Abstract
Experimental cross sections for single and double ionisation of helium by 0.13-15 MeV AMU-1 ions of charge 1-8 are presented. Where necessary, coincidence techniques were applied to distinguish between pure ionisation and ionisation involving electron capture. For high ion velocities, double ionisation takes place either through a shake-off or through a two-step process. The perturbative shake-off mechanism dominates only for the lowest ion charges and projectile energies above 10 MeV AMU-1. A reliable semi-empirical expression which determines the ratio between the double and single ionisation cross sections over a wide region of ion charge and velocity is given. The difference between double ionisation by equi-velocity electrons and positive ions is discussed. It is concluded that no reliable theoretical description of double ionisation exists for combinations of ion charge and ion velocity which give cross sections close to the maximum values.
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