When an atom is ionized by ion impact, the electron is ejected into a final continuum state of a two-centre potential due to the Coulomb fields of the projectile and ionized atom. The related effects on the electron yield or energy and angular distributions are referred to as lwo-cenlre electron emisrion (TCEE). The present report is devoted CO a discussion of experimental and theoretical evidence of this TCEE. The use of heavy ions or antiprotons as projectiles allows t o unravel these effects by monitoring the two centre potential. On the theoretical side, the continuum distorted wave-eikonal initial state (COW-EIS) theory accounts for the TCEE thus allowing a detailed interpretation of the experimental findings.095;-40iSiFl/i43O9i i29S05.50 0 i49i IOP Publishing Lrd 309i

We have c-ed out a generalization of the continuum-distoned-we~ikonal-initialstate (CDW-EJS) approximation for ion-impact single ionization where the interaction of the active e l e m n with the target is represented by a Hxtree-Fock potential We apply this model to the ionization of He, Ne and AI by proton and multiply-charged bare-ion impact. Doubly differential and total cross sections me calculated from each subshell. These cross sections summed over all subshells show a much better agreement with experimental data than those obtained from the previous formulations of the c ~i v a s appmximation which use hydrogenic wavefuncrions with effective charges.

A comprehensive analysis of the stopping power of antiprotons and negative muons in He and gas targets for projectile velocities (equivalent antiproton energies) ranging from about 0.1 to 10 au (0.25 keV to 2.5 MeV) is performed. Recent experimental data are contrasted with theoretical results obtained from different approaches. The electronic stopping power is evaluated within the coupled-state atomic-orbital method and the distorted-wave Born approximation as well as, for low projectile velocities, within a generalized adiabatic-ionization model that takes into account collisional-broadening effects. The departure of the antiproton stopping power from the proton stopping power (`Barkas effect'), observed for intermediate projectile velocities, is discussed. The contribution to the stopping power arising from energy transfer to the translational degrees of freedom of the target system (`nuclear stopping') is evaluated. Our analysis results in a good understanding of the stopping mechanisms of negative heavy particles in gases, in particular in He. Discrepancies between theory and experiment in the case are attributed to effects of the molecular structure of the target.

The continuum-distorted-wave-eikonal-initial-state model is extended to describe single-electron ionisation by impact of bare projectiles on multielectronic targets. Applications are given for collisions between multicharged ions and helium. Double differential, single differential and total cross sections are calculated. Experimental data and present theoretical results show deviations from the square of the projectile charge dependence predicted by the first Born approximation.