G. G. Ryzhikh

The structure of exotic atoms containing positrons and positronium

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, , LiPs, , , NaPs and have been predicted. The binding energy for HPs, 0.038 1944 Hartree, is the largest attained so far. Two of the species, and , 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 or positively charged core. The 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 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.

Positron and positronium binding to atoms

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(3Se), 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.

Positronic Lithium, an Electronically Stable Li-e+ Ground State

Calculations of the positron-Li system were performed using the Stochastic Variational Method and yielded a minimum energy of -7.53208 Hartree for the L=0 ground state. Unlike previous calculations of this system, the system was found to be stable against dissociation into the Ps + Li

Configuration interaction calculations of positronic atoms and ions

^+

The structure of e+LiH

channel with a binding energy of 0.00217 Hartree and 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 and form an electronically stable bound state.

Positron scattering from atomic sodium

Abstract The configuration interaction (CI) method is one of the most commonly used methods for the calculation of the electronic structure of atoms. The standard CI method based on a linear combination of orthonormal orbitals centered on the nucleus has been adapted to the calculation of the structure of exotic atoms containing one or two electrons and a positron. Results of calculations on a number of systems, including positronium hydride (PsH), e + Cu , e + Li , e + Be , e + Cd and CuPs show both the strengths and limitations of the CI approach for positron binding atoms.

A metastable state of positronic helium

The stochastic variational method (SVM) has been used to confirm the prediction of a quantum Monte Carlo (QMC) calculation (Bressanini et al 1998 J. Chem. Phys. 109 1716) that a positron can be bound to the lithium hydride molecule (LiH). The adiabatic approximation was not made and the nuclear motion was explicitly included in the calculation. A completely ab initio calculation gave a positron binding energy of 0.613 eV and constitutes an explicit demonstration of binding not restricted to the Born-Oppenheimer approximation. The fixed-core variant of the SVM was also used with a Li+ 1s2 core. The positron binding energy in the fixed-core SVM was 0.909 eV, which is approximately the same as that previously reported in the QMC calculation. The mean inter-nuclear radius was 3.96 a0 and the 2γ annihilation rate of the e+LiH ground state was 1.64×109 s-1.

The formation of antihydrogen by the charge transfer reaction

Close-coupling calculations of positron scattering from atomic sodium are performed from threshold to 100 eV. The alkali atom is represented by a frozen-core model based upon the Hartree - Fock approximation. The close-coupling calculations are performed in a model containing multiple sodium (3s, 3p, 4s, 3d, 4p) and positronium states (1s, 2s, 2p, 3s, 3p, 3d). Subsidiary calculations with three and ten positronium states were performed at selected energies to test the convergence of the positronium formation cross section. The predicted total cross sections agree with the experimental data of Kwan et al but there are differences with the data of Kauppila et al. The net positronium formation cross section underestimates the experimental data of Zhou et al.

Positron binding to atomic silver

A metastable state of helium with an attached positron is predicted to exist with a total energy of . The state is stable against dissociation into with a binding energy of . This state is formed by the attachment of a positron to the metastable He 1s2s state. The He ( state has a decay rate of . The state can only decay by a process with an approximate rate of . The structure of these states are best characterized as a positronium atom orbiting a 1s core at large distances.

The stability of Mg and NaPs

The cross sections for antihydrogen formation in the n = 1, 2 and 3 levels from the reaction are computed in the close-coupling approximation. A large basis of positron - hydrogen channels (28 states) is supplemented by the Ps(1s), Ps(2s) and Ps(2p) channels. The present results represent the most accurate calculations of this cross section yet undertaken. It is seen that the net cross section for antihydrogen formation decreases extremely rapidly as a function of increasing energy and beyond about 5 Ryd is essentially negligible.