The positronium negative ion: Classical properties and semiclassical quantization

Properties of collinear and planar periodic orbits for the positronium nega tive ion are examined with respect to the possibilities for semiclassical q uantization. In contrast to other two-electron atomic systems as helium and H- the relevant orbits for quantization are fully stable and permit a full torus quantization. However, for lower excitations the area of stability i n phase-space is too small for a reliable torus quantization. Instead, a qu asi-separability of the three-body system is used to apply effective one-di mensional (WKB) quantization.