Resonante Anregung astrophysikalischer Röntgen-Übergänge in hochgeladenen Eisenionen mit dem Freie-Elektronen-Laser LCLS

Das Spektrum von Fe16+, beobachtet in vielen astrophysikalischen Objekte und in Laborexperimenten, wird nur schlecht von Plasma-Modellen wiedergegeben. Insbesondere für die beiden, 3C und 3D genannten, stärksten 2p-3d-Übergänge um 800 eV ist das Intensitätsverhältnis 3C/3D immer kleiner als vorhergesagt. Es war bisher nicht möglich, dieses Rätsel zu lösen, da die Anregung immer durch Stöße erfolgte, was die Interpretation der Daten erschwerte. In der vorliegenden Arbeit wurde resonante Anregung dieser und anderer Übergänge mit hochbrillanten Röntgen-Pulsen von einem Freie-Elektronen-Laser dazu verwendet, dieses Problem zu enträtseln. In einer Elektronenstrahlionenfalle wurden die Ionen bestrahlt, während die Photonenenergie durchgestimmt wurde. So war es erstmals möglich, das Verhältnis der Oszillatorenstärken unabhängig von Stoßprozessen zu messen. Das Ergebnis widerspricht Rechnungen der elektronischen Strukturen, die bisher für ausreichend genau gehalten wurden. So wird demonstriert, daß die Problematik bereits auf der Ebene der Elektronenwellenfunktionen auftritt und nicht primär in der Behandlung von Stoßprozessen oder Plasmaeffekten. Weiterhin wurden die Übergangsenergien von Resonanzen aus Fe13+, Fe14+, Fe15+ und Fe16+ zwischen 795 und 830 eV mit Unsicherheiten bis hinuter zu 240 meV gemessen, indem sie mit nahegelegenen Übergängen Wasserstoff-artiger Ionen verglichen wurden. Das neuartige Experiment weitet den für Laserspektroskopie zugänglichen Spektralbereich bis in die Röngten-Region aus und eröffnet neue Möglichkeiten für Grundlagenexperimente und Anwendungen

Polarization of K-shell dielectronic recombination satellite lines of Fe XIX-XXV and its application for diagnostics of anisotropies of hot plasmas

We present a systematic measurement of the X-ray emission asymmetries in the K-shell dielectronic, trielectronic, and quadruelectronic recombination of free electrons into highly charged ions. Iron ions in He-like through O-like charge states were produced in an electron beam ion trap, and the electron-ion collision energy was scanned over the recombination resonances. Two identical X-ray detectors mounted head-on and side-on with respect to the electron beam propagation recorded X-rays emitted in the decay of resonantly populated states. The degrees of linear polarization of X-rays inferred from observed emission asymmetries benchmark distorted-wave predictions of the Flexible Atomic Code (FAC) for several dielectronic recombination satellite lines. The present method also demonstrates its applicability for diagnostics of energy and direction of electron beams inside hot anisotropic plasmas. Both experimental and theoretical data can be used for modeling of hot astrophysical and fusion plasmas.Comment: 22 pages, 4 figures, 10 tables, accepted for publication in The Astrophysical Journal Supplement Serie

On the Transition Rate of the Fe X RED Coronal Line

We present a lifetime measurement of the 3s 23p 5 2 Po 1/2 first excited fine-structure level of the ground state configuration in chlorine-like Fe X, which relaxes to the ground state through a magnetic dipole (M1) transition (the so-called red coronal line) with a wavelength accurately determined to 637.454(1) nm. Moreover, the Zeeman splitting of line was observed. The lifetime of 14.2(2) ms is the most precise one measured in the red wavelength region and agrees well with advanced theoretical predictions and an empirically scaled interpolation based on experimental values from the same isoelectronic sequence

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.Comment: 8 pages, 3 figure

Complete measurements of anisotropic x-ray emission following recombination of highly charged ions

Synopsis We report the first systematic measurement of x-ray emission anisotropies following dielectronic, trielectronic and quadroelectronic recombination into He-like through O-like iron and krypton ions. Using the experimental data we calculated the polarization of the Kα x rays in an anisotropic plasma as a function of the plasma temperature. Unexpectedly, we found that the degree of polarization is dominated by previously neglected trielectronic and quadroelectronic recombination resonances

Measurement of the angular distribution of Dielectronic Recombination into highly charged Krypton ions

Angular distribution of x-rays emitted in the process of Dielectronic Recombination (DR) was studied at the Electron Beam Ion Trap. For this the photon emission spectra were observed along and perpendicular the electron beam propagation direction. X-ray line intensities differ drastically between the two acquired spectra. This indicates a strong alignment of the total angular momentum vector of the excited states populated by DR with respect to the electron beam propagation direction

Polarization measurement of dielectronic recombination transitions in highly charged krypton ions

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 $K\alpha$ 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.Comment: 11 pages, 4 figures (Note that the Eqs. (5) and (10) of the original paper are corrected in Physical Review A 94, 029905(E) (2016). doi:10.1103/PhysRevA.94.029905