We demonstrate that recent experiments for positron impact ionization of $\mathrm{He}$ and ${\mathrm{H}}_2$ can be interpreted by extending Wannier theory to higher energies. Anharmonicities in the expansion of the three-particle potential around the Wannier configuration give rise to corrections in the threshold behavior of the breakup cross section. These corrections are taken into account perturbatively by employing the hidden crossing theory. The resulting threshold law is $\sigma \left(E\right)\propto E^{2.640}\exp [-0.73\mathrm{}\sqrt{E}]$. The actual energy range for which the Wannier law is valid is found to be smaller for positron impact ionization than for electron impact ionization.

• Received 21 January 1997

DOI:https://doi.org/10.1103/PhysRevLett.78.4027