S-Wave Shape Resonances in the Ps− System

We have investigated the S-wave shape resonance states of the positronium negative ion (Ps−) system. The resonance poles are traced from H− system to Ps− by systematically varying the mass of the positively charged particle from infinitely heavy to one unit of the electron mass. The shape resonances that associated with and lying above the Ps(N = 2, 3, 4 and 5) thresholds are located by employing the complex-coordinate rotation method with highly correlated Hylleraas-type wave functions. It has been shown that the Ps−(N = 3) shape resonance lies at an energy which is higher than the Ps(N = 4) thresholds and even the Ps−(N = 4) shape resonance. An explanation was given to shed light on such phenomena.