Electron-electron interaction studied in strong central fields by resonant transfer and excitation with H-like U ions

Electron-electron interaction is studied in the strongest possible atomic fields (Zα⇒1) in the presence of only two electrons. A quasifree electron from a hydrogen gas target is resonantly captured into an L_j subshell of a fast H-like U⁹¹⁺ ion by simultaneous excitation of the strongly bound K electron also into an L_j^′ subshell of the projectile, with j and j^′ the total angular momenta of 1/2 or 3/2 for the electron of concern. This resonant transfer and excitation process (RTE) KL_jL_j^′ is mediated by electron-electron interaction. It is equivalent to dielectronic recombination in ion-electron collisions and leads to a doubly excited He-like U^90+** ion, which stabilizes—almost exclusively—via the emission of two successive K x rays, first a K hypersatellite (Kα_i-H) and then a K satellite (Kα_i^′-S) transition. The K x-ray emission characteristics associated with one-electron capture in collisions of U⁹¹⁺ ions with a hydrogen target are studied for the three resonance groups of the KL_jL_j^′ RTE and one off-resonance energy, i.e., in the energy range between 100 and 135 MeV/u. The total cross section for the first resonance group KL_1/2L_1/2 confirms the importance of the Breit contribution to the interaction. The angular distribution for the Kα₂-H transition (j=1/2) is isotropic in the projectile system, whereas the Kα₁-H transition (j=3/2) indicates a strong alignment for the 3/2 electrons in the doubly excited states for the second resonance group KL_1/2L_3/2. The experimental results are in agreement with fully relativistic calculations including the generalized Breit interaction.