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. The authors have used a target basis set of the for 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.

The close-coupling approximation has been used to investigate positronium formation into the n=1 and n=2 levels and the excitation of the target in positron-sodium and positron-potassium collisions at impact energies up to 60 eV. The interaction between the positron and the alkali target atom is represented by a model potential obtained using a single-electron approximation. The 3s, 3p, 4s and 4p states of sodium and the 4s, 4p, 5s and 5p states of potassium have been included. The effect on elastic scattering and excitation due to the coupling to positronium formation channels has been studied by adding the positronium 1s, 2s and 2p eigenstates to the expansion. For impact energies between 2 and 20 eV positronium formation significantly reduces the Na(3s to 3p) excitation cross section, while the elastic scattering cross section increases. For potassium the positronium formation reduces the K(4s to 4p) cross section at all energies below 30 eV. A reduction is also seen in the elastic cross section for energies below 7 eV. The calculated total integrated cross sections are in excellent agreement with the experimental data of Kwan et al.(1989, 1991) in the energy range 5-60 eV provided allowance is made for the incomplete elastic discrimination in the experiment.

The close-coupling approximation (CCA) has been used to investigate positronium formation into the n = 1 and n = 2 levels for the positron-hydrogen system. The solution has been obtained by solving integral Lippmann-Schwinger equations in momentum space. We have used two basis sets (H( Is, 2s, 2p) + Ps( Is, 2s, 2p) and H( Is, 2s, 26) + Ps(ls, 2s, 2p)), which make some allowance for short-and long-range correlation effects in both the incident and positronium channels. These are the first close-coupled results for Ps(n = 2) formation. The effect of adding polarization in the positronium channel is found to be significant for Ps formation. A new feature of the present calculations is the use of Gaussian basis functions in the evaluation of the rearrangement matrix elements.

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The close-coupling approximation has been used to investigate positronium formation into the n=1 and n=2 levels and helium excitation (11S-21S, 21P) in positron-helium collisions at energies between 31.3 eV and 200 eV. The authors have used a target basis set of the form He(1s, 2s, ks, 2p, kp) and investigated the effect upon excitation due to the addition of the positronium basis set Ps(1s, 2s, 2p). An independent electron model has been combined with a model potential to represent the helium atom. Positronium formation reduces the 21S excitation below 50 eV but the 21P excitation is significantly reduced for all energies up to 150 eV. The present excitation results are in closer agreement with previous close-coupled calculations than with other theoretical methods. The He(n=2) excitation total is in agreement with experimental data below 36 eV. The calculated total positronium formation cross section is in agreement with experiment below 100 eV.