Photodetachment of the positronium negative ion

Recent interest in the properties of the negative ion of positronium has encouraged us to compute its lifetime against photodetachment. To simplify the calculation, we have used the asymptotic form of the bound-state wave function and a plane wave for the final-state wave function, following the work of Ohmura and Ohmura in the case of photodetachment of ${\mathrm{H}}^{\mathrm{-}}$. We have obtained the needed normalizaton constant from a very precise and extensive Hylleraas wave function for the three-particle bound state.