SCATTERING OF LOW-ENERGY POSITRONS BY LITHIUM

Coupled-state calculations are reported for s-, p- and d-wave scatteri ng by lithium of positrons with momentum k less than or equal to 0.45 atomic units (au). Comparison is made with the accurate variational re sults of Watts and Humberston for s and p waves. Very good convergence towards the variational numbers is found. In the most sophisticated a pproximation, Ps(1s, 2s, ($$$) over bar 3s, ($$$) over bar 4s, 2p, ($$ $) over bar 3p, ($$$) over bar 4p, ($$$) over bar 3d, ($$$) over bar 4 d) + Li(2s, 2p, 3s, 3p. 3d), where a bar denotes a pseudostate, resona nces are seen in the s- and d-wave cross sections near k = 0.30 au and k = 0.27 au respectively. Experiments in the range k less than or sim ilar to 0.45 au (energies less than or similar to 2.75 eV) are strongl y urged.