The 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 (Z.alpha => 1) in the presence of only two electrons. A quasi-free electron from a hydrogen gas target is resonantly captured into a L sub j subshell of a fast H-like U sup 9 sup 1 sup + ion by simultaneous excitation of the strongly bound K electron also into a L sub j sub ' 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 capture and excitation process, KL sub j L sub j sub ' -RTE, is mediated by electron-electron interaction. It is equivalent to dielectronic recombination (DR) in ion-electron collisions and leads to a doubly excited He-like U sup 9 sup 0 sup + sup * sup * ion, which stabilizes - almost exclusively - via the emission of two successive K X-rays, first a K hypersatellite (K alpha i-H) and then a K satellite (K alpha i'-S) transition. The K X-ray emission characteristics associated with one-electron capture in collisions of U sup 9 sup 1 sup + ions with a hydrogen target is studied for the three resonance groups of the KL sub j L sub j sub ' -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 sub 1 sub / sub 2 L sub 1 sub / sub 2 confirms the importance of the Breit contribution to the interaction. The angular distribution for the K alpha 2-H transition (j=1/2) is isotropic in the projectile system whereas the K alpha 1-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 sub 1 sub / sub 2 L sub 3 sub / sub 2. The experimental results are in agreement with fully relativistic calculations including the generalized Breit interaction.