S. Kieslich

Experimental Mg IX photorecombination rate coefficient

The rate coefficient for radiative and dielectronic recombination of beryllium-like magnesium ions was measured with high resolution at the Heidelberg heavy-ion storage ring TSR. In the electron-ion collision energy range 0-207 eV reso- nances due to 2s → 2p (∆N = 0) and 2s → 3l (∆N = 1) core excitations were detected. At low energies below 0.15 eV the recombination rate coefficient is dominated by strong 1s 2 (2s 2p 3 P) 7l resonances with the strongest one occuring at an energy of only 21 meV. These resonances decisively influence the Mg  recombination rate coefficient in a low temperature plasma. The experimentally derived Mg  dielectronic recombination rate coefficient (±15% systematical uncertainty) is compared with the recommendation by Mazzotta et al. (1998, A&AS, 133, 403) and the recent calculations by Gu (2003, ApJ, 590, 1131) and by Colgan et al. (2003, A&A, 412, 597). These results deviate from the experimental rate coefficient by 130%, 82% and 25%, respectively, at the temperature where the fractional abundance of Mg  is expected to peak in a photoionized plasma. At this temperature a theoretical uncertainty in the 1s 2 (2s 2p 3 P) 7l resonance positions of only 100 meV would translate into an uncertainty of the plasma rate coefficient of almost a factor 3. This finding emphasizes that an accurate theoretical calculation of the Mg  recombination rate coefficient from first principles is challenging.

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.

Dielectronic recombination of lithiumlike Xe51+ ions: Storage ring experiment and theoretical calculations

Absolute rate coefficients for dielectronic recombination (DR) of Li-like 136Xe51+ have been measured at the heavy-ion storage ring ESR. The experimental results are compared with relativistic distorted-wave calculations employing the multiconfiguration Dirac-Fock method. Based on the DR measurements the 2s-2p1/2 and 2s-2p3/2 excitation energies in Li-like Xe51+ were determined with a relative accuracy of ~400 ppm.

Dielectronic recombination of berylliumlike Xe50+ ions: Measurement and theoretical calculations

Absolute rate coefficients for dielectronic recombination (DR) of Be-like 136Xe50+ have been measured at the heavy-ion storage ring ESR. The experimental results are compared with relativistic distorted-wave calculations employing the multiconfiguration Dirac-Fock method. Based on the DR measurements, multiple intra-L-shell excitation energies were determined.

Dielectronic recombination of metastable berylliumlike xenon ions

Absolute rate coefficients for dielectronic recombination of Be-like Xe50+ have been measured by employing the electron-ion merged beam technique at GSIs heavy-ion storage ring ESR in the center-of-mass energy range 0 – 600 eV. In addition to ground state ions, resonances from metastable Xe50+ (2s2p3P0) parent ions have been observed. These may be exploited in future measurements of the E1M1 two photon 2s2p3P0 → 2s2 1S0 transition rate.

Trielectronic recombination in Be-like ions

Recombination between Be-like ions and free electrons was measured using the merged-beams technique at the TSR storage ring. Trielectronic recombination via triply excited intermediate states was found to contribute significantly to the recombination spectrum. The strength of this process relies strongly on the absence of continua of singly-excited core states. Calculations corroborate the experimental findings.

Dielectronic recombination at low energies: Spectroscopy of doubly excited states in beryllium-like Na7+ ions

Abstract Absolute rate coefficients for the dielectronic recombination (DR) of Na 8+ ions producing doubly excited states in beryllium-like Na 7+ have been measured at the Heidelberg heavy-ion test storage ring (TSR). The investigated electron energy range covers all DR resonances related to 2s→2p 1/2 and 2s→2p 3/2 Δ n =0 core excitations. Resonances related to Δ n =1 and Δ n =2 transitions of the valence electron are found at relative energies between 50 and 250 eV. At very low energies the resonance group associated with Na 7+ (1s 2 2p j 7l j ′ ) intermediate states has been investigated with an energy resolution and precision that will challenge state-of-the-art theory.