Pick-off annihilation in positronium scattering from alkali-metal ions

Abstract

When orthopositronium collides with a closed-shell atom or ion the ``pick-off'' process is the dominant process for positron annihilation. In pick-off annihilation the positron annihilates with electrons in the target but not with the electron that is part of the positronium atom. The rate for pick-off annihilation, expressed in terms of the dimensionless parameter ${}^{1}{Z}_{\mathrm{eff}},$ for orthopositronium scattering from ${\mathrm{Li}}^{+},{\mathrm{Na}}^{+},$ and ${\mathrm{K}}^{+}$ was computed with an ${L}^{2}$ variant of the fixed-core stochastic variational method. The threshold ${}^{1}{Z}_{\mathrm{eff}}$ for ${\mathrm{Li}}^{+}$ and ${\mathrm{Na}}^{+},$ which can form a stable bound state with Ps were found to be 0.067 and 0.73, respectively, while that for ${\mathrm{K}}^{+},$ which does not bind Ps, was 0.064.