Cross section for photoionization of the positronium negative ion at the lowest Feshbach resonance

The photoionization cross section for the positronium (Ps) negative ion, Ps-, at the lowest n = 2 Feshbach resonance, estimated by neglecting the influences of the weakly bound outer electron, is σ_F = 1.4 x 10^-12 cm², about 3 x 10³ times the existing lower limit calculated by Igarashi and co-workers (New J. Phys. 2, 17 (2000)). Although the estimated cross section is 200 times smaller than the analogous cross section for photoexcitation of the Ps Lyman-α transition, including the effect of the broad line width of the resonance shows it will be feasible to observe this resonance to obtain precision information about the three body Ps- system. (author)