Theoretical aspects of ionization in ion-atom collisions

We present theoretical studies of electron emission spectra resulting from ionizing collisions between an ion (or atom) and an atom. Mechanisms for ionization are discussed and analyzed for slow and fast collisions and for two different collision systems. In the first collision system, we consider an exactly solvable three-body model in which an electron moves in the field of two centers of zero range potentials travelling at constant speeds. The exact electron emission spectrum shows two important features: (a) We find evidence for the existence of the so-called "v/2" (or ridge) electrons at intermediate collision speed. These electrons are emitted with about half the speed of the incident ion. It is shown that they are due to promotion to the continuum of the molecular orbitals. v/2 electron emission is strongly influenced by the relative interaction strengths of the electron with the two centers as a function of the the charge asymmetry parameter Z_p/Z_T. (b) For fast collisions multiple scattering peaks can be seen to be a dominant feature in the ionization spectrum. Three-body effects are found to be responsible for generation of the peaks. In the second collision system we study the ionization spectrum at large ejection angles resulting from electron loss by the incident ions (atoms). The ionization spectrum is shown to be composed of two parts: one is caused by the interaction of the projectile electron with the mean field of the target core and the other by explicit electron-electron interaction. Mean field ionization is treated in an impulse approximation. It is shown that proper treatment of the off-energy-shell scattering matrix element is required to describe experimental data. For the correlated ionization we use the concept of double scattering represented by a second Born approximation. We show that this two-step mechanism is essential in describing the electron angular and energy distribution, especially on the low energy side of the spectrum for electron loss from atomic hydrogen. For other incident ions (like He⁺) we find that in addition to double scattering, three-body effects are also very important. They enhance the electron loss cross section and improve significantly the agreement between theory and experiment. This thesis is based in part on the following publications: [18-20, 39, 40].