X-ray spectra following radiative recombination of free electrons with bare uranium ions (U92+) were measured at the electron cooler of the ESR storage ring. The most intense lines observed in the spectra can be attributed to the characteristic Lyman ground-state transitions and to the recombination of free electrons into the K shell of the ions. Our experiment was carried out by utilizing the deceleration technique which leads to a considerable reduction of the uncertainties associated with Doppler corrections. This, in combination with the 0 degree observation geometry, allowed us to determine the ground-state Lamb shift in hydrogenlike uranium (U91+) from the observed x-ray lines with an accuracy of 1%. The present result is about 3 times more precise than the most accurate value available up to now and provides the most stringent test of bound-state quantum electrodynamics for one-electron systems in the strong-field regime.

A method for solving the time-dependent two-center Dirac equation is developed. The time-dependent Dirac wave function is represented as a sum of atomiclike Dirac-Sturm orbitals, localized at the ions. The atomic orbitals are generated by solving numerically the one-center Dirac and Dirac-Sturm equations by means of a finite-difference approach with the Coulomb potential taken as the sum of the exact reference-nucleus potential and of the other nucleus within the monopole approximation. An original procedure for calculating the two-center integrals with these orbitals is proposed. As a first test of the approach developed here, calculations of the charge-transfer and ionization cross sections for the H(1s)-proton collisions at proton energies from 1 to 100 keV are performed. The obtained results are compared with related experimental and other theoretical data. To investigate the role of the relativistic effects, the charge-transfer cross sections in collisions of Ne 9+ (1s)-Ne 10+ (at energies from 0.1 to 10 MeV/u) and U 91+ (1s)-U 92+ (at energies from 6 to 10 MeV/u) are calculated for both relativistic and nonrelativistic cases.

The three-body Coulomb problem has been explored in kinematically complete experiments on single ionization of helium by 100 MeV/u C(6+) and 3.6 MeV/u Au(53+) impact. Low-energy electron emission ( E(e)<150 eV) as a function of the projectile deflection theta(p) (momentum transfer), i.e., the Bethe surface [15], has been mapped with Delta theta(p)+/-25 nanoradian resolution at extremely large perturbations ( 3.6 MeV/u Au(53+)) where single ionization occurs at impact parameters of typically 10 times the He K-shell radius. The experimental data are not in agreement with state-of-the-art continuum distorted wave-eikonal initial state theory.

For radiative electron capture into the K shell of bare uranium ions, a study of the polarization properties has been performed. For this purpose a position sensitive germanium detector has been used as an efficient Compton polarimeter. This enabled us to measure the degree of linear polarization by analyzing Compton scattering inside the detector and to determine the orientation of the polarization plane. Depending on the observation angle and the beam energy used, the radiation is found to be linearly polarized by up to 80%. In all cases studied, the plane of polarization coincides with the collision plane. The results will be discussed in the context of rigorous relativistic calculations, showing that relativistic effects tend to lead to a depolarization of the radiation emitted. DOI: 10.1103/PhysRevLett.97.223202 PACS numbers: 34.80.Lx, 32.30.Rj, 32.80.Cy, 32.80.Fb Radiative capture of free or quasifree electrons (REC) into high-Z ions has been found to provide a wealth of information on the electron-photon interaction in the presence of strong fields [1,2]. Most important, today, REC is an established tool for studying the time-reversed process, the elementary photoeffect, for one-and few-electron ions at high Z which can otherwise not be addressed in experiments [2]. As a consequence, most properties of the REC were the subject of detailed experimental and theoretical studies (see [2 -5] and references therein). However, a very important aspect of linear polarization so far has not been the subject of experimental investigations although the results of the angular-differential studies already indicate that REC may exhibit distinctive polarization features. Even for the time-reversed process and neutral heavy elements only very few experimental data are available [6,7]. There, the results are strongly affected by electron scattering in the target and except at very high photon energies (above 1 MeV) an unambiguous interpretation of the data is hampered by distortion effects caused by the solid targets used [7]. In contrast, these effects are completely absent in REC studies. Moreover, the polarization features of K-REC radiation have recently attracted particular attention because of its predicted sensitivity to a possible spin polarization of the particles involved in the collision (electrons or ions) [8]. As a consequence, the detection of the linear polarization of K-REC might be applied as an important tool for the control and diagnostics of the degree of the spin polarization of heavy ion beams confined in storage rings. Currently, such a technique is much needed for experiments aiming on a detection of parity nonconservation effects in highly charged ions and for the search of the electric dipole moment of heavy nuclei as proposed for a test of the standard model [9][10][11]. In this context, we like to emphasize that REC into the K shell is the most important charge exchange channel for heavy ions in collisions with light target atoms and, therefore, represents an intense source of radiation.In ...