IONIZATION OF HYDROGEN BY POSITRON-IMPACT NEAR THE FRAGMENTATION THRESHOLD

The semiclassical approximation of Feynman's path integral is used to calculate the S matrix for the positron-impact ionization of hydrogen. The formulation provides a full scattering amplitude, and more import antly does not require knowledge of the asymptotic three-body Coulomb state in the continuum. In the limit of vanishing excess energy, the r esults confirm Wannier's classical model for fragmentation [Phys. Rev. 90, 817 (1953)]. The experimentally observable ratio of fragmentation versus total ionization (including positronium formation) is predicte d.