LOW-ENERGY ELECTRONS IN SLOW ION-ATOM COLLISIONS

We have investigated velocity distributions of electrons ejected in ad iabatic atomic collisions, both experimentally and theoretically, in s earch for saddle point electrons. These electrons reside on/near the t op of the internuclear potential barrier on the outgoing way of the co llision. The so-called theory of hidden crossings is used to treat thi s 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 collis ion energies of 4 and 6 keV/amu, the good agreement between theory and experiment, for both electron velocity distributions and integrated c ross sections, strongly suggests the actual existence of a saddle poin t ionization mechanism. At collision energies below 2 keV/amu the expe rimental data suggest that the recently proposed radial decoupling ion ization mechanism might be effective. Besides the H+-H system we have also studied the He2+-H (theory) and H+-He (experimental) collision sy stems.