J. Jacobi

Electron-impact single ionization of Xe10 + ions

Absolute cross sections for electron-impact single ionization of Xe10 + ions have been measured by employing the crossed-beam technique in the electron–ion collision energy range from threshold up to 1000 eV. In addition, fine-step energy-scan measurements of the cross-section function have been performed revealing resonant structures associated with the excitation of an electron from inner electron subshells. For better understanding of the relative importance of direct and indirect ionization contributions to the measured cross section, configuration-averaged distorted-wave calculations have been performed. Significant contributions of indirect processes involving inner-shell excitation with subsequent autoionization could be identified. Furthermore, the calculations reveal that an ≈1% fraction of metastable ions present in the parent Xe10 + ion beam has been analysed.

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.

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.

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.

High-power electron gun for electron-ion crossed-beams experiments

A high-performance electron gun for crossed-beams electron-ion collision experiments is presented. First test measurements demonstrated high electron currents, e.g., 1 A at 3.5 keV electron energy verifying the results of electron optics simulations carried out prior to the mechanical construction of the gun. Presently, the device is implemented into an ultra-high vacuum setup that will allow for its integration into an ion beamline for cross section measurements.

Multiple ionization of Sc+-ions by electron impact

Electron-impact n-fold (n= 2–5) ionization of Sc+ ions has been studied covering an energy range from threshold to 1000 eV. An electron-ion crossed-beams set-up was employed for the measurement of absolute cross sections as well as for high-resolution energy-scans by which fine details in the energy dependence of the cross section could be uncovered. Contributions of indirect resonant and non-resonant processes to the partial ionization cross sections such as excitation or ionization of an inner shell electron with subsequent autoionization processes have been observed.

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.