H. Knopp

Radiative recombination of bare Bi 83+ : Experiment versus theory

Electron-ion recombination of completely stripped Bi83+ was investigated at the Experimental Storage Ring (ESR) of the GSI in Darmstadt. It was the first experiment of this kind with a bare ion heavier than argon. Absolute recombination rate coefficients have been measured for relative energies between ions and electrons from 0 up to about 125 eV. In the energy range from 15 meV to 125 eV a very good agreement is found between the experimental result and theory for radiative recombination (RR). However, below 15 meV the experimental rate increasingly exceeds the RR calculation and at Erel = 0 eV it is a factor of 5.2 above the expected value. For further investigation of this enhancement phenomenon the electron density in the interaction region was set to 1.6E6/cm3, 3.2E6/cm3 and 4.7E6/cm3. This variation had no significant influence on the recombination rate. An additional variation of the magnetic guiding field of the electrons from 70 mT to 150 mT in steps of 1 mT resulted in periodic oscillations of the rate which are accompanied by considerable changes of the transverse electron temperature.

Electron-impact ionization of Ti3+ ions

Absolute cross sections for electron-impact single ionization of potassium-like titanium have been measured from threshold to 1000 eV using a crossed-beams experimental set-up. A 0.5 eV resolution energy-scan technique revealed structures in the cross section in the range from 40 eV to 150 eV due to dominant contributions of indirect ionization mechanisms. A large fraction of the cross section is due to excitation–autoionization proceeding via intermediate excited states 3p5(3d2 3F) 2F, 3p5(3d2 3P) 2P, 3p5(3d2 3F) 2D which subsequently decay by super-Coster–Kronig transitions. The results of the measurements are in satisfactory agreement with previous advanced theoretical calculations.

Electron-impact single and multiple ionization of Mg+ ions

Absolute cross sections for single, double and triple electron-impact ionization of Mg+ ions have been measured by employing a crossed-beams experimental set-up. Contributions of indirect resonant processes to the total cross section for single ionization such as resonant-excitation–double-autoionization and resonant-excitation–auto-double-ionization have been observed utilizing a high-resolution energy scan technique with statistical uncertainties as low as 0.04%. The experimental cross sections are compared with results of recent R-matrix calculations showing agreement in the overall features but pronounced differences in the details of indirect ionization contributions. Double ionization was found to be dominated by the direct removal of a 2p-electron with subsequent autoionization. There are also indications of indirect processes in the triple ionization of Mg+ but no clear evidence could be established.

Autoionizing high-Rydberg states of very heavy Be-like ions: A tool for precision spectroscopy

Abstract Autoionizing high-Rydberg states formed during the initial electron capture phase of dielectronic recombination (DR) provide a unique access to spectroscopic information of highly charged heavy ions. This paper summarizes recent experimental and theoretical studies on the low energy 1s22pjnlj′ (j=1/2, 3/2) Be-like DR resonances associated with the 2s1/2→2pj core excitations of the Li-like ions 197 79 Au76+, 208 92 Pb79+ and 238 92 U89+. An extrapolation of the resonance energies of the 1s22p1/2nlj′ (n⩾20) Rydberg series to the series limits (n→∞) yields the 2s1/2–2p1/2 transition energies. The experimental precision of below 100 meV for all three ions obtained with this photonfree spectroscopic method is comparable to the best optical measurements available and allows to test quantum electrodynamics of strong central fields on a level of below 7% of the α2 contributions. Furthermore, our theoretical calculations show that the observed DR resonance positions sensitively depend on the charge distribution within the atomic nucleus.

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.

Electron-impact ionization of B3+ ions

The electron-impact total ionization cross sections for B3+ have been determined both experimentally and theoretically. The experiment was carried out using a parent beam with B3+ ions in a mix of ground 11S and metastable 23S states. The convergent close-coupling (CCC) method has been used to calculate the corresponding cross sections and, by matching with experiment, the ratio of the initial target states in the beam has been determined. Good agreement with experiment across a broad energy range is obtained, particularly when indirect ionization processes are included in the calculations.

Electron-impact multiple ionization of Baq+ ions (1⩽q⩽13) via resonant 3d excitation

Abstract One ionization mechanism of barium ions is the resonant excitation of a 3d inner-shell electron with the subsequent decay of the resulting highly excited intermediate state by multiple autoionization processes. We have experimentally studied this process by observing associated resonance structures – in the energy range 700–800 eV – in the multiple (up to sevenfold) ionization cross-sections of Baq+ ions with 1⩽q⩽13. The 3d-resonance strength is determined quantitatively as a function of initial and final ion charge state. The results suggest a statistical description of multiple ionization via the excitation of inner-shell electrons.

Dielectronic Recombination of Very Heavy Lithiumlike Ions

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 storage ring ESR at GSI utilizing the electron cooler as an electron target. In particular, results for low energy dielectronic recombination of Au76+ and U89+ are discussed. In the energy range of 0 to 400 eV which can be covered with our present set-up dielectronic resonances associated with Δn=0 core excitations from 2s1/2→2p1/2,3/2 were found. The experimental results can only be explained within a fully relativistic treatment and are sensitive to QED contributions of the order α2 as well as to the finite size of the atomic nucleus.

Spectroscopy of berylliumlike xenon ions using dielectronic recombination

Be-like ions have been investigated employing the resonant electron–ion collision process of dielectronic recombination (DR) as a spectroscopic tool. The experiments were performed at the experimental storage ring in Darmstadt, Germany, using its electron cooler as a target for free electrons. DR Rydberg resonance series for the associated intra-L-shell transitions and were observed with high resolution. In addition to these excitations from the ground state we determined resonances associated with excitations of ions initially in the metastable state. The corresponding excitation energies were determined to be and and . These excitation energies are compared with previous measurements and with recent state-of-the-art atomic structure calculations.

Towards a measurement of the 2s2p 3P0 ? 2s2 1S0 E1M1 two photon transition rate in Be-like xenon ions

Past and ongoing research activities at the Heidelberg heavy-ion storage-ring TSR are reviewed which aim at providing accurate absolute rate coefficients and cross sections of atomic collision processes for applications in astrophysics and magnetically confined fusion. In particular, dielectronic recombination and electron impact ionization of iron ions are discussed as well as dielectronic recombination of tungsten ions.Dielectronic recombination (DR) of Be-like 136Xe50+ was measured by employing the electron-ion merged-beam technique at the storage ring ESR of GSI in Darmstadt, Germany. We present results covering the center-of-mass energy range 0 − 4 eV. In addition to Rydberg series of resonances associated with ground-state intra L-shell excitations, resonances were found which are formed during DR of initially metastable 136Xe50+(2s2p 3P0) parent ions. This state is the first excited state and is long lived because the J=0 → J=0 transition and the absence of nuclear spin make the rare E1M1 two-photon transition the lowest-order decay channel. Future measurements which will monitor the strength of the resonances as function of storage time may be exploited for a measurement of the E1M1 two photon transition rate.