Electron capture in collisions between bare heavy ions at ultrarelativistic 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+ + U92+, U92+ + H+ and U92+ + U92+ systems are presented. Saturation effects are predicted for ultra-high collisions velocities. It is shown that at sufficiently high impact energy the mechanism studied here can dominate the K-shell-K-shell electron capture process. (author)