Positronic Lithium, an Electronically StableLi−e+Ground State

Abstract

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...

Resonance and Threshold Phenomena in Low-Energy Electron Collisions with Molecules and Clusters

“…However, very little is known about such systems. Even for positron-atom bound states, calculations became possible only recently (Dzuba et al, 1995;Ryzhikh and Mitroy, 1997), and virtually nothing is known about molecules, although some results were obtained for the relation between positron affinities and molecular dipole moments (Tachikawa et al, 2001). However, experimental data do not indicate any significance of the dipole moment for enhanced annihilation.…”

Stability of few-charge systems in quantum mechanics

“…The study of stability provides very useful information for various physical systems, e.g., positronic atoms [3,4,5,6,7], antimatter compounds [8], systems of atoms and antiatoms [9,10], or charged excitons (system of electrons and holes) in semiconductors [11]. New experimental investigations [11] of the fine structure of charged excitons in semiconductor quantum dots have shown the existence of bound states of three electrons and a positive charge or four electrons plus two-positive charges.…”

Effect of the atomic electric quadrupole moment on positron binding

“…Only when correlations between positron and atomic electrons are taken into account, positron can be bound to a majority of atoms [1] (see also [2][3][4][5][6][7][8][9][10] and references therein). A simple explanation of the positron binding is polarization of the atom by positron field.…”