Double ionization of noble gases by positron impact

Abstract

The total double ionization cross sections for helium and neon by positron impact have been measured at energies from threshold to 900 eV. The cross sections exhibit a surprising absence or suppression of positronium (Ps) formation in the near-threshold region. This is a remarkable difference to what is seen in single ionization and in contrast to what has previously been suggested by other experiments. The absolute cross sections for the two targets are compared to a modified Rost-Pattard parametrization. At threshold this model incorporates Wannier theory for double ionization, while it at high energies uses an E−1.5 dependence. Good agreement is found, showing that this type of model can be extended from single to double ionization.