POSITRONIUM FORMATION IN POSITRON HELIUM COLLISIONS AT INTERMEDIATE ENERGIES

Positronium formation cross sections in the collision of a positron with a helium atom are calculated in the energy range from 50 to 300 eV. The target continuum distorted-wave (TCDW) amplitude is evaluated exactly within the framework of the single-electron approximation. The present TCDW cross sections are much smaller than the TCDW values of Deb et al., who obtained reasonable agreement with experiments employing some further approximations. We show that no existing theory for particle transfer can explain the high-energy behavior of the measured cross sections and further extensive studies are required both theoretically and experimentally.