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[en] After an extended upgrading period at the Experimental Storage Ring (ESR) of the GSI in Darmstadt, an experiment on recombination of lithium-like uranium has been carried out. A 107.1 MeV/u U⁸⁹⁺ beam with up to 500 μA effective current was stored and cooled. In addition to measurements of radiative recombination (RR) and dielectronic recombination (DR) with high energy resolution in the range of 0-90 eV, the diagnostic properties of DR have been utilized to characterize the new cooler setup. Preliminary experimental results for the 1s²2p_3/25lj(j=3/2,5/2) resonance manifolds are presented and compared with theoretical predictions

[en] The origin of the widely observed enhancement of rates for electron-ion recombination at very low energies is still unknown. We investigated the recombination of Au²⁵⁺ with free electrons in a merged-beams experiment at the UNILAC accelerator of the GSI in Darmstadt. At E_rel= 0 eV we found an enormous enhancement factor of 365 compared to the theory of radiative recombination. An increase of the electron density by a factor of 10 had not much influence on the measured rate coefficient

[en] The storage ring TSR is used to cool and accumulate a wide range of heavy-ion beams produced at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany. With ionization stages reaching up to 50 (bare nuclei up to Cl¹⁷) and beam energies of typically 5-8 MeV/u, storage lifetimes amount to minutes up to hours; repeated cycles of injection and electron cooling (reducing the ion beam diameter Tom centimeters to millimeters in about 2 sec) make it possible to accumulate beam currents up to 1 mA in the ring. With the circulating beam, in particular the interaction of multicharged atomic ions with electrons of variable, well defined energies (10 MeV to 3 keV) is studied, detecting recombination and electron impact ionization. High-resolution spectra of the dielectronic recombination cross section have been obtained, which provide a sensitive test for atomic-structure calculations describing the large number of doubly excited states contributing to the recombination rate. For lithiumlike ions (Si¹¹⁺, Cl¹⁴⁺) all possible outer- and inner-shell excitations were investigated[1]. Low-energy di- electronic recombination measurements are presently being extended[2] also to more complex open-L-shell ions (F¹¹⁺-F²³⁺). Very recently the enhancement of dielectronic recombination by ambient electric fields was studied with Cl¹⁴⁼; at well-controlled fields of order 100 V /cm the enhancement of the cross section by up to a factor of 3 in certain energy regions could be followed in detail[3] . In addition to the information on doubly excited levels from dielectronic recombination experiments, spectroscopic results on multicharged ions were also obtained by laser-stimulated recombination[4] , allowing highly excited levels in multicharged ions to be reached by laser transitions from continuum states populated by surrounding free electrons. Moreover, for a number of metastable levels in multicharged ions the natural lifetimes in the millisecond to second range could be determined with high accuracies of about 0.2%, using dielectronic resonances to identify metastable ions and also direct optical observation of vacuum-ultraviolet emission

[en] Using an electron beam merged with an energetic ion beam in a storage ring, non-resonant recombination between multiply charged ions and free electrons is studied by measuring directly the rate of recombination products leaving the interaction zone. The relative electron-ion energy was varied from meV energies up to ∝50 eV. Above ∝0.5 eV the measured rates are well represented by the calculated RR into final states up to the experimental Rydberg-ion detection limit. Between ∝0.01 and 0.5 eV the calculated RR rates are 10% experimental data. As the relative energy is decreased below 0.01 eV, the observed recombination rates rise strongly (to < or∼2.2 times the predicted RR rate), reproducing the low-energy recombination enhancement found in other recent experiments. (orig.)

[en] Dielectronic recombination (DR) and radiative recombination (RR) of lithium-like gold in the energy range of 0 to 225 eV have been studied at the Experimental Storage Ring (ESR) of the GSI in Darmstadt. Main objective of the measurements is the precise determination of the 2s_1/2-2p_1/2 energy splitting as an additional QED test. This novel method, developed at the ESR, is based on the extrapolation of a multitude of measured resonances Au⁷⁵⁺ (1s²2p_1/2nl_j) up to the series limit (n = ∞). Furthermore experimental data for the Au⁷⁵⁺ (1s²2p_3/26l_j) resonance manifold are presented

[en] An introduction to electron-ion recombination processes is given and recent measurements are described as examples, focusing on low collision energies. Discussed in particular are fine-structure-mediated dielectronic recombination of fluorine-like ions, the moderate recombination enhancement by factors of typically 1.5-4 found for most ion species at relative electron-ion energies below about 10 meV, and the much larger enhancement occurring for specific highly charged ions of complex electronic structure, apparently caused by low-energy dielectronic recombination resonances. Recent experiments revealing dielectronic resonances with very large natural width are also described

[en] Iron ions provide many valuable plasma diagnostics for cosmic plasmas. The accuracy of these diagnostics, however, often depends on an accurate understanding of the ionization structure of the emitting gas. Dielectronic recombination (DR) is the dominant electron-ion recombination mechanism for most iron ions in cosmic plasmas. Using the heavy-ion storage ring at the Max-Planck-Institute for Nuclear Physics in Heidelberg, Germany, we have measured the low temperature DR rates for Fe^q+ where q = 15, 17, 18, and 19. These rates are important for photoionized gases which form in the media surrounding active galactic nuclei, X-ray binaries, and cataclysmic variables. Our results demonstrate that commonly used theoretical approximations for calculating low temperature DR rates can easily under- or over-estimate the DR rate by a factor of ∼ 2 or more. As essentially all DR rates used for modeling photoionized gases are calculated using these approximations, our results indicate that new DR rates are needed for almost all charge states of cosmically abundant elements. Measurements are underway for other charge states of iron

[en] New recombination experiments with merged cold beams of electrons and atomic ions have been carried out at the storage ring facilities TSR in Heidelberg, ESR in Darmstadt, and CRYRING in Stockholm. A brief overview is given on the recent activities in which the Giessen group was engaged. Topics of this research were dielectronic recombination (DR) of astrophysically relevant ions, recombination of highly charged ions with respect to cooling losses in storage rings, field effects on DR, search for interference effects in photorecombination of ions, correlation effects in DR of low-Z ions, spectroscopy of high-Z ions by DR, and lifetimes of metastable states deduced from DR experiments

[en] Radiative and dielectronic recombination of F⁶⁺ have been studied at the heavy ion storage ring TSR in Heidelberg. For a detailed investigation of rate enhancement effects at very low electron-ion center-of-mass energies experimental parameters such as the magnetic guiding field, the electron density and the adiabatic expansion factor of the electron beam have been varied systematically. Whereas measurements at different electron densities show no influence on the enhancement and while a variation of the expansion factor evokes the predicted behaviour, we see an increase of the enhancement with increasing axial magnetic field between 20 mT and 70 mT. (orig.)