Scattering of orthopositronium off a helium atom

Abstract

Scattering of orthopositronium off helium target has been investigated using close-coupling method in the energy range 0--110 eV. Two basis sets, (a) $\mathrm{Ps}(1s)+\mathrm{He}{(1s}^{2},{1s2}^{1}s,{1s2}^{1}p)$ and (b) $\mathrm{Ps}(1s,2p)+\mathrm{He}{(1s}^{2},{1s2}^{1}s,{1s2}^{1}p),$ have been employed to find the scattering parameters. Low-order phase shifts, scattering length, and elastic and excitation cross section up to $n=2$ are reported using close-coupling approximation. Total cross section is also given by adding other partial cross section and compared with available measured data and existing theoretical predictions. Our total cross section at zero energy is very close to the theoretical prediction of Drachman and Houston [J. Phys B 3, 1657 (1970)] and measured data of Canter et al. [Phys. Rev. A 12, 375 (1975)]. Present total cross section is in qualitative agreement with measured data of the University College London group in the energy range considered. In particular, present predictions are in good agreement with the UCL group in the energy range 20--30 eV. It has been found that elastic cross section is dramatically reduced, at zero or near zero energies, with the inclusion of target excitation in the expansion scheme. Moreover, ionization cross section of the Ps atom is found to be a major contributor to the total cross section above the ionization threshold.