Recombination of Au25+ with free electrons at low energies

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. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dielectronic Recombination of Very Heavy Lithiumlike Ions

Hyperfine Interactions - An overview of measurements of dielectronic recombination (DR) with the heaviest lithiumlike ions is presented. The experiments have been carried out at the heavy ion...     

Recombination of U89+ ions with free electrons at the ESR

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. This revised version was published online in July 2006 with corrections to the Cover Date.     

Precise determination of the 2s(1/2)-2p(1/2) splitting in very heavy lithiumlike ions utilizing dielectronic recombination

The 2s(1/2)-2p(1/2) energy splittings DeltaE(L) of the lithiumlike ions 19779Au76+, 20882Pb79+, and 23892U89+ have been measured at the Experimental Storage Ring, utilizing low energy dielectronic recombination. The resonance energies in total 41 different 1s(2) 2p(1/2)nl(j(')) (n > or =20) autoionizing Rydberg states populated in the dielectronic capture process have been determined. The 2s(1/2)-->2p(1/2) excitation energies have been obtained by extrapolation of these resonance energies to the associated series limits n--> infinity. The combined analysis of the experimental data for all three ions yields DeltaE(L)=216.134(96) eV for Au76+, 230.650(81) eV for Pb79+, and 280.516(99) eV for U89+.     

Recombination of U 92 + ions with electrons

Recombination of fully stripped U⁹²⁺ ions with electrons has been investigated at the Experimental Storage Ring (ESR) in Darmstadt. Absolute recombination rate coefficients have been measured for relative energies from 0 to 33 eV. For energies greater than 20 meV the experimental result is well described by the theory for radiative recombination (RR). Below 20 meV the experimental rate increasingly exceeds the RR calculation as observed previously in the recombination of light bare ions as well as of Bi⁸³⁺. This low-energy rate enhancement is shown to scale as Z^2.6 for bare ions, where Z is the atomic number of the ion. The U⁹²⁺ recombination rate enhancement is insensitive to changes of the electron density. Variation of the magnetic guiding field strength from 80 mT to 120 mT resulted in oscillations of the recombination rate at 0 eV. The oscillations are partly attributed to changes of the transverse electron temperature accompanying the change of the magnetic guiding field strength; partly they may be caused by uncompensated small changes of the interaction angle between the two beams. Received 1st March 2001 and Received in final form 20 April 2001     

Recent dielectronic recombination experiments

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. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dielectronic recombination of lithium-like gold: Towards QED tests

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 [1], is based on the extrapolation of a multitude of measured resonances Au⁷⁵⁺ (1s²2p_1/2 nl_j) up to the series limit (n = ∞). Furthermore experimental data for the Au⁷⁵⁺ (1s²2p_3/26l_j) resonance manifold are presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

High Rydberg resonances in dielectronic recombination of pb(79+)

Dielectronic recombination resonances of Pb (79+) associated with 2s(1/2)-->2p(1/2) excitations were measured at the heavy-ion storage ring ESR at GSI. The fine structure of the energetically lowest resonance manifold Pb (78+)(1s(2)2p(1/2)20l(j)) at around 18 eV could partially be resolved, and rate coefficients on an absolute scale were obtained. A comparison of the experimental data with results of a fully relativistic theoretical approach shows that high-angular-momentum components up to j=31/2 significantly contribute to the total resonance strength demonstrating the necessity to revise the widespread notion of negligible high-angular-momentum contributions at least for very highly charged ions.     

Influence of magnetic fields on electron-Ion recombination at very low energies

Radiative recombination (inverse photoionization) is believed to be well understood since the beginning of quantum mechanics. Still, modern experiments consistently reveal excess recombination rates at very low electron-ion center-of-mass energies. In a detailed study on recombination of F6+ and C6+ ions with magnetically guided electrons we explored the yet unexplained rate enhancement, its dependence on the magnetic field B, the electron density n(e), and the beam temperatures T( perpendicular) and T( ||). The excess scales as T(-1/2)( perpendicular) and, surprisingly, as T(-1/2)( ||), increases strongly with B, and is insensitive to n(e). This puts strong constraints on explanations of the enhancement.