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

Abstract

Electron-electron interaction is studied in the strongest possible atomic fields $(Z\ensuremath{\alpha}\ensuremath{\Rightarrow}1)$ 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 ${\mathrm{U}}^{91+}$ ion by simultaneous excitation of the strongly bound $K$ electron also into an ${L}_{{j}^{\ensuremath{'}}}$ subshell of the projectile, with $j$ and ${j}^{\ensuremath{'}}$ the total angular momenta of 1/2 or 3/2 for the electron of concern. This resonant transfer and excitation process (RTE) ${\mathrm{KL}}_{j}{L}_{{j}^{\ensuremath{'}}}$ is mediated by electron-electron interaction. It is equivalent to dielectronic recombination in ion-electron collisions and leads to a doubly excited He-like ${\mathrm{U}}^{90+**}$ ion, which stabilizes---almost exclusively---via the emission of two successive $K$ x rays, first a $K$ hypersatellite $(K{\ensuremath{\alpha}}_{i}\ensuremath{-}H)$ and then a $K$ satellite $(K{\ensuremath{\alpha}}_{{i}^{\ensuremath{'}}}\ensuremath{-}S)$ transition. The $K$ x-ray emission characteristics associated with one-electron capture in collisions of ${\mathrm{U}}^{91+}$ ions with a hydrogen target are studied for the three resonance groups of the ${\mathrm{KL}}_{j}{L}_{{j}^{\ensuremath{'}}}$ 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 ${\mathrm{KL}}_{1/2}{L}_{1/2}$ confirms the importance of the Breit contribution to the interaction. The angular distribution for the $K{\ensuremath{\alpha}}_{2}\ensuremath{-}H$ transition $(j=1/2)$ is isotropic in the projectile system, whereas the $K{\ensuremath{\alpha}}_{1}\ensuremath{-}H$ transition $(j=3/2)$ indicates a strong alignment for the 3/2 electrons in the doubly excited states for the second resonance group ${\mathrm{KL}}_{1/2}{L}_{3/2}.$ The experimental results are in agreement with fully relativistic calculations including the generalized Breit interaction.