State-Selective Electron Capture Studied for U90+ Ions in Collisions with Gaseous Targets

In relativistic collisions involving high-Z projectiles and heavy target atoms, non-radiative electron capture (NRC) is a very important charge exchange process [1], a process mediated by three-body interaction. In general, a precise theoretical treatment of this process is very difficult to achieve since the Coulomb field of the fast moving projectile leads to distortions of the atomic target wave functions even at infinite distances. For high-Z ions and relativistic energies the experimental information about NRC is restricted to total cross sections which are quite insensitive to the details of this process. An experiment aiming at a study of state-selective electron capture was conducted at the gas-jet area of the ESR, by utilizing the spectroscopy of projectile x-ray transitions following electron capture into excited projectile states. This method benefits in particular from the large fine structure splitting present in such heavy ion systems. In the experiment, the Balmer radiation produced by electron capture in collisions of 223 MeV/u U ions on N2, Ar, Kr, and Xe targets have been measured in coincidence with the down-charged uranium ions (here Balmer transition refers to the transitions from higher levels to n=2 states). The recorded x-ray spectra are shown in figure 1. As the target varies from light to heavy atoms, the relative intensities of the Balmer lines exhibit a significant change. This feature already indicates a strong influence of the target charge on the relative population of the various (n, l, j) projectile sublevels by electron capture.