Michael W. J. Bromley

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.

Configuration interaction calculations of positronic atoms and ions

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.

Higher-order Cn dispersion coefficients for hydrogen

The complete set of second-, third-, and fourth-order van der Waals coefficients C-n up to n=32 for the H(1s)-H(1s) dimer have been determined. They are computed by diagonalizing the nonrelativistic Hamiltonian for hydrogen to obtain a set of pseudostates that are used to evaluate the appropriate sum rules. A study of the convergence pattern for n <= 16 indicates that all the C-n <= 16 coefficients are accurate to 13 significant digits. The relative size of the fourth-order C-n((4)) to the second-order C-n((2)) coefficients is seen to increase as n increases and at n=32 the fourth-order term is actually larger.

POSITRON BINDING TO A MODEL ALKALI ATOM

The fixed core stochastic variational method is used to investigate positron binding to a model alkali atom with a continuously adjustable ionization potential. Positron binding is possible for model atoms which have ionization potential (IP) ranging from 0.1767 to 0.479 Hartree (corresponding to dipole polarizabilities ranging from 209 to 23.5 a03). Results of the model indicate that positron binding is likely for gold, but not for potassium, rubidium or caesium. The annihilation rate was largest (1.97 × 109 s-1) when the IP is smallest and smallest (0.07 × 109 s-1) when the IP is largest. The presence of a positronium (Ps) cluster configuration in the wavefunction is found to be important for an accurate estimate of the annihilation rate even in circumstances when the positron and electron are located large distances apart.

Higher-order Cn dispersion coefficients for the alkali-metal atoms

The van der Waals coefficients, from C11 through to C16 resulting from second-, third-, and fourth-order perturbation theory are estimated for the alkali-metal sLi, Na, K, and Rbd atoms. The dispersion coefficients are also computed for all possible combinations of the alkali-metal atoms and hydrogen. The parameters are determined from sum rules after diagonalizing a semiempirical fixed core Hamiltonian in a large basis. Comparisons of the radial dependence of the Cn/ r n potentials give guidance as to the radial regions in which the various higher-order terms can be neglected. It is seen that including terms up to C10/ r 10 results in a dispersion interaction that is accurate to better than 1% whenever the inter-nuclear spacing is larger than 20 a0. This level of accuracy is mainly achieved due to the fortuitous cancellation between the repulsive sC11, C13, C15d and attractive sC12, C14, C16d dispersion forces.

Large-dimension configuration-interaction calculations of positron binding to the group-II atoms

The configuration-interaction (CI) method is applied to the calculation of the structures of a number of positron binding systems, including e{sup +}Be, e{sup +}Mg, e{sup +}Ca, and e{sup +}Sr. These calculations were carried out in orbital spaces containing about 200 electron and 200 positron orbitals up to l=12. Despite the very large dimensions, the binding energy and annihilation rate converge slowly with l, and the final values do contain an appreciable correction obtained by extrapolating the calculation to the l{yields}{infinity} limit. The binding energies were 0.00317 hartree for e{sup +}Be, 0.0170 hartree for e{sup +}Mg, 0.0189 hartree for e{sup +}Ca, and 0.0131 hartree for e{sup +}Sr.

Properties of the triplet metastable states of the alkaline-earth-metal atoms

The static and dynamic properties of the alkaline-earth-metal atoms in their metastable state are computed in a configuration interaction approach with a semiempirical model potential for the core. Among the properties determined are the scalar and tensor polarizabilities, the quadrupole moment, some of the oscillator strengths, and the dispersion coefficients of the van der Waals interaction. A simple method for including the effect of the core on the dispersion parameters is described.

Bose–Einstein condensation in large time-averaged optical ring potentials

Interferometric measurements with matter waves are established techniques for sensitive gravimetry, rotation sensing, and measurement of surface interactions, but compact interferometers will require techniques based on trapped geometries. In a step towards the realization of matter wave interferometers in toroidal geometries, we produce a large, smooth ring trap for Bose-Einstein condensates using rapidly scanned time-averaged dipole potentials. The trap potential is smoothed by using the atom distribution as input to an optical intensity correction algorithm. Smooth rings with a diameter up to 300

Generating Phase Shifts from Pseudostate Energy Shifts

\mu

Effective oscillator strength distributions of spherically symmetric atoms for calculating polarizabilities and long-range atom–atom interactions

m are demonstrated. We experimentally observe and simulate the dispersion of condensed atoms in the resulting potential, with good agreement serving as an indication of trap smoothness. Under time of flight expansion we observe low energy excitations in the ring, which serves to constrain the lower frequency limit of the scanned potential technique. The resulting ring potential will have applications as a waveguide for atom interferometry and studies of superfluidity.