Simultaneous excitation and ionization of He-like uranium ions in relativistic collisions with gaseous targets

The process of simultaneous excitation and ionization is investigated for He-like uranium $\left({\mathrm{U}}^{90+}\right)$ ions colliding with Ar, Kr, and Xe targets at an incident energy of 223.2 MeV/u. The two-electron transitions, where one of the ground-state electrons is promoted into the continuum and the other into the L-subshell states of the projectile, are identified by the coincident observation of U Lyman-series radiation and ${\mathrm{U}}^{91+}$ ions. The experimental cross sections are compared to relativistic calculations based on the independent particle approximation and first-order perturbation theory. It is shown, that simultaneous excitation-ionization occurs preferably at small impact parameters, for which the excitation process is dominated by the monopole part of the interaction potential and the perturbation potential is largest. Good agreement is found between experimental data and calculations for the Ar target. For heavier targets the experimental results are generally smaller than predicted pointing to the invalidity of the first-order perturbation theory in this energy–target atomic number domain.