Electron capture in collisions between bare heavy ions at ultra relativistic impact energies

Capture into the K shell of the projectile of an electron corresponding to a e - -e + pair created in the target nucleus is studied at ultrarelativistic impact energies. A plane-wave Born approximation is developed. Analytical double differential cross sections as a function of the scattering angle and of the final energy of the positron are obtained. Calculations for the H + +H + , H + +U 92+ , U 92+ +H + and U 92+ +U 92+ systems are presented. Saturation effects are predicted for ultra-high collision velocities. It is shown that at sufficiently high impact energy the mechanism studied here can dominate the K-shell-K-shell electron capture process.