Ionisation of molecular hydrogen by positrons

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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.

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