THE EFFECT OF POSITRONIUM FORMATION ON CLOSE-COUPLING CALCULATIONS OF POSITRON LITHIUM COLLISIONS

The close-coupling approximation has been used to investigate positronium formation into the n = 1 and n = 2 levels and the excitation of lithium in positron-lithium collisions at impact energies up to 50 eV. The lithium atom is described within a single-electron approximation using a model potential. We have used a target basis set of the form Li(2s, 3s, 2p, 3p), and investigated the effect upon excitation and elastic scattering due to the coupling of positronium formation channels by adding the positronium states Ps(1s, 2s, 2p) to the expansion. The convergence of the cross sections was studied with respect to the addition of a Li(p) pseudostate. The sensitivity of the results to the choice of model potential was also examined. In the impact energy range 2-20 eV, we find that Ps formation significantly reduces the Li(2s --> 2p) excitation cross section, while the elastic scattering cross section is reduced for energies below 3 eV. We obtain Ps(1s) formation cross sections within the coupled-static approximation which are in satisfactory agreement with previous calculations. When coupling to the Li(2p) state is included the cross sections for both direct scattering and Ps formation become significantly different to previous close-coupled cross sections for impact energies up to 40 eV.