We consider the positron-impact ionization (breakup) of atomic hydrogen utilizing the full and $S$-wave model calculations, concentrating on the near-threshold energy region. Unlike the corresponding electron-impact case, the $S$-wave model does support the Wannier-like threshold law predicted by Ihra et al. [Phys. Rev. Lett. 78, 4027 (1997)]. It is found that convergent $S$-wave model cross sections are obtained only if complete expansions are utilized on both the atomic and the positronium centers. Furthermore, we suggest that, in the model and full calculations, the separate contributions to the breakup cross section from both centers become equal at threshold.

• Received 14 March 2007

DOI:https://doi.org/10.1103/PhysRevLett.98.263202