The scattering of positronium by lithium

Abstract

The scattering of positronium (Ps) by lithium (Li) is investigated using the static-exchange approximation and a two-state close-coupling approximation (CCA) including exchange. The major effect of very high polarizability of Li, at least 30 times higher than hydrogen and more than four times higher than Ps, has been considered in the present CCA scheme in a dynamic way by using the Li(2p) state of the target Li atom in the basis set. Different scattering parameters, e.g. s-, p- and d-wave phase shifts below the excitation threshold, the partial wave elastic and excitation cross sections for s-, p- and d-waves, the integrated elastic and excitation cross sections, the quenching cross sections, and the ortho-para conversion ratios are presented in the incident energy range from 0.068≤E≤100 eV. The present study reveals that the long-range dipole correlation potential has an appreciable effect in the cases of Ps and Li scattering in the lower energy region 0.068≤E≤30 eV. In the incident energy region above 30 eV, both the results almost coincide with the first Born results. A sharp maximum in the elastic cross sections near 3 eV and three sharp peaks in excitation cross sections near the incident energies of 0.425, 13.0 and 19.0 eV appear in the present results, which are consistent with the minima in the conversion ratios. The p-wave singlet elastic phase shifts calculated by the CCA method are higher by a factor of nearly π/2 than the corresponding static exchange results below 0.439 eV.