Dominance of the Breit interaction in the x-ray emission of highly charged ions following dielectronic recombination

Effect of the configuration mixing on the polarization and angular distribution of x-ray line emissions following electron-impact excitation of Ne-like ions

We present a systematic theoretical study on the angular distribution and linear polarization of x-ray line emissions of neon-like ions following the electron-impact excitation from the ground state to the excited levels [(2p5)1/23d3/2]J=1, [(2p5)3/23d5/2]J=1, [(2p5)3/23d3/2]J=1, and [(2p5)1/23s]J=1. The cross sections are calculated by using the flexible atomic code under configuration-interaction plus many-body perturbation theory method. The angular distribution and linear polarization are obtained based on density matrix theory. Emphasis has been placed on the effect of the configuration mixing on the angular distribution and polarization. It has been proved that the strong mixing of configuration [(2p5)3/23d3/2]J=1 with configuration [(2p5)1/23s]J=1 can result in the abrupt change of Z-dependence of angular distribution and polarization. It indicates that angular distribution and polarization can be expected to serve as a tool for investigation of configuration mixing effect.

Strong Polarization of a J=1/2 to 1/2 Transition Arising from Unexpectedly Large Quantum Interference

We experimentally show that the 1s^{2}2s^{2}2p_{1/2}-1s2s^{2}2p_{1/2}^{2} transition in Pb^{77+} emitted in dielectronic recombination of Pb^{78+} is strongly polarized, although it is an intrinsically unpolarized J=1/2 to 1/2 transition. This unanticipated polarization is shown to be due to quantum interference with radiative recombination. The interference effect has been studied on an asymmetric resonance profile but has never been studied on polarization. In this Letter, we show that the effect on polarization can arise from a different cross term than that responsible for asymmetry, resulting in unexpectedly large polarization even for a nearly symmetric resonance suggesting a small interference.

On the development of relativistic distorted wave approximation for the energies and collision dynamics of atoms or ions subjected to the outside plasma

In this manuscript, we present the development of a relativistic distorted wave method for determining the energies and collision dynamics of plasma-immersed atoms or ions. The methodology is based on the Dirac–Coulomb Hamiltonian, in which contributions from relativity and higher order effects, such as quantum electrodynamics and Breit interaction, are incorporated. The key element in this method is that a modified Debye–Hückel approximation is employed to represent the effect of plasma screening. In order to correctly describe the (bound and continuous state) wave functions, a self-consistent field calculation incorporating the shielding potential is performed within the fully relativistic framework. The particle interaction within the scattering matrix element of the excitation process is described by the shielded Coulomb interaction. The present technique is illustrated by calculations of energy, line shift, transition probability, electron-impact excitation/ionization cross section, and photoionization cross section of a few-electron system confined in plasma environments. The present model is tested and validated against a number of known cases (simulations are made for the He-like Al11+ ion) in the literatures. Numerical results demonstrate that the modifications to the Coulomb potential proposed in the spatial and temporal criteria of the Debye–Hückel approximation allow us to improve the theoretical description of the plasma shielding and thus the dynamical processes in dense plasmas. Comparisons of our computational predictions and the recent experimental measurements are performed. The current work not only has far-reaching implications for our understanding of the dense plasma screening, but also has potential applications in fusion, laboratory astrophysics, and related disciplines.

Apparent change of the 3C/3D line intensity ratio in neonlike ions

The resonance 3C ([(2p⁵)_1/23d_3/2]_J=1 → [2p⁶]_J=0) to intercombination 3D ([(2p⁵)_3/23d_5/2]_J=1 → [2p⁶]_J=0) line intensity ratio of neonlike ions has been studied. The measured line intensity ratio for neonlike Xe⁴⁴⁺ ions shows an apparent change, which is reproduced by the calculations using the relativistic configuration interaction plus many-body perturbation theory. It is clearly elucidated that the change in the 3C/3D line intensity ratio is caused by strong configuration mixing between the upper levels of the 3D and 3F ([(2p⁵)_1/23s]_J=1 → [2p⁶]_J=0) lines. The present measurement allows us to discuss the 3C/3D line intensity ratio for the highest-Z ions hitherto, which suggests that the experiment-theory discrepancy in the 3C/3D line intensity ratio of neonlike ions diminishes with increasing atomic number Z and further trends to vanish at higher-Z ions. Furthermore, the present study provides benefits to better understand configuration mixing effect in the radiative opacity of hot plasmas.

Theoretical evaluation of excitation cross section and fluorescence polarization of a solid-density Si plasma

In this article, a fully relativistic approach is proposed to precisely predict the electronic structures, spectral properties, cross sections, and degrees of linear polarization of light emitted after excitation of plasma-embedded ions by electron impact, taking into account the plasma shielding effects on the atomic structures and collision dynamics, in addition to the contributions of Breit interaction and quantum electrodynamics effects. The scheme employs the effective shielding potential deduced from a solution of Poisson equation, based on the self-consistent field ion-sphere simulations to explain the interactions among the charged particles, where the perturbation correlation Dirac–Coulomb Hamiltonian is constructed. The simple and understandable form makes it a good substitute for complex self-consistent field calculation. As an illustrative example, a comparative investigation regarding the influences of different plasma temperature and density parameters on the level energies, transition rates, integrated total/magnetic sublevel cross sections, and linear polarizations of the radiation following electron-impact excitation of Si XIII (a solid-density Si plasma) is carried out. Numerical results show that the plasma density effect can significantly affect the atomic structures and collision cross sections, yet has limited influence on the polarization characteristics. A comparison of our calculations with other results, when available, is made. The advanced approach presented here not only opens a novel window for exploring the atomic dynamics processes in hot and/or dense plasmas, but also provides important information about polarization of the line emission. This study is beneficial for the high energy density physics, laser-produced plasmas, and astrophysical applications.

Level Structure and Properties of Open f-Shell Elements

Open f-shell elements still constitute a great challenge for atomic theory owing to their (very) rich fine-structure and strong correlations among the valence-shell electrons. For these medium and heavy elements, many atomic properties are sensitive to the correlated motion of electrons and, hence, require large-scale computations in order to deal consistently with all relativistic, correlation and rearrangement contributions to the electron density. Often, different concepts and notations need to be combined for just classifying the low-lying level structure of these elements. With Jac, the Jena Atomic Calculator, we here provide a toolbox that helps to explore and deal with such elements with open d- and f-shell structures. Based on Dirac’s equation, Jac is suitable for almost all atoms and ions across the periodic table. As an example, we demonstrate how reasonably accurate computations can be performed for the low-lying level structure, transition probabilities and lifetimes for Th2+ ions with a 5f6d ground configuration. Other, and more complex, shell structures are supported as well, though often for a trade-off between the size and accuracy of the computations. Owing to its simple use, however, Jac supports both quick estimates and detailed case studies on open d- or f-shell elements.

An application of a Si/CdTe Compton camera for the polarization measurement of hard x rays from highly charged heavy ions

Methods to measure the polarization of x rays from highly charged heavy ions with a significantly higher accuracy than that of the existing technology are needed to explore relativistic and quantum electrodynamics effects, including the Breit interaction. We developed an Electron Beam Ion Trap Compton Camera (EBIT-CC), a new Compton polarimeter with pixelated multi-layer silicon, and cadmium telluride counters. The EBIT-CC detects the three-dimensional position of Compton scattering and photoelectric absorption, and thus, the degree of polarization of incoming x rays can be evaluated. We attached the EBIT-CC on the Tokyo Electron Beam Ion Trap (Tokyo-EBIT) in the University of Electro-Communications. An experiment was performed to evaluate its polarimetric capability through an observation of radiative recombination x rays emitted from highly charged krypton ions, which were generated by the Tokyo-EBIT. The CC of the EBIT-CC was calibrated for the ∼75 keV x rays. We developed event reconstruction and selection procedures and applied them to every registered event. As a result, we successfully obtained the polarization degree with an absolute uncertainty of 0.02. This uncertainty is small enough to probe the difference between the zero-frequency approximation and full-frequency-dependent calculation for the Breit interaction, which is expected for dielectronic recombination x rays of highly charged heavy ions.

Circular polarization of X-ray radiation emitted by longitudinally polarized electron impact excitation: Under a screened Coulomb interaction

Longitudinally polarized electron impact excitation from the ground state 1s2 to the excited state 1s2l (l =s,p) levels of highly charged He-like Fe24+ ions in weakly coupled hot-dense plasmas is investigated using a fully relativistic distorted-wave method. The Debye-Hückel potential is used to describe the plasma screening. Benchmark results such as the total cross sections, the magnetic sublevels cross sections, and the circular polarizations of corresponding X-ray radiations are presented. For the excitation process, results show that the plasma screening has an effect in reducing both the total and magnetic sublevels cross sections. For the de-excitation process, it is found that while the plasma screening as a slightly effect on the circular polarizations of radiations for the 1s2s 3S1 → 1s2 1S0,1s2p 3P2 → 1s2 1S0, and 1s2p 1P1 → 1s2 1S0 transition lines, it gives a substantial contribution for the same properties of the 1s2p 3P1 → 1s2 1S0 line.

Strong higher-order resonant contributions to x-ray line polarization in hot plasmas

We studied angular distributions of x rays emitted in resonant recombination of highly charged iron and krypton ions, resolving dielectronic, trielectronic, and quadruelectronic channels. A tunable electron beam drove these processes, inducing x rays registered by two detectors mounted along and perpendicular to the beam axis. The measured emission asymmetries comprehensively benchmarked full-order atomic calculations. We conclude that accurate polarization diagnostics of hot plasmas can only be obtained under the premise of inclusion of higher-order processes that were neglected in earlier work.

Observation of coherence in the time-reversed relativistic photoelectric effect

The photoelectric effect has been studied in the regime of hard x rays and strong Coulomb fields via its time-reversed process of radiative recombination (RR). In the experiment, the relativistic electrons recombined into the 2p_{3/2} excited state of hydrogenlike uranium ions, and both the RR x rays and the subsequently emitted characteristic x rays were detected in coincidence. This allowed us to observe the coherence between the magnetic substates in a highly charged ion and to identify the contribution of the spin-orbit interaction to the RR process.

Relativistic transfer ionization and the Breit interaction

We consider correlated transfer ionization in relativistic collisions between a highly charged ion and a light atom. In this process two quasifree electrons of the atom interact with each other during the short collision time that results in the capture of one of them by the ion and emission of the other. We show that this process is strongly influenced by the generalized Breit interaction already at modest relativistic impact energies.

Experimental demonstration of the Breit interaction which dominates the angular distribution of x-ray emission in dielectronic recombination

We report the experimentally determined angular distribution of the [1s2s(2)2p(1/2)](1)→[1s(2)2s(2)](0) transition in dielectronic recombination of Li-like Au. Recently, Fritzsche et al. [Phys. Rev. Lett. 103, 113001 (2009)] predicted that the Breit interaction plays a dominant role in the angular distribution of this transition. However, the predicted phenomenon has not yet been observed experimentally due to technical difficulties in conventional methods. To overcome the difficulties, we combine two different measurements with an electron beam ion trap (EBIT) to obtain the x-ray angular distribution. One is the x-ray measurement at 90° and another is the integral resonant strength measurement through the ion charge abundance in the EBIT. Our measurements agree well with the theoretical prediction and confirm the dominance of the Breit interaction.