Annihilation of the electron-positron pairs in the positronium ion Ps^- and bi-positronium Ps₂

Rates of the two-, three-, four- and five-photon annihilations of the electron-positron pairs are determined numerically for the three-body positronium ion Ps$^-$ ($e^- e^+ e^-$) and four-body bi-positronium `molecule' Ps$_2$ ($e^- e^+ e^- e^+$). The values obtained in our computations are $\Gamma_{2 \gamma}($Ps$^-) \approx$ 2.08048530525$\cdot 10^{9}$ $sec^{-1}$, $\Gamma_{3 \gamma}($Ps$^-) \approx$ 5.6364151550$\cdot 10^{6}$ $sec^{-1}$, $\Gamma_{4 \gamma}($Ps$^-) \approx$ 3.075$\cdot 10^{3}$ $sec^{-1}$, $\Gamma_{5 \gamma}($Ps$^-) \approx 5.383$ $sec^{-1}$ and $\Gamma_{2 \gamma}($Ps$_2) \approx$ 4.4385952$\cdot 10^{9}$ $sec^{-1}$, $\Gamma_{3 \gamma}($Ps$_2) \approx$ 1.202497$ \cdot 10^7$ $sec^{-1}$, $\Gamma_{4 \gamma}($Ps$_2) \approx$ 6.562$\cdot 10^3$ $sec^{-1}$, $\Gamma_{5 \gamma}($Ps$_2) \approx$ 11.484 $sec^{-1}$. The four- and five-photon annihilation rates are significantly smaller than the corresponding two- and three-photon annihilation rates known for these systems. We also determine the rates of one- and zero-photon annihilation for the Ps$^{-}$ ion and Ps$_2$ system. The corresponding numerical values are $\Gamma_{1 \gamma}($Ps$^-) \approx$ 3.82491$\cdot 10^{-2}$ $sec^{-1}$, $\Gamma_{1 \gamma}($Ps$_2) \approx$ 1.94188$\cdot 10^{-1}$ $sec^{-1}$ and $\Gamma_{0 \gamma}($Ps$_2) \approx$ 2.32197$\cdot 10^{-9}$ $sec^{-1}$. Pusblished almost `as is' in Phys. Rev. A {\bf 80}, 014502 (2009).