to determine the δr2136,124 difference in the mean-square nuclear charge radii of the two xenon isotopes. In the present work, δr2136,124 of 0.276 ±more »0.030 fm2 was obtained from the weighted average of the Na-like D1, Mg-like 3s2-3s3p and Al-like 3s23p-3s23p and 3s23p-3s23d transitions. This result confirms the value previously determined from the Na-like D1 transition of 0.269 ± 0.042 fm2. The uncertainty of our result is half of that of previous results for the same isotopes obtained from x-ray spectroscopy of muonic atoms, laser spectroscopy of neutral xenon atoms, and a global evaluation of charge radii. Our result is slightly outside the uncertainty of the value obtained from a King plot analysis of comparable precision. The present work illustrates that extreme-ultraviolet spectroscopy of highly charged ions is a viable approach for measurements of charge nuclear radii differences and can be used to benchmark conventional methods.« less
  4. We present spectroscopic measurements and detailed theoretical analysis of inner-shell LMn and LNn (n  4) dielectronic resonances in highly charged M-shell ions of tungsten. The x-ray emission from W49+ through W64+ was recorded at the electron-beam ion trap (EBIT) facility at the National Institute of Standards and Technology with a high-purity Ge detector for electron-beam energies between 6.8 and 10.8 keV. The measured spectra clearly show the presence of strong resonance features as well as direct excitation spectral lines. The analysis of the recorded spectra with large-scale collisional-radiative modeling of the EBIT plasma allowed us to unambiguously identify numerousmore »dielectronic resonances associated with excitations of the inner-shell 2s1/2, 2p1/2, and 2p3/2 electrons.« less