Electron–electron interaction in strong central fields studied by resonant transfer and excitation with two-photon processes in U91+ – H2 collisions

Abstract

We studied the angular distribution of the X-ray emission associated with KLL-RTE for H-like U91+ ions colliding with quasi-free electrons from an H2 target. In this resonant transfer and excitation process an intermediate doubly excited He-like U90+** with two electrons in the L-shell is produced by electron–electron interaction; this exotic state decays by the emission of two photons, a Kα-hypersatellite and a Kα-satellite X-ray. According to the different j values (1/2 and 3/2) for the L-shell electrons three different KLL-RTE resonance groups can be distinguished by the corresponding ion energies. At the three resonance energies and one off-resonance energy (116.6, 124.9, 133.1 and 102.0 MeV/u, respectively) X-ray spectra were taken simultaneously at seven different observation angles. The Kα2 satellite lines (j=1/2–j=1/2 transitions) are isotropic, whereas the Kα1 hypersatellite lines (3/2–1/2 transitions) for the second resonance display a pronounced anisotropy pointing to a strong alignment of the involved doubly excited states. Our data agree with full relativistic theory in its general features.