[en] Recombination of Ar-like Sc³⁺ ions has been studied experimentally at the TSR for center of mass energies ranging from approximately 12-18 eV. In this energy range rather broad resonances from short lived Sc²⁺(3s²3p⁵3d²²F) intermediate states are expected. For these resonances recent theoretical work predicts strongly asymmetric lineshapes due to quantum mechanical interferences between radiative and dielectronic recombination channels. The study of this heavy, low-charged ion suffered from a high background due to electron capture and the outcome of our experiment does not yet allow for a conclusive test of the theoretical predictions

[en] Some recent new developments in dielectronic recombination (DR) measurements at heavy-ion storage rings are presented. A DR spectrum measured at CRYRING in Stockholm of F⁶⁺ is used to illustrate a few points regarding this kind of measurements. From the spectrum a transversal temperature of the adiabatically expanded electron beam of 3.0(5) meV is deduced, which is slightly higher than expected

[en] In search for interference between radiative and dielectronic recombination (RR and DR) absolute recombination rate coefficients for Ar-like Sc³⁺ and Ti⁴⁺ ions have been measured at the Heidelberg heavy ion storage ring TSR. Whereas the Sc³⁺ experiment suffers from statistics too poor for a line shape analysis, the Ti⁴⁺ recombination spectrum clearly does not exhibit asymmetric lineshapes due to quantum mechanical interferences as recently predicted theoretically for Sc³⁺ ions. The broad Ti³⁺(3s²3p⁵3d²²F) DR resonance expected on the basis of multi-configuration Hartree-Fock calculations at 3.0 eV with a width of 1.3 eV appears to be shifted towards zero center-of-mass (c. m.) energy, such that its width is larger than its distance to threshold. At zero c. m. energy an unexplained recombination rate enhancement of a factor of 2 beyond the sum of RR and DR rates is observed. (orig.)

[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] 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.)

[en] The enhancement of dielectronic recombination of multiply charged ions by external electric fields has been studied under controlled conditions. The heavy ion storage rings CRYRING at Stockholm University and the heavy ion storage ring TSR at the Max-Planck-Institute in Heidelberg were used to store beams of Si¹¹⁺ (ion energy 280 MeV) and Cl¹⁴⁺ (ion energies 250 MeV and 110 MeV), respectively. Recombination in the electron cooler has been measured over energy ranges covering all resonances due to 2s → 2p core excitation. External electric fields up to 183 V/cm at CRYRING and up to 379 V/cm at the TSR have been applied. In the TSR experiment the influence of different magnetic guiding fields as well as the influence of different electron densities in the electron cooler on the recombination rate was investigated. A significant rate enhancement was found for 1s² 2p nl Rydberg resonances with n > 25 for Si¹¹⁺ ions and n > 20 for Cl¹⁴⁺ ions. (orig.)

[en] We report the first experimental observation of magnetic field effects on dielectronic recombination (DR) via highly excited Rydberg levels. Crossed static electric and magnetic fields E_y and B_z were imposed on the collision region in high resolution DR measurements with Li-like Cl¹⁴⁺ ions at the heavy ion storage ring TSR in Heidelberg. Enhancement of DR rate coefficients α for the group of high Rydberg states attached to the 2p_1/2 and 2p_3/2 series limits was observed when motional electric fields E_y up to 380thinspthinspV/cm were introduced. The associated enhancement rate dα/dE_y which we found to be constant at least for E_y≤100 V/cm decreased by almost a factor of 2 when the longitudinal field B_z was increased from 20 to 69thinspthinspmT. copyright 1999 The American Physical Society