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

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 1Zeff, for orthopositronium scattering from Li+,Na+, and K+ was computed with an L2 variant of the fixed-core stochastic variational method. The threshold 1Zeff for Li+ and Na+, which can form a stable bound state with Ps were found to be 0.067 and 0.73, respectively, while that for K+, which does not bind Ps, was 0.064.