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He(21S, 21P) excitation and positronium formation in positron-helium collisions at intermediate energies

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.