Scattering of orthopositronium off a helium atom - art. no. 052503

Scattering of orthopositronium off helium target has been investigated usin g close-coupling method in the energy range 0-110 eV. Two basis sets, (a) P s(1s)+He(1s(2), 1s2(1) s, 1s2(1) p) and (b) Ps(1s,2p)+He(1s(2), 1s2(1), 1s2 (1) p), have been employed to find the scattering parameters. Low-order pha se shifts, scattering length, and elastic and excitation cross section up t o n = 2 are reported using close-coupling approximation. Total cross sectio n is also given by adding other partial cross section and compared with ava ilable measured data and existing theoretical predictions. Our total cross section at zero energy is very close to the theoretical prediction of Drach man and Houston [J. Phys B 3, 1657 (1970)] and measured data of Canter et a l. [Phys. Rev. A 12, 375 (1975)]. Present total cross section is in qualita tive agreement with measured data of the University College London group in the energy range considered. In particular, present predictions are in goo d agreement with the UCL group in the energy range 20-30 eV. It has been fo und that elastic cross section is dramatically reduced, at zero or near zer o 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.