Introduction

During the last several decades the ion-atom collisions, occurring at impact velocities substantially exceeding the typical orbiting velocities of outer-shell atomic electrons, have been a subject of extensive research, both experimental and theoretical. The studies of different processes, accompanying such collisions, are of great interest not only for the basic atomic physics research but also have many applications in other fields of physics such as plasma physics, astrophysics and radiation physics.

With the advent of accelerators of relativistic heavy ions much higher impact energies and projectile charge states had become accessible for the explorations in experiments on ion-atom collisions. This, as well as the variety of physical processes governing atomic collisions at high impact energies, have triggered both great interest from and, simultaneously, become a source of substantial challenges for atomic physics theorists.

Three basic atomic physics processes can occur in collisions between a bare projectile-nucleus and a target-atom. (i) The atom can be excited or ionized by the interaction with the projectile. (ii) One or more atomic electrons can be picked up by the projectile-nucleus and form bound or low-lying continuum states of the corresponding projectile-ion. The pick-up process can proceed with or without emission of radiation and is called radiative or nonradiative electron capture, respectively. A combination of (i) and (ii) can also occur. Besides, in relativistic collisions the pair production becomes possible with cross sections reaching quite substantial values in the case of extreme relativistic impact energies when the collision velocity approaches very closely the speed of light in vacuum.