Positron scattering and annihilation on noble-gas atoms

Abstract

Positron scattering and annihilation on noble gas atoms is studied ab initio using many-body theory methods for positron energies below the positronium formation threshold. We show that in this energy range the many-body theory yields accurate numerical results and provides a nearcomplete understanding of the positron-noble-gas-atom system. It accounts for positron-atom and electron-positron correlations, including the polarization of the atom by the positron and the nonperturbative process of virtual positronium formation. These correlations have a large effect on the scattering dynamics and result in a strong enhancement of the annihilation rates compared to the independent-particle mean-field description. Computed elastic scattering cross sections are found to be in good agreement with recent experimental results and Kohn variational and convergent close-coupling calculations. The calculated values of the annihilation rate parameter Z eff (effective number of electrons participating in annihilation) rise steeply along the sequence of noble gas atoms due to the increasing strength of the correlation effects, and agree well with experimental data.

Positron-electron correlation-polarization potentials for the calculation of positron collisions with atoms and molecules*

“…The data by Jones et al [7] includes some angular correction and presents the total cross sections without Positronium formation. Figure 1 shows the calculations done by Fursa and Bray [5] using the convergentclose-coupling (CCC) method, by Green et al [4] using many-body perturbation theory (MBPT), and by Jones et al [7] using the relativistic optical potential (ROP) method. Our calculations and those by Green et al [4] and Jones et al [7] are done for elastic cross sections.…”

“…Over the past ten years the experimental and calculated scattering cross sections for low-energy collisions of positrons with rare gas atoms, like argon and krypton, have converged within the experimental uncertainties [2][3][4][5][6][7][8][9][10]. For positron collisions with (polar and non-polar) molecules the situation is less favourable.…”

Do positrons measure atomic and molecular diameters?

“…Thus it defines the distance from the nucleus where valence electrons form a covalent bond. It is expected that at this distance the incoming positron strongly interacts with the valence electrons, for example through the virtual Positronium formation [21], what determines the output of scattering experiment at impact energies just below the threshold for positronium formation.…”

“…The molecular orbitals, which were used in the scattering calculations to represent the target electron density, are generated with the B3LYP (Becke [44], three-parameter, Lee-Yang-Parr [45]) exchange-correlation functional, using the double-zeta basis set of Dunning and Hay [46] with polarization and diffuse functions. [62] (polarized orbital), Jones et al [61] (relativistic optical potential), Fursa and Bray [22] (convergent close-coupling) and Green et al [21] (many-body theory). Experiments: Kauppila et al [6], Sinapius et al [63], Karwasz et al [13], Jones et al [61] and Zecca et al [60].…”

“…For example, citing some most recent works on the atoms: McEachran et al (see [20] and references therein) used different forms of polarized-orbital calculations. Gribakin and co-workers [21] used many-body theory that model also formation of virtual positronium and multi-electron correlation. Bray and Fursa [22], for the Ne-Xe series, considered only one-electron excitations out of six p-electrons above an inert Hartree-Fock core; in their approach formation of virtual positronium is taken into account via excitation to continuum-like pseudostates.…”