Experimental evidence of the asymmetry of the soft electron peak in ion-atom ionization

We have measured the doubly differential energy and angular distribution of low-energy electrons emitted in collisions of H⁺ and ³He²⁺ on Ne at 106 keV/u. In this way, we are able to obtain information about the shape of the soft electron ionization peak. Against current belief, but in accordance with a two-Coulomb-center interaction of the emitted electron, we find it to be strongly asymmetric in the forward-backward direction. We discuss the shape of the cross section and introduce a parametric expression for fitting the data.