Subshell-selective x-ray studies of radiative recombination of U92+{\mathrm{U}}^{92+} ions with electrons for very low relative energies

Radiative recombination (RR) into the K shell and L subshells of U92+ ions interacting with cooling electrons has been studied in an x-ray RR experiment at the electron cooler of the Experimental Storage Ring at GSI. The measured radiative recombination rate coefficients for electron-ion relative energies in the range 0–1000 meV demonstrate the importance of relativistic effects. The observed asymmetry of the measured K-RR x-ray emission with respect to the cooling energy, i.e., zero average relative velocity (⟨vrel⟩=0), are explained by fully relativistic RR calculations. With our new approach, we show that the study of the angular distribution of RR photons for different relative energies opens new perspectives for detailed understanding of the RR of ions with cooling electrons in cold magnetized plasma

Relativistic quantum dynamics in strong fields: Photon emission from heavy, few-electron ions

Recent progress in the study of the photon emission from highly-charged heavy ions is reviewed. These investigations show that high-$Z$ ions provide a unique tool for improving the understanding of the electron-electron and electron-photon interaction in the presence of strong fields. Apart from the bound-state transitions, which are accurately described in the framework of Quantum Electrodynamics, much information has been obtained also from the radiative capture of (quasi-) free electrons by high-$Z$ ions. Many features in the observed spectra hereby confirm the inherently relativistic behavior of even the simplest compound quantum systems in Nature.Comment: Version 18/11/0

First Measurement of the Linear Polarization of Radiative Electron Capture Transitions

For radiative electron capture into the K shell of bare uranium ions, a study of the polarization properties has been performed. For this purpose a position sensitive germanium detector has been used as an efficient Compton polarimeter. This enabled us to measure the degree of linear polarization by analyzing Compton scattering inside the detector and to determine the orientation of the polarization plane. Depending on the observation angle and the beam energy used, the radiation is found to be linearly polarized by up to 80%. In all cases studied, the plane of polarization coincides with the collision plane. The results will be discussed in the context of rigorous relativistic calculations, showing that relativistic effects tend to lead to a depolarization of the radiation emitted

The electron-electron interaction studied in strong central fields by resonant transfer and excitation with H-like U ions

Electron-electron interaction is studied in the strongest possible atomic fields (Z.alpha => 1) in the presence of only two electrons. A quasi-free electron from a hydrogen gas target is resonantly captured into a L sub j subshell of a fast H-like U sup 9 sup 1 sup + ion by simultaneous excitation of the strongly bound K electron also into a L sub j sub ' subshell of the projectile, with j and j' the total angular momenta of 1/2 or 3/2 for the electron of concern. This resonant capture and excitation process, KL sub j L sub j sub ' -RTE, is mediated by electron-electron interaction. It is equivalent to dielectronic recombination (DR) in ion-electron collisions and leads to a doubly excited He-like U sup 9 sup 0 sup + sup * sup * ion, which stabilizes - almost exclusively - via the emission of two successive K X-rays, first a K hypersatellite (K alpha i-H) and then a K satellite (K alpha i'-S) transition. The K X-ray emission characteristics associated with one-electron capture in collisions of U sup 9 sup 1 sup + ions with a hydrogen target is studied for the three resonance groups of the KL sub j L sub j sub ' -RTE and one off-resonance energy, i.e. in the energy range between 100 and 135 MeV/u. The total cross section for the first resonance group KL sub 1 sub / sub 2 L sub 1 sub / sub 2 confirms the importance of the Breit contribution to the interaction. The angular distribution for the K alpha 2-H transition (j=1/2) is isotropic in the projectile system whereas the K alpha 1-H transition (j=3/2) indicates a strong alignment for the 3/2 electrons in the doubly excited states for the second resonance group KL sub 1 sub / sub 2 L sub 3 sub / sub 2. The experimental results are in agreement with fully relativistic calculations including the generalized Breit interaction

Angular correlation and polarization studies for radiative electron capture into high-Z ions

Recent photon correlation studies for Radiative Electron Capture into high-Z projectiles are reviewed. Emphasis is given to the investigation of polarization phenomena which are now accessible due to recent developments in position sensitive solid-states detectors. It is shown, that REC may provide a tool for the diagnostics and detection of the spinâ"polarization of particles involved in atomic collisions. Also the impact of REC studies for atomic structure studies is outlined. Here the strong alignment of excited states induced by REC allowed us to observe an interference between competing decay branches for the case of the Lyman-α1 transition in hydrogen-like ions