STABLE BOUND-STATES OF E(+)+LI AND E(+)+NA

Using model potentials to describe the ionic cores, we have approximat ed the e(+) +Li and e(+) + Na systems as quasi-three-body problems and performed adiabatic hyperspherical calculations to search for the exi stence of bound states. We have confirmed the existence of a bound sta te for e(+) + Li that was first predicted by Ryzhikh and Mitroy [Phys. Rev. Lett. 21, 4124 (1997)] with a binding energy of 58 meV. Further, we predict the existence of a stable bound state for e(+) + Na with a binding energy of 7 meV and explain why bound states exist for these two systems but not for the e(+) + H system, despite the fact that H- has a higher binding energy than either Li- or Na-. Based on this work , we find that it is unlikely that positrons can form stable bound sta tes with any other of the alkali-metal atoms.