Table of contents

The spectrum of quadruply-ionized molybdenum Mo V was observed from 200 to 4700 Å with sliding spark discharges on 10.7 m normal- and grazing-incidence spectrographs. The existing analyses of this spectrum (Tauheed et al 1985 Phys. Scr.31 369; Cabeza et al 1986 Phys. Scr.34 223) were extended to include the 5s², 5p², 5s5d, 5s6s, 4d5f, and 4d5g configurations as well as the missing ³H₆ level of 4d4f and about 75 levels of the core-excited configuration 4p⁵4d³. The values of the 4d5d ¹S₀, 5s5p ¹P₁, and 4d6p ³P₀ levels were revised. There are now about 900 lines classified as transitions between 66 even parity and 191 odd parity energy levels. Of these, about 600 lines and 130 levels are new. From the optimized energy level values, Ritz-type wavelengths were determined for about 380 lines, with uncertainties varying from 0.0003 to 0.002 Å. The observed configurations were theoretically interpreted by means of Hartree–Fock calculations and least-squares fits of the energy parameters to the observed levels. The fitted parameters were used to calculate oscillator strengths for all classified lines. A few unclassified lines and undesignated levels are also given. An improved value for the ionization energy was obtained by combining the observed energy of the 4d5g configuration with an ab initio calculation of its term value. The adopted value is 438 900 ± 150 cm⁻¹ (54.417 ± 0.019 eV).

The recent theoretical and experimental studies show that the Breit interaction plays a dominant role in the dielectronic recombination for some particular transitions. The detailed mechanism of why the Breit interaction is dominant for such a process is still unknown. In this work, we performed a simulation and decomposed each individual term in the transition matrix level and found that the Breit interaction is dominant when the leading term ($1/{{r}_{\gt }}$ with ${{r}_{\gt }}$ the larger of r₁ and r₂) contribution of the two-electron Coulomb interaction is vanished. Based on this mechanism, we explained why the dielectronic capture strength to $1{\rm s}2{{{\rm s}}^{2}}2{{{\rm p}}_{1/2}}\ {{J}_{d}}=1$ state is much stronger than the one to $1{\rm s}2{\rm s}2{\rm p}_{1/2}^{2}\ {{J}_{d}}=1$ as well as why the Breit interaction plays a dominant role in the anisotropic parameters. Furthermore, the present finding may guide us to search some physical processes in which the Breit interaction is dominant by simply analyzing the coupling coefficients for a given isoelectronic sequence.

Triple photoionization of Xe³⁺, Xe⁴⁺ and Xe⁵⁺ ions has been studied in the energy range 670–750 eV, including the 3d ionization threshold. The photon-ion merged-beam technique was used at a synchrotron light source to measure the absolute photoionization cross sections. These cross sections exhibit a progressively larger number of sharp resonances as the ion charge state is increased. This clearly visualizes the re-ordering of the $\epsilon {\rm f}$ continuum into a regular series of (bound) Rydberg orbitals as the ionic core becomes more attractive. The energies and strengths of the resonances are extracted from the experimental data and are further analysed by relativistic atomic-structure calculations.

The finite nuclear size corrections to the relativistic recoil effect in H-like ions are calculated within the Breit approximation. The calculations are performed for the 1s, 2s, and 2p_1/2 states in the range Z = 1–110. The obtained results are compared with previous evaluations of this effect. It is found that for heavy ions the previously neglected corrections amount to about 20% of the total nuclear size contribution to the recoil effect calculated within the Breit approximation.

The degree of linear polarization and angular distribution of the x-ray photoemission of highly charged He-like and Li-like uranium ions following electron-impact excitation and dielectronic recombination processes are calculated using a fully relativistic distorted-wave method. A detailed investigation is carried out regarding the contribution of the magnetic quadropole (M2) term to the subsequent characteristic x-ray emission from the above two different processes. It is found that while the M2 term has a slight effect on the angular distribution and linear polarization of electron-impact excitation, it has a substantial effect on the same properties of dielectronic recombination.

We have studied the excitation of H-like and He-like uranium (U⁹¹⁺ and U⁹⁰⁺) in relativistic collisions with gaseous targets by observing the subsequent x-ray emission. The experiment was conducted at the ESR storage ring of the GSI accelerator facility in Darmstadt, Germany. The measurements were performed with a newly developed multi-phase target at different collision energies. This enabled us to explore the proton (nucleus) impact excitation as well as the electron impact excitation processes in the relativistic collisions. The large fine-structure splitting in uranium allowed us to unambiguously resolve excitation to different L-shell levels. Moreover, information about the population of different magnetic sublevels has been obtained via an angular differential study of the decay photons associated with the subsequent de-excitation process. The experimental results are compared with calculations performed within the relativistic framework including excitation mechanisms due to both protons (nucleus) and electrons.

It is demonstrated that the effect of the electric dipole moment of electron and of the P,T-odd electron–nucleus interaction can be well distinguished in the experiments with H-like (or Li-like) highly charged ions in storage rings. These two effects cannot be separated in the experiments with a certain atom or molecule. Possible experiments for the search of P,T-odd effects with H-like ions in storage rings are discussed and the parameters of these experiments (observation time, field control) are estimated.

Be-like $^{136}{\rm X}{{{\rm e}}^{50+}}$ ions have been investigated employing the resonant electron–ion collision process of dielectronic recombination (DR) as a spectroscopic tool. The experiments were performed at the experimental storage ring in Darmstadt, Germany, using its electron cooler as a target for free electrons. DR Rydberg resonance series $2{{{\rm s}}^{2}}+{{e}^{-}}\to 2{\rm s}2{{{\rm p}}_{{{j}^{\prime }}}}n{{l}_{j}}$ for the associated intra-L-shell transitions $2{{{\rm s}}^{2}}{{\;}^{1}}{{{\rm S}}_{0}}-2{\rm s}2{{{\rm p}}_{1/2}}{{\;}^{3}}{{{\rm P}}_{1}},2{{{\rm s}}^{2}}{{\;}^{1}}{{{\rm S}}_{0}}-2{\rm s}2{{{\rm p}}_{3/2}}{{\;}^{3}}{{{\rm P}}_{2}}$ and $2{{{\rm s}}^{2}}{{\;}^{1}}{{{\rm S}}_{0}}-2{\rm s}2{{{\rm p}}_{3/2}}{{\;}^{1}}{{{\rm P}}_{1}}$ were observed with high resolution. In addition to these excitations from the ground state we determined resonances associated with excitations $2{\rm s}2{{{\rm p}}_{1/2}}{{\;}^{3}}{{{\rm P}}_{0}}\to 2{{{\rm p}}_{1/2}}2{{{\rm p}}_{3/2}}{{\;}^{3}}{{{\rm P}}_{1}}$ of ions initially in the metastable $2{\rm s}2{{{\rm p}}_{1/2}}{{\;}^{3}}{{{\rm P}}_{0}}$ state. The corresponding excitation energies were determined to be $E{{(}^{1}}{{{\rm S}}_{0}}\to {{\;}^{3}}{{{\rm P}}_{1}})=127.269(46)\;{\rm eV},$$E{{(}^{1}}{{{\rm S}}_{0}}\to {{\;}^{3}}{{{\rm P}}_{2}})=469.474(81)\;{\rm eV}$ and $E{{(}^{1}}{{{\rm S}}_{0}}\to {{\;}^{1}}{{{\rm P}}_{1}})=532.801(16)\;{\rm eV},$ and $E{{(}^{3}}{{{\rm P}}_{0}}\to 2{{{\rm p}}_{1/2}}2{{{\rm p}}_{3/2}}{{\;}^{3}}{{{\rm P}}_{1}})=533.733(22)\;{\rm eV}$. These excitation energies are compared with previous measurements and with recent state-of-the-art atomic structure calculations.

In this paper we report on results obtained with a compact double-stage molybdenum x-ray laser (XRL), operated with a total pump energy of 600 mJ. The two gain regions were pumped using the double-pulse grazing incidence pumping technique, which includes travelling wave excitation for both the seed- and the amplifier-target. In addition, the influence of an additional pre-pulse has been studied. Seeded XRL operation has been demonstrated in both schemes, resulting in XRL pulses with a divergence of 2 × 2 mrad. The peak brilliance of the amplified XRL of 4 × 10²³ photons/s/mm²/mrad² in 5 × 10⁻⁵ relative bandwidth was more than two orders of magnitude larger compared to the original seed pulses. The presented experimental concept provides an alternative approach to the currently more common use of high-order harmonic pulses as a seed source, well suited for applications like laser spectroscopy of highly-charged ions at a storage ring.

The experimental investigation of quantum-electrodydamic contributions to the binding energies of inner shells of highly charged heavy ions requires an accurate spectroscopy in the region of hard x-rays suitable at a limited source strength. For this purpose the focusing compensated asymmetric Laue crystal optics has been developed and a twin-spectrometer assembly has been built and commissioned at the experimental storage ring of the GSI Helmholtzzentrum Darmstadt. We characterize the crystal optics and demonstrate the usefulness of the instrumentation for accurate spectroscopy of both stationary and fast moving x-ray sources. The experimental procedures discussed here may also be applied for other spectroscopic studies where a transition from conventional germanium x-ray detectors to crystal spectrometers seems too demanding because of low source intensity.

The characteristics of soft x-ray emission from optically thin high-Z plasmas of gold, lead and bismuth were investigated with the large helical device. Compared to optically thicker laser-produced plasmas, significantly different spectral structures were observed due to the difference in opacities and electron temperatures. Peak structures appearing in unresolved transition arrays were identified by calculations using atomic structure codes. The main contributors of discrete line emission in each case were Pd-, Ag-, and Rh-like ion stages. The present calculations point to the overestimation of contributions for 4p–4d transitions based on intensity estimates arising purely from gA distributions that predict strong emission from 4p–4d transitions. Understanding of such spectral emission is not only important for the completion of databases of high-Z highly ion charge states but also the development of promising high brightness sources for biological imaging applications.

We have observed a number of different types of extreme ultraviolet (EUV) spectra from highly charged gadolinium (Gd), terbium (Tb) and dysprosium (Dy) ions in optically thin plasmas produced in the Large Helical Device at the National Institute for Fusion Science. Temporal changes in EUV spectra in the 6–9 nm region subsequent to the injections of solid pellets were measured by a grazing incidence spectrometer. The spectra rapidly change from discrete features into unresolved transition arrays (UTAs) following a drop in the electron temperature after the heating power is reduced. In particular, extremely narrowed UTA features, which comprise spectral lines of Ag-like, Pd-like and neighboring ion stages, are observed when the peak electron temperature is less than 0.45 keV due to the formation of hollow plasmas. Some discrete spectral lines of Cu-like and Ag-like ions have been identified in the high and low temperature plasmas, respectively, some of which are experimentally identified for the first time.

X-ray spectra of H- and He-like ions (and satellites) from argon, calcium and chlorine have been measured in Alcator C-Mod tokamak plasmas using a high wavelength resolution imaging x-ray spectrometer system. For H-like charge states, the Ly_α doublet intensity ratio is found to be slightly greater than 1/2 due to interaction with the ²S_1/2 fine structure sub-level. Neighbouring satellites with spectator electrons occupying n = 2 through n = 7 have been modelled. The Ly_α doublet ratio scalings with electron density and temperature are shown to be in good agreement with collisional–radiative modelling. For He-like ions, the n = 2 satellite intensities are in good agreement with calculations and can be used to determine the electron temperature. The high n Rydberg series has been resolved up to 1s²–1s14p, and satellites with spectator electrons up to the n = 12 level have been identified.

The Los Alamos suite of relativistic atomic physics codes is a robust, mature platform that has been used to model highly charged ions in a variety of ways. The suite includes capabilities for calculating data related to fundamental atomic structure, as well as the processes of photoexcitation, electron-impact excitation and ionization, photoionization and autoionization within a consistent framework. These data can be of a basic nature, such as cross sections and collision strengths, which are useful in making predictions that can be compared with experiments to test fundamental theories of highly charged ions, such as quantum electrodynamics. The suite can also be used to generate detailed models of energy levels and rate coefficients, and to apply them in the collisional-radiative modeling of plasmas over a wide range of conditions. Such modeling is useful, for example, in the interpretation of spectra generated by a variety of plasmas. In this work, we provide a brief overview of the capabilities within the Los Alamos relativistic suite along with some examples of its application to the modeling of highly charged ions.

A theoretical analysis is presented for the elastic Rayleigh scattering of x-rays by many-electron atoms and ions. Special emphasis is placed on the angular distribution and linear polarization of the scattered photons for the case when the incident light is completely (linearly) polarized. Based on second-order perturbation theory and the independent particle approximation, we found that the Rayleigh angular distribution is strongly affected by the charge state and shell structure of the target ions or atoms. This effect can be observed experimentally at modern synchrotron facilities and might provide further insight into the structure of heavy atomic systems.

We report on studies of projectile excited states produced by electron capture in both low and high velocity regimes, and when highly charged ions (HCIs) collide either with dilute or dense matter. Quantum effects in the interaction dynamics are probed via high-resolution x-ray spectroscopy for Ar¹⁷⁺ at 7 keV u⁻¹ and for Ar¹⁸⁺ at 13.6 MeV u⁻¹ on Ar, N₂ or CH₄ gas targets and on carbon solid foils. Relevant comparison between those two collision velocity regimes, and between gaseous and solid targets reveal specific features. In particular, the effect of multiple capture process occurring within a single-collision with gaseous target can be compared with the consequence of multistep collisions arising at surfaces and in solid-bulk at low velocity. At high velocity, beside evidence for collective response of the target electrons due to the wake field induced by HCI passing through the solid-bulk, we demonstrate that excitation and ionization collision processes damp the populations of projectile excited states for long ion transit times. The evolution of the np population as a function of n in solid is at variance from the 1/n³ law found in gas, and the disagreement increases with solid target thickness. We have also tackled studies of HCIs in collision with clusters showing that x-ray spectroscopy provides a powerful tool to sign the presence of clusters in a supersonic gas jet.

Highly charged ions play a crucial role in magnetic fusion plasmas. These plasmas are excellent sources for producing highly charged ions and copious amounts of radiation for studying their atomic properties. These studies include calibration of density diagnostics, x-ray production by charge exchange, line identifications and accurate wavelength measurements, and benchmark data for ionization balance calculations. Studies of magnetic fusion plasmas also consume a large amount of atomic data, especially in order to develop new spectral diagnostics. Examples we give are the need for highly accurate wavelengths as references for measurements of bulk plasma motion, the need for accurate line excitation rates that encompass both electron-impact excitation and indirect line formation processes, for accurate position and resonance strength information of dielectronic recombination satellite lines that may broaden or shift diagnostic lines or that may provide electron temperature information, and the need for accurate ionization balance calculations. We show that the highly charged ions of several elements are of special current interest to magnetic fusion, notably highly charged ions of argon, iron, krypton, xenon, and foremost of tungsten. The electron temperatures thought to be achievable in the near future may produce W⁷⁰⁺ ions and possibly ions with even higher charge states. This means that all but a few of the most highly charged ions are of potential interest as plasma diagnostics or are available for basic research.

We investigated Pm-, Nd-, and Pr-like spectra in the extreme ultra-violet region around 20 nm of Pt, Ir, Os, and Re (Z = 78–75) produced in the Heidelberg electron beam ion trap. Identification of the transitions was supported by several theoretical calculations, including collisional radiative modeling of the observed spectra. Special attention is given to the identifications of the alkaline-like $5{{{\rm s}}_{1/2}}$–$5{{{\rm p}}_{3/2}}$ resonance lines in promethium-like highly charged ions. Previous identifications of these lines have been tentative at best due to disagreements with theory and doubts about the experimental charge state identifications. Our experimental results for the $5{{{\rm s}}_{1/2}}$–$5{{{\rm p}}_{3/2}}$ wavelengths are accurate at the 0.005%-level. Understanding the level-structure of ions near the $4{\rm f}$–$5{\rm s}$ level crossing is of particular importance for future searches of a possible fine-structure constant variation, and new optical clocks.

Time-dependent close-coupling methods are used to calculate energy differential cross sections for the single and double ionization of He by impact with F⁹⁺ ions at 4.0 MeV amu⁻¹. Single ionization energy differential cross sections using both a one active electron method and a two active electron method are compared with recent experimental results. Double ionization energy differential cross sections using a two active electron method are presented to guide future experiments.

Wavelengths and transition probabilities have been computed for forbidden lines within the 3d^k(k = 1-9) ground configurations in ions of hafnium, tantalum, tungsten and gold, employing the second-order relativistic many-body perturbation theory (RMBPT). For comparison, the relativistic configuration-interaction (RCI) calculations have also been performed. Comparing to the previously published theoretical wavelengths and the present RCI ones, the present RMBPT results are in much better agreement with the experimental values measured recently by using the electron beam ion trap facility, reproducing these observed wavelengths to within 0.2% for all transitions only with one exception. In addition, it removes dramatically the systematic underestimation/overestimation for shorter/longer wavelengths existing in the present RCI and other calculations.

Electron emissions in transfer ionization processes were studied for 75 keV u⁻¹ He²⁺, and 80 keV u⁻¹ Ne⁸⁺on He collisions, using the well-developed reaction microscope techniques. Momentum distributions in the scattering plane, doubly differential distributions as a function of longitudinal momentum and emission angles of the ejected electrons were obtained. An apparent enhancement of electrons distributed around the projectile in the scattering plane was found for the Ne⁸⁺ incident case. Furthermore, we report the ratio of doubly differential distributions at the emission angle of 0° between these two transfer ionization processes, in which an abrupt rise is found at and above the electron capture to the continuum peak. This rise qualitatively agrees with the prediction within the framework of Dettmann's theory. We conclude that this kind of enhancement was caused by the charge state effect of the projectile.

We report an improved measurement of the hyperfine splitting in hydrogen-like bismuth (²⁰⁹Bi⁸²⁺) at the experimental storage ring ESR at GSI by laser spectroscopy on a coasting beam. Accuracy was improved by about an order of magnitude compared to the first observation in 1994. The most important improvement is an in situ high voltage measurement at the electron cooler (EC) platform with an accuracy at the 10 ppm level. Furthermore, the space charge effect of the EC current on the ion velocity was determined with two independent techniques that provided consistent results. The result of ${{\lambda }_{0}}=243.821(6)$ nm provides an important reference value for experiments testing bound-state quantum electrodynamics in the strong magnetic field regime by evaluating the specific difference between the splittings in the hydrogen-like and lithium-like ions.

The ${{{\rm W}}^{45+}}$ and ${{{\rm W}}^{46+}}$ 3p–4d inner shell excitation lines in addition to ${\rm M}{{{\rm o}}^{32+}}$ 2p–3s lines have been identified from the spectrum taken by an upgraded high-resolution x-ray spectrometer. It is found from analysis of the absolute intensities of the ${{{\rm W}}^{46+}}$ and ${\rm M}{{{\rm o}}^{32+}}$ lines that W and Mo concentrations are in the range of $\sim {{10}^{-5}}$ and $\sim {{10}^{-6}}$, respectively, with a ratio of ∼5% in JET with the ITER-like wall configuration for ELMy H-mode plasmas with a plasma current of 2.0–2.5 MA, a toroidal magnetic field of 2.7 T and a neutral beam injection power of 14–18 MW. For the purpose of checking self-consistency, it is confirmed that the W concentration determined from the ${{{\rm W}}^{45+}}$ line is in agreement with that from the ${{{\rm W}}^{46+}}$ line within 20% and that the plasma effective charge determined from the continuum of the first order reflection spectrum is also in agreement with that from the second order within 50%. Further, the determined plasma effective charge is in agreement with that determined from a visible spectroscopy, confirming that the sensitivity of the x-ray spectrometer is valid and that the W and the Mo concentrations are also likely to be valid.

Highly charged radioactive ions can be stored for extended periods of time in storage rings which allows for precision measurements of their decay modes. The straightforward motivation for performing such studies is that fully ionised nuclei or few-electron ions can be viewed as clean quantum-mechanical systems, in which the interactions of the many electrons can be either excluded or treated precisely. Thus, the influence of the electron shell on the decay probability can be investigated. Another important motivation is stellar nucleosynthesis, which proceeds at high temperatures and the involved atoms are therefore highly ionised. Presented here is a compact review of the relevant experiments conducted at heavy-ion storage rings. Furthermore, we outline the perspectives for future experiments at new-generation storage-ring facilities.

The primary requirement for the development of tools for extreme ultraviolet lithography (EUVL) has been the identification and optimization of suitable sources. These sources must be capable of producing hundreds of watts of extreme ultraviolet (EUV) radiation within a wavelength bandwidth of 2% centred on 13.5 nm, based on the availability of Mo/Si multilayer mirrors (MLMs) with a reflectivity of ∼70% at this wavelength. Since, with the exception of large scale facilities, such as free electron lasers, such radiation is only emitted from plasmas containing moderately to highly charged ions, the source development prompted a large volume of studies of laser produced and discharge plasmas in order to identify which ions were the strongest emitters at this wavelength and the plasma conditions under which their emission was optimized. It quickly emerged that transitions of the type 4p⁶4dⁿ − 4p⁵4dⁿ⁺¹ + 4dⁿ⁻¹4f in the spectra of Sn IX to SnXIV were the best candidates and work is still ongoing to establish the plasma conditions under which their emission at 13.5 nm is maximized. In addition, development of other sources at 6.X nm, where X ∼ 0.7, has been identified as the wavelength of choice for so-called Beyond EUVL (BEUVL), based on the availability of La/B based MLMs, with theoretical reflectance approaching 80% at this wavelength. Laser produced plasmas of Gd and Tb have been identified as potential source elements, as n = 4 − n = 4 transitions in their ions emit strongly near this wavelength. However to date, the highest conversion efficiency (CE) obtained, for laser to BEUV energy emitted within the 0.6% wavelength bandwidth of the available mirrors is only 0.8%, compared with values of 5% for the 2% bandwidth relevant for the Mo/Si mirrors at 13.5 nm. This suggests a need to identify other potential sources or the selection of other wavelengths for BEUVL. This review deals with the atomic physics of the highly-charged ions relevant to EUV emission at these wavelengths. It considers the developments that have contributed to the realization of the 5% CE at 13.5 nm which underpins the production of high-volume lithography tools, and those that will be required to realize BEUV lithography.

We report on the selective production of the 2s metastable state of 390-MeV u⁻¹ H-like Ar¹⁷⁺ ions by the resonant coherent excitation (RCE). The RCE to the 2s state was achieved by using Stark mixing of n = 2 states under planar channeling conditions in a 1-$\mu$m thick Si crystal. The population of the 2s state was quantitatively estimated from the ionization probability in a stripper foil installed 50 mm downstream of the crystal. The 2s population was found to be 0.05 of the total number of Ar¹⁷⁺ ions at the stripper. The result is in good agreement with a theoretical calculation performed with the density matrix approach.

Electron impact ionization cross sections for the U⁸⁸⁺, U⁸⁹⁺, U⁹⁰⁺ and U⁹¹⁺ ions were calculated with the relativistic binary encounter Bethe model (RBEB), the modified RBEB (MRBEB) and the new MRBEB corrected by the ionic factor (MRBR–IF). Our results were compared with the available three sets of experimental data and the most used theoretical results. The MRBEB–IF results are the ones that better agree with the experimental data of the four analysed ions.

The upgraded x-ray crystal spectrometer KX1 on the Joint European Torus (JET) can now measure the M x-ray lines from tungsten with sufficiently high resolution to evaluate how much tungsten may sputter from the plasma-facing tungsten wall planned for the International Thermonuclear Experimental Reactor (ITER). However, a test run on JET found that the L x-ray lines of a molybdenum impurity, which happen to occur in the targeted wavelength region of 5.00–5.35 Å, must be taken into account together with the radiation from the M x-ray lines of tungsten to match the high-resolution spectra. Such detailed radiation modeling is expected to be needed for ASDEX Upgrade tokamak, and for other tokamaks such as ITER and tungsten (W) Environment in Steady-state Tokamak (WEST), which will implement tungsten plasma-facing components and a high-resolution spectrometer to keep track of with a similar x-ray diagnostics.

We present an overview of recent work on the spectroscopy of tungsten ions, related to tokamak edge plasma. The spectra were recorded from the newly-built low energy electron beam ion traps (EBITs) in the Shanghai EBIT laboratory. By analyzing the spectra with the help of accurate theoretical calculations, using state-of-the-art techniques, we were able to identify term and fine structure splittings in the ground and the first excited configuration for a number of charge states. The theoretical models included a careful study of correlation and showed an excellent agreement with our experimental results for transition energies and rates. Some metastable levels which have extremely long lifetime and high population were found, and the influences of these levels on the charge state distribution of tungsten ions in tokamaks are discussed.

Detailed calculations using a fully relativistic distorted-wave method incorporating the Debye–Hückel potential are carried out for the electron-impact excitation cross sections from the ground state 1s²2s² (J = 0) to the individual magnetic sublevels of 1s2s²2p $^{1}{{P}_{1}}$ and $^{3}{{P}_{1}}$ states of highly charged Be-like ${\rm S}{{{\rm c}}^{17+}}$ ions in the presence of a plasma environment. The effects of plasma screening on the cross section and linear polarization of the corresponding x-ray emission are investigated. For the $^{3}{{P}_{1}}-{{}^{1}}{{S}_{0}}$ line, it is found that the plasma screening has a pronounced effect and causes the cross section to decrease over the whole energy range considered. This dramatic influence also leads to a remarkable decrease in the linear polarization of subsequent x-ray radiation. However, for the $^{1}{{P}_{1}}-{{}^{1}}{{S}_{0}}$ line, the plasma screening effect on the cross sections is relatively larger than for the $^{3}{{P}_{1}}$ excitations, while its influence on the linear polarization of the radiation is very small.

By applying novel-type position sensitive x-ray detectors as Compton polarimeters we recently performed a study of the linear polarization of Lyman-${{\alpha }_{1}}$ radiation following radiative electron capture into initially bare uranium ions. It was found that a model-independent determination of the ratio of the E1 and M2 transition amplitudes, and consequently of the corresponding transition rates, is feasible by combining the linear polarization data with a measurement of the angular distribution of the emitted radiation. In this work a detailed description of the underlying experimental technique for combined measurements of the linear polarization and the angular distribution of characteristic transitions in high-Z ions is presented. Special emphasis is given to the application of two, two-dimensional position-sensitive x-ray detectors for Compton polarimetry of hard x-rays. Moreover, we demonstrate the polarimeter efficiency of such detector systems can be significantly improved if events, where the charge is spread over neighboring segments, are reconstructed to be used in the polarization analysis.

The electron mass in atomic mass units has been determined with a relative uncertainty of $2.8\times {{10}^{-11}}$ (Sturm et al 2014 Nature506 467–70), which represents a 13-fold improvement of the 2010 CODATA value (Mohr et al 2012 Rev. Mod. Phys.84 1527–605). The underlying measurement principle combines a high-precision measurement of the Larmor-to-cyclotron frequency ratio on a single hydrogen-like carbon ion in a Penning trap with a corresponding very accurate g-factor calculation. Here, we present the measurement results in detail, including a comprehensive discussion of the systematic shifts and their uncertainties. A special focus is set on the various sources of phase jitters, which are essential for the understanding of the applied line-shape model for the g-factor resonance.

The charge-exchange (CX) rate coefficients for highly ionized impurity ions play a crucial role in fusion plasma diagnostics. However, till today a substantial difference exists in data for the nl-resolved cross-sections based on the different approximations underlying the classical trajectory Monte Carlo (CTMC) calculations either based on the standard initial momentum distribution of target electron orbits (pCTMC, as in the CX data provided by Whyte et al (1998 Phys. Plasmas5 3694) and Schultz et al (2010 J. Phys. B: At. Mol. Opt. Phys.43 144002) or based on the alternate initial radial distribution of orbits (rCTMC, as in the calculations of Errea et al (2006 J. Phys. B: At. Mol. Opt. Phys.39 L91). In this paper, results of new pCTMC and rCTMC calculations for CX in 16.7, 25, and 50 keV/u Ar¹⁷⁺ + H(1s), H(2s), and H(2p) are compared against X-ray line measurements performed at the tokamak TEXTOR. The Rydberg series (1snp–1s²) and the K_α-spectrum (1s2l–1s²) of He-like argon were measured directly in the beam-line of a 16.7–50 keV/u hydrogen injector. The intensities of the spectral lines are compared to the effective CX rate coefficients utilizing both sets of cross sections. While both data sets show good agreement with respect to the observed impact on the K_α transition, only the pCTMC data allow a consistent description of the CX 'resonance' observed on the Rydberg lines around n ≈ 8, 9. Similar to the case of low energy ion–atom interactions reported from different tokamaks, the observed influence of CX is separable into contributions from beam particles in the ground and excited states, respectively. It is shown, that the number of beam excited states n_h contributing to the CX signal, where n_h is the principal quantum number, is limited to n_h ≲ 10, confirming the results of recent collisional-radiative models of beam atoms in parabolic states.