Radiative double electron capture in collisions of O8+ ions with carbon.

Multielectron capture processes observed in low energy collisions of bare ions with target atoms open insight into electron correlations in electromagnetic fields. Radiative double electron capture (RDEC) provides the simplest tool for investigation of such processes. Here, the experimental observation of the RDEC process in collisions of O8+ ions with thin carbon films is presented and the RDEC cross section value obtained is compared with recent theoretical calculations.

1s lamb shift in hydrogenlike uranium measured on cooled, decelerated ion beams.

The Lyman- alpha transitions of hydrogenlike uranium associated with electron capture were measured in collisions of stored bare U (92+) ions with gaseous targets at the storage ring ESR. By applying the deceleration technique, the experiment was performed at slow collision energies in order to reduce the uncertainties associated with Doppler corrections. From the measured centroid energies, a ground state Lamb shift of 468 eV+/-13 eV is deduced which gives the most precise test of quantum electrodynamics for a single electron system in the strong field regime. In particular, the technique applied paves the way towards the 1 eV precision regime.

Near-threshold photoionization of hydrogenlike uranium studied in ion-atom collisions via the time-reversed process.

Radiative electron capture, the time-reversed photoionization process occurring in ion-atom collisions, provides presently the only access to photoionization studies for very highly charged ions. By applying the deceleration mode of the ESR storage ring, we studied this process in low-energy collisions of bare uranium ions with low- Z target atoms. This technique allows us to extend the current information about photoionization to much lower energies than those accessible for neutral heavy elements in the direct reaction channel. The results prove that for high- Z systems, higher-order multipole contributions and magnetic corrections persist even at energies close to the threshold.

Strong evidence for enhanced multiple electron capture from surfaces in 46 MeV/u Pb81+ collisions with thin carbon foils.

Strong evidence has been found for enhanced multiple electron capture into 46 MeV/u Pb81+ with a significant contribution from the entrance surface of thin carbon foils. Capture of up to five electrons has been observed. The multiple electron capture yield is found to increase with decreasing target thickness for thin targets. A simple model describing the data and showing the importance of capture from surfaces is discussed. Further evidence is found for a pronounced asymmetry between electron capture at the entrance and the exit surfaces. Absolute yields for multiple electron capture and projectile ionization are presented. The experimental total cross sections for single capture and ionization agree well with theory.

Electron-electron interaction in strong electromagnetic fields: the two-electron contribution to the ground-state energy in He-like uranium.

Radiative recombination transitions into the ground state of cooled bare and hydrogenlike uranium ions were measured at the storage ring ESR. By comparing the corresponding x-ray centroid energies, this technique allows for a direct measurement of the electron-electron contribution to the ionization potential in the heaviest He-like ions. For the two-electron contribution to the ionization potential of He-like uranium we obtain a value of 2248+/-9 eV. This represents the most accurate determination of two-electron effects in the domain of high-Z He-like ions, and the accuracy reaches already the size of the specific two-electron radiative QED corrections.