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A number of resonances in the L=0, 1 and 2 channels have been found in close-coupling calculations of positron-alkali collisions. In contrast to the corresponding electron scattering case the s-wave phaseshifts for positrons colliding with Li, Na and K start from a positive multiple of pi rad at zero energy which indicates the existence of true alkali-positron bound states. The widths for the various resonances found in the region of the atomic excitation threshold vary between 0.2 and 130 meV. The inclusion of the electronic 3d orbital is observed to be crucial in order to obtain fully developed resonances below the 'resonant' ns-np threshold.

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Five-state close-coupling calculations are presented for e+-Li (2s-2p-3s-3p-3d), e+-Na (3s-3p-4s-3d-4p) and e+-K (4s-4p-5s-3d-5p) scattering in the energy range 0.5-50 eV. An investigation was made of (i) the convergence of the cross sections with respect to the number of target states, and (ii) the sensitivity of the cross sections to the target wavefunctions employed. A sharp minimum is present in the elastic differential cross section in the vicinity of 40-50' for energies less than 10 eV for all three atoms. Good agreement is obtained between the five-state (effective) total integrated cross sections and experimental results for both e+-Na and e+-K scattering.

We investigate the four-body Coulomb process of low-energy elastic positronium-hydrogen (Ps-H) scattering below the Ps(n=2) excitation threshold using scattering wavefunctions that include Hylleraas-type correlation terms. Using the complex Kohn variational method, we compute phase shifts through the 1,3 H-wave and obtain highly accurate 1,3 Sand 1,3 P-wave phase shifts. The complex Kohn variational results compare well to a number of other calculations for this system. We present elastic differential, elastic integrated, and momentum transfer cross sections, and for the singlet, resonances through the 1 F-wave. The differential cross section exhibits interesting features, including a change from slightly backward peaked to forward peaked scattering as the energy of the incident positronium increases and rich structure due to multiple resonances near the Ps(n=2) threshold. We also give a detailed analysis of the scattering lengths and effective ranges using multiple effective range theories.