We experimentally re-evaluate the fine structure of Sn 11+...14+ ions. These ions are essential in bright extreme-ultraviolet (EUV) plasma-light sources for next-generation nanolithography, but their complex electronic structure is an open challenge for both theory and experiment. We combine optical spectroscopy of magnetic dipole M 1 transitions, in a wavelength range covering 260 nm to 780 nm, with charge-state selective ionization in an electron beam ion trap. Our measurements confirm the predictive power of ab initio calculations based on Fock space coupled cluster theory. We validate our line identification using semi-empirical Cowan calculations with adjustable wavefunction parameters. Available Ritz combinations further strengthen our analysis. Comparison with previous work suggests that line identifications in the EUV need to be revisited. arXiv:1605.04236v1 [physics.atom-ph]

The reported observations of an unidentified X-ray line feature at ∼3.5 keV have driven a lively discussion about its possible dark matter origin. Motivated by this, we have measured the K -shell X-ray spectra of highly ionized bare sulfur ions following charge exchange with gaseous molecules in an electron beam ion trap, as a source of or a contributor to this X-ray line. We produce S 16+ and S 15+ ions and let them capture electrons in collision with those molecules with the electron beam turned off while recording X-ray spectra. We observed a charge-exchanged-induced X-ray feature at the Lyman series limit (3.47 ± 0.06 keV). The inferred X-ray energy is in full agreement with the reported astrophysical observations and supports the novel scenario proposed by Gu and Kaastra (A & A 584, L11 (2015)).

We report linear polarization measurements of x rays emitted due to dielectronic recombination into highly charged krypton ions. The ions in the He-like through O-like charge states were populated in an electron beam ion trap with the electron beam energy adjusted to recombination resonances in order to produce K α x rays. The x rays were detected with a newly developed Compton polarimeter using a beryllium scattering target and 12 silicon x-ray detector diodes sampling the azimuthal distribution of the scattered x rays. The extracted degrees of linear polarization of several dielectronic recombination transitions agree with results of relativistic distorted-wave calculations. We also demonstrate a high sensitivity of the polarization to the Breit interaction, which is remarkable for a medium-Z element like krypton. The experimental results can be used for polarization diagnostics of hot astrophysical and laboratory fusion plasmas.

We analyze the complex level structure of ions with many-valence-electron open [Kr] 4d m subshells (m=7-4) with ab initio calculations based on configuration-interaction many-body perturbation theory (CI+MBPT). Charge-state-resolved optical and extreme ultraviolet (EUV) spectra of Sn 7+ -Sn 10+ ions were obtained using an electron beam ion trap. Semi-empirical spectral fits carried out with the orthogonal parameters technique and cowan code calculations lead to 90 identifications of magnetic-dipole transitions and the determination of 79 energy ground-configuration levels, questioning some earlier EUV-line assignments. Our results, the most complete data set available to date for these ground configurations, confirm the ab initio predictive power of CI+MBPT calculations for the these complex electronic systems.

We present a comprehensive experimental and theoretical study on superfluorescence in the extreme ultraviolet wavelength regime. Focusing a high-intensity free-electron laser pulse in a cell filled with Xe or Kr gas, the medium is quasi instantaneously population-inverted by inner-shell ionization on the giant resonance followed by Auger decay. On the timescale of 100 ps a macroscopic polarization builds up in the medium, resulting in superfluorescent emission of several Xe and Kr 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 shows an exponential growth of over 4 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.Superfluorescence [1] is the spontaneous, collective decay of an extended ensemble of atoms that have been prepared in a population-inverted state, resulting in collimated, highintensity radiation pulses. The pulses are emitted at a certain delay following excitation and have a duration that can be several orders of magnitude smaller than the typical upper-state lifetimes. Long before the advent of short-wavelength free-electron lasers (FELs), strong superfluorescence in optically thick media was proposed as a source of highly intense and pulsed extreme-ultraviolet (XUV) or X-ray radiation [2]. Strong X-ray K-α superfluorescence following ionization of the 1s shell with a focused X-ray FEL (XFEL) beam was demonstrated in neon gas [3,4], solid copper [5] and manganese salts in aqueous solution [6]. Extremely high gains were observed in these experiments [3,6], with exponential amplification factors surpassing 20 compared to spontaneous emission. In the vacuum ultraviolet and XUV regions, superfluorescence following inner-shell ionization has so far not been demonstrated.The difficulty to obtain transient gain in this wavelength regime is a consequence of the very different time scales of two competing processes: on the one hand, short (fs) Auger lifetimes of inner-valence vacancies, and, on the other hand, comparatively long (ns) radiative transition times -a highly unfavorable combination to sustain a sizable population inversion and gain. Here, we present combined experimental and theoretical work, giving strong evidence for XUV superfluorescence of Xe and Kr gases. Population inversion is