Low energy electrons in slow ion-atom collisions

Abstract We have investigated velocity distributions of electrons ejected in adiabatic atomic collisions, both experimentally and theoretically, in search for saddle point electrons. These electrons reside on/near the top of the internuclear potential barrier on the outgoing way of the collision. The so-called theory of hidden crossings is used to treat this unusual and seemingly unstable ionization mechanism. We will discuss the experimental set-up, in particular our newly designed “magnetic time-of-flight electron spectrometer”. For the H + H system, at collision energies of 4 and 6 keV/amu, the good agreement between theory and experiment, for both electron velocity distributions and integrated cross sections, strongly suggests the actual existence of a saddle point ionization mechanism. At collision energies below 2 keV/amu the experimental data suggest that the recently proposed radial decoupling ionization mechanism might be effective. Besides the H + H system we have also studied the He 2+ H (theory) and H + He (experimental) collision systems.