Saddle point electrons in slow ion‐atom collisions

Ionization in atomic collisions at adiabatic impact energies is discussed. Special attention is given to the saddle point ionization mechanism. This mechanism can be described by a quantum mechanical theory based on the concept of hidden crossings. The fundamental H+‐H collision system is studied as a test case. For 1–6 keV collisions both the experimental velocity distributions and the relative total ionization cross sections of this system can be explained by the theory in a consistent way. At collision energies of 4 keV and higher there is strong evidence for the existence of a saddle point ionization mechanism. Saddle point effects in the He2+‐H (theory) and H+‐He (experiment) collision systems are briefly commented upon.