The Kohn variational method is used with a configuration-interaction-type wave function to determine the J = 0 and J = 1 phase shifts and annihilation parameter Z eff for positron-hydrogenic ion scattering. The phase shifts are within 1 -2% of the best previous calculations. The values of Z eff are small and do not exceed unity for any of the momenta considered. At thermal energies Z eff is minute with a value of order 10 −50 occurring for He + at k = 0.05a 0 −1 . In addition to the variational calculations, analytic expressions for the phase shift and annihilation parameters within the Coulomb wave Born approximation are derived and used to help elucidate the dynamics of positron collisions with positive ions.

When positrons are injected into a gas, 75% of the positronium (Ps) is likely to be formed as longlived ortho-Ps. The main decay mechanisms for the ortho-Ps have been assumed to be natural decay of ortho-Ps and pickoff annihilation of the positron during Ps-atom collisions. A third possibility for annihilation is ortho-Ps ! para-Ps conversion due to the spin-orbit interaction between the atom and colliding Ps. This extra quenching mechanism may explain a number of phenomena observed in the annihilation spectrum of Kr and Xe, including the very small Ps fraction of 3% seen for Xe.

The non-relativistic absolute values and shapes of the cross sections for the process of two-photon one-electron excitation-ionization of the 1s shell of highly charged positive atomic ions Ne6+ and Ne8+ are calculated using second-order quantum-mechanical perturbation theory, taking into account the many-particle effect of relaxation of the ion residue in the field of the virtual vacancies created. The many-particle effect of stabilization of the deep vacancy when changing target from the neutral atom to the highly charged positive ions is also taken into account. The calculations are fulfilled for both linearly and circularly polarized incident photons.

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