The structures of a number of exotic atoms with an attached positron or positronium atom are studied using a large-scale variational expansion in terms of a basis of explicitly correlated Gaussian functions. The binding energies and annihilation rates for seven exotic species with electronically stable ground states, namely HPs, Lie + , LiPs, Bee + , Nae + , NaPs and Mge + have been predicted. The binding energy for HPs, 0.038 1944 Hartree, is the largest attained so far. Two of the species, Lie + and Nae + , with approximate binding energies of 0.0024 and 0.0005 Hartree respectively, are seen to have structures best described as a positronium atom orbiting a residual Li + or Na + positively charged core. The Bee + atom with an approximate binding energy of 0.0028 Hartree is best characterized as a positron orbiting a polarized Be core. The binding energy of the Mge + ground state, 0.014 Hartree, is larger than that of any other positronic atom (a neutral atom with an attached positron). The LiPs and NaPs atoms, with approximate binding energies of 0.012 and 0.0072 Hartree respectively, have structures similar to HPs although the binding energies are smaller and the valence electrons and the positron are found at larger distances from the nucleus.

Recent research has shown that there are a number of atoms and atomic ions that can bind a positron. The number of atoms known to be capable of binding a positron has expanded enormously in recent years, with Li, He(3 S e), Be, Na, Mg, Ca, Cu, Zn, Sr, Ag and Cd all capable of binding a positron. The structure of these systems is largely determined by the competition between the positron and the nucleus to bind the loosely bound valence electrons. Some systems, such as e + Li and e + Na, can be best described as a Ps cluster orbiting a charged Li + or Na + core, while others such as e + Be consist of a positron orbiting a polarized Be atom. In addition, a number of atoms (Li, C, O, F, Na, Cl, K, Cl, Cu, Br) can bind positronium and a few systems capable of binding two positrons have also been identified. These positron-binding systems decay by electron-positron annihilation with the annihilation rate for e + A systems largely determined by the parent atom ionization potential.

Calculations of the positron-Li system were performed using the stochastic variational method, yielding a minimum energy of 27.532 08 hartree for the L 0 ground state. In contrast with previous calculations of this system, the system was found to be stable against dissociation into the Ps 1 Li 1 channel with a binding energy of 0.002 17 hartree; it is therefore electronically stable. This is the first instance of a rigorous calculation predicting that it is possible to combine a positron with a neutral atom to form an electronically stable bound state. [S0031-9007(97)04578-X] PACS numbers: 36.10. Dr, 31.15.Ar One of the most tantalizing questions of positron physics is: Is it possible for a positron to bind itself to a neutral atom and form an electronically stable state [1,2]? This question can be answered only by a sophisticated calculation (or experimentation) as the mechanisms responsible for binding the positron to the atom are polarization potentials present in the positron-atom complex. The accurate computation of the polarization potential for a positron-atom (or electron-atom) system is of course a challenging exercise in many-body physics.While the question of whether it is possible to bind a positron to a neutral atom is an open question, the ability of positronium to attach itself to atoms has been known for a long time. A number of previous works have demonstrated that the positronium-hydride (PsH) species [3][4][5][6][7][8] is stable against dissociation into the Ps 1 H or the e 1 1 H 2 channels. In this case, binding is more likely since the positron is binding itself to a species with an overall negative charge.The question of whether a positron can form an electronically stable bound state with a neutral atom is more vexing. Dzuba et al. [9] have made calculations suggesting that it is possible to bind a positron to atomic species with two valence electrons such as Mg, Zn, Cd, and Hg. These calculations were performed in the framework of the many-body perturbation theory, and their results, while suggestive, cannot be regarded as providing proof to the existence of electronically stable positronic atoms.In their work, Dzuba et al. [9] did not consider the possibility of positrons forming bound states with alkali atoms such as Li, Na, K, . . . even though the polarization potential for these species should be stronger than for the alkaline and alkaline earth atoms and therefore the possibility of binding should be improved. One difficulty in binding positrons to alkali atoms is that the ionization energy of the alkali atoms is smaller than the binding energy of positronium. Therefore the binding energy of the positron to the neutral atom must exceed a particular value for the species to be stable against dissociation into positronium 1 ion. For example, for the Li-e 1 species to be stable against dissociation into positronium plus Li 1 requires that the binding energy of the e 1 with respect to the Li ground state be greater than ͑0.25 0.198 15͒ hartree. In this respect, it is more appropriate...

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