IS DOUBLY DIFFERENTIAL ELECTRON-EMISSION PRODUCED IN ION ATOM COLLISIONS DOMINANT AT THE SADDLE-POINT VELOCITY

We present evidence that doubly differential distributions of electrons produced by ion-atom ionizing collisions of intermediate energies do not show a concentration in the sense of a dominant emission centred at the velocity of the saddle point of the potential extended between the two Coulomb centres, as given by the projectile ion and the residual target ion. This evidence is sustained by measurements performed with protons of 52 and 103 keV interacting with He. Our electron distributions are represented in terms of the doubly differential cross section given by the number of electrons per unit volume in velocity space, that is not affected by transformation into different reference frames. We do, however, not deny the important role that the two-centre saddle potential plays in the ionization mechanism. This leads to an enhanced forward emission which, however, is distributed in the velocity interval between zero and the projectile velocity.