The K shell excitation of H-like uranium (U(91+)) in relativistic collisions with different gaseous targets has been studied at the experimental storage ring at GSI Darmstadt. By performing measurements with different targets as well as with different collision energies, we were able to observe for the first time the effect of electron-impact excitation (EIE) process in the heaviest hydrogenlike ion. The large fine-structure splitting in H-like uranium allowed us to unambiguously resolve excitation into different L shell levels. State-of-the-art calculations performed within the relativistic framework which include excitation mechanisms due to both protons (nucleus) and electrons are in good agreement with the experimental findings. Moreover, our experimental data clearly demonstrate the importance of including the generalized Breit interaction in the treatment of the EIE process.
We present calculated and experimentally derived electron-ion recombination rate coefficients for Na-like Si IV, recombining into Mg-like Si III, and provide accurate spectroscopic data for doubly excited states located above the ionization threshold of Si III. The experimental recombination rate coefficients were measured in a merged-beam-type experiment at the heavy-ion storage ring CRYRING at the Manne Siegbahn Laboratory in Stockholm. Changing the electron-ion relative energy from 0 to 20 eV we covered the energy region from the first to the third ionization threshold. We find that even for the low-charged Si 2+ ion, a relativistic many-body perturbation theory calculation is necessary, to describe the recombination rate coefficients in the low-energy region, up to 1.5 eV, satisfactorily. Doubly excited states, forbidden to form in LS coupling, are responsible for the most prominent dielectronic recombination resonances at low energies and contribute with 40% to the strength. Several wide resonances give rise to a plateau-like formation in the recombination spectrum. A broader energy range, up to 6.7 eV, was covered with a non-relativistic many-body calculation. This range contains, in addition to 3pn resonances, several resonances of the type 3dn , with the LS-forbidden 3d 2 3 F states giving rise to a strong, isolated peak at 2.976 eV. The NIST database lists eleven doubly excited states of Si III with energy positions deviating considerably from our determination. Since the listed lines are also not fully matching those with the largest fluorescence yields it must be concluded that they are misidentified.
Aims. Absolute, total recombination rate coefficients for Si iv were determined using the CRYRING heavy-ion storage ring.Calculated rate coefficients were used to estimate recombination into states that could not be detected in the experiment because of field ionization. Total, as well as separate, radiative and dielectronic plasma recombination rate coefficients were determined. Methods. Stored ions were merged with an expanded electron beam in the electron cooler section of the storage ring. Recombined ions were separated from the stored ion beam in the first dipole magnet after the electron cooler and were detected with unity efficiency. The absolute radiative and dielectronic recombination rate coefficients were obtained over a center-of-mass energy range of 0−20 eV, covering ∆ n = 0 core excitations up to the 3s → 3d series limit. The results of an intermediate coupling autostructure calculation were compared with the experiment. The theoretical results were also used to estimate the contribution to dielectronic recombination by high Rydberg states, which were not detected because of field ionization. The spectra were convoluted with Maxwell-Boltzmann energy distributions in the 10 3 −10 6 K temperature range. Results. The resulting plasma recombination rate coefficients are presented and compared with theoretical results frequently used for plasma modeling. In the 10 3 −10 4 K range, a significant underestimation of the calculated dielectronic recombination plasma rate coefficients was observed. Above 3 × 10 4 K, the agreement between our dielectronic recombination plasma rate coefficients and two of the previously published rate coefficients is better than 20%. Conclusions. The observed differences between the experimental and calculated recombination rate coefficients at low temperatures reflect the need for benchmarking experiments. Our experimentally-derived rate coefficients can guide the development of better theoretical models and lead to more accurately-calculated rate coefficients.
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