Ion–Atom Collisions – High Energy

Abstract

This chapter deals with inelastic processes that occur in collisions between fast, often highly charged, ions and atoms. Fast collisions are here defined to be those for which V ∕ ve ≥ 1, where V is the projectile velocity and ve the orbital velocity of this electron. For processes involving outer-shell target electrons, this implies V ≳ 1 a.u., or the projectile energy $$\gtrsim{\mathrm{25}}\,{\mathrm{ke{\mskip-2.0mu}V/a.m.u}}$$ . For inner-shell electrons, typically, V ≳ Z2 ∕ n a.u., where Z2 is the target nuclear charge and n the principal quantum number of the active electron. A useful relationship is $$V=6.35\sqrt{E/M}$$ , where V is in a.u., E is in Me V, and M is in a.m.u. Fast collisions involving outer-shell processes can be studied using relatively small accelerators, while those involving inner-shell processes require larger van de Graaffs, LINACs (linear accelerators), etc. Because the motion of the inner-shell electrons is dominated by the nuclear Coulomb field of the target, and because transitions involving these electrons take place rather independently of what transpires with the outer-shell electrons, it has proven to be somewhat easier to understand one-electron processes involving inner-shell electrons. Thus, for a long time, a great deal of the work on fast ion–atom collisions concentrated on inner-shell processes involving heavy-target atoms. However, more recently, new experimental techniques have led to a shift of this focus to inelastic processes involving light-target atoms. Furthermore, present investigations go beyond the one-electron picture to include the influence of the electron–electron interaction. The present chapter outlines some of the developments in this area over a very active past few decades. The literature is vast, and only a small sampling of references is given. Emphasis is on experimental results (for the theory, see Part D of this book).