Positron scattering by lithium

Positron scattering by lithium in the energy range 0.5-60 eV is studied within a frozen-core model of the atomic target using six different levels of approximation: (i) Li(2s, 2p); (ii) Li(ls. 2p, 3s. 3p, 3d); (iii) Ps(1s, 2s, 2p) + Li(2s, 2p); (iv) Ps(1s, 2s, 2p) + Li(2s, 2p, 3s, 3p, 3d); (v) Ps(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d) + Li(2s, 2p, 3s, 3p, 3d); (vi) Ps(1s, 2s, 2p) + Li(2s, 3s, 4s-9s, 2p, 3p, 4p-9p, 3d, 4d-9d, 4f-9f); where a bar denotes a pseudostate. The results demonstrate the importance of including the positronium formation channel in the approximation scheme. The best approximation, (vi), which is modelled upon the highly successful treatment of Kernoghan et al for positron scattering by atomic hydrogen, and which uses pseudostates only on the atom centre, is believed to give the main cross sections for positron scattering off ground state lithium to a very good degree of accuracy. This confidence stems from the corroboration provided by the 'complementary' approximation (v) which employs pseudostates only on the positronium centre. A new prediction from (vi) is the ionization cross section. This turns out to be relatively small, as had been assumed in earlier calculations. The idea that positron scattering by lithium might be treatable in a simple eigenstate approximation including the 2s and 2p states of lithium and a few positronium eigenstates seems to be largely correct for elastic scattering, 2s --> 2p excitation of the atom and total scattering. For these cross sections the simple eigenstate approximations (iii) and (iv) agree with the best results (vi) to better than 20%. However, agreement on total positronium formation is not quite so good, the eigenstate approximations (iii) and (iv) predicting a cross section which is generally larger than that given by (vi). It is found that at low energies the total cross section is dominated by elastic scattering, this dominance being directly taken over by 2s --> 2p excitation with increasing impact energy. Positronium formation is significant at energies less than 20 eV, although it never exceeds elastic scattering in the energy range studied.