Abstract
We consider double ionization by positron impacts near the break-up threshold. Threshold exponents are evaluated making use of the Newtonian classical method. Our results agree with those due to Kuchiev and Ostrovsky (1998 Phys. Rev. A 58 321–35), obtained by Hamiltonian formalism. We address the question of deducing threshold law in the presence of several guiding configurations, as the case with positron impact arises. General formulae for these degenerate cases are proposed and the issue of the comparison with experimental data near threshold is discussed. Finally, we argue that the disagreement between Wannier's threshold laws and the experimental evidence for the multiple fragmentation functions may be resolved on the basis of the shift from the proper threshold region.
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