Orthopositronium-lithium scattering using a close-coupling approximation at low and medium energies

Abstract

The scattering of orthopositronium off atomic lithium (Li) target has been investigated using the close-coupling approximation (CCA) with different basis sets. The predictions of different projectile-elastic and target-elastic CCA models have been presented along with static-exchange results. We report the scattering length, the effective range, and the zero-energy cross section for singlet and triplet scattering. The spin averaged zero-energy cross section is significantly improved over the static-exchange prediction by the inclusion of the virtual Ps excitation channels in the basis set. All the present models indicate a binding of lithium-positride (LiPs) in the attractive singlet channel. The low-energy (below first Li excitation threshold) and low order (up to $l=2)$ phase shifts are also reported and are found to be variationally consistent. Except the s-wave singlet scattering, other phase shifts are influenced more by the target distortion effect than the projectile inelastic effect. Estimates of the different single-atom excitation cross sections have also been reported along with an estimate of the total cross section. The peak in the total cross-section curve for the present system is not prominent at energies around the Ps inelastic threshold, unlike the Ps-Na case. For incident energy 20 eV or more the Ps ionization is the dominant contributor to the total cross section.