Abstract
The way in which atoms and molecules are ionized by the impact of charged particles has important consequences for the behaviour of many physical systems, from gas lasers to astrophysical plasmas. Much of our understanding of this process has come from ionization measurements of the energy and angular distribution of electrons ejected in the same plane as the trajectory of the incident ionizing beam. Such studies suggest that the mechanisms governing the ionization of atoms and molecules are essentially the same. But by measuring the electrons ejected from a gas in a plane perpendicular to the incident beam, we show this is not always the case. Experiments and quantum mechanical calculations enable us to construct a remarkably accurate classical picture of the physics of charged-particle ionization. This model predicts that the differences in ionization behaviour arise in molecules that do not have nuclei at their centres of mass.