Ionisation of molecular hydrogen by positrons

Abstract

The authors measure the positron impact ionisation cross section, sigma _ion⁺, and the positronium (Ps) formation cross section, sigma _Ps, from their thresholds up to energies where sigma _ion⁺ merges with the electron impact ionisation cross section, sigma _ion^-, and sigma _Ps is negligible. The maximum values are 1.7*10^-16 cm² for sigma _ion⁺ around 63 eV (as compared with 1.1*10^-16 cm² for sigma _ion^- near 65 eV) and 2.3*10^-16 cm² for sigma _ps, around 20 eV. Above 120 eV the measurements of sigma _Ps are consistent with zero. Above 250 eV sigma _ion⁺ and sigma _ion^- are equal within the experimental errors. At intermediate energies sigma _ion⁺ exceeds sigma _ion^- whereas near threshold sigma _ion⁺< sigma _ion^-, a fact which is qualitatively in accordance with the Wannier theory of threshold behaviour. The authors' data, together with literature values for the positron total scattering cross section, give evidence for a structure in the elastic cross section at the Ps-formation threshold.