γ -Ray Spectra and Enhancement Factors for Positron Annihilation with Core Electrons

2018

Phys. Rev. A

Positronium (Ps) scattering by noble-gas atoms (He, Ne, Ar, Kr, and Xe) is studied in the frozen-target approximation and with inclusion of the van der Waals interaction. Single-particle electron and positron states in the field of the target atom are calculated, with the system enclosed by a hard spherical wall. The two-particle Ps wave function is expanded in these states, and the Hamiltonian matrix is diagonalized, giving the Ps energy levels in the cavity. Scattering phase shifts, scattering lengths, and cross sections are extracted from these energies and compared with existing calculations and experimental data. Analysis of the effect of the van der Waals interaction shows that it cannot explain the recent experimental data of Brawley et al. for Ar and Xe [Phys. Rev. Lett. 115, 223201 (2015)].

Section: Discussionmentioning

confidence: 99%

Calculations of positronium-atom scattering using a spherical cavity

2018

Phys. Rev. A

“…We also used the wave function of the bound state to calculate the annihilation rate, including important short-range correlation enhancement factors. 80,81 Although our description of the bound positron-molecule system is not ab initio, its simplicity enables clear physical insight into the problem. The model correlation potential contains at most one free parameter for each type of atom in the molecule: the cutoff radius (assuming that the values of the hybrid polarizabilities of the atoms are known).…”

Section: Discussionmentioning

confidence: 99%

Calculations of positron binding and annihilation in polyatomic molecules

The Journal of Chemical Physics

2018

A model-potential approach to calculating positron-molecule binding energies and annihilation rates is developed. Unlike existing ab initio calculations, which have mostly been applied to strongly polar molecules, the present methodology can be applied to both strongly polar and weakly polar or nonpolar systems. The electrostatic potential of the molecule is calculated at the Hartree-Fock level, and a model potential that describes short-range correlations and long-range polarization of the electron cloud by the positron is then added. The Schrödinger equation for a positron moving in this effective potential is solved to obtain the binding energy. The model potential contains a single adjustable parameter for each type of atom present in the molecule. The wave function of the positron bound state may be used to compute the rate of electron-positron annihilation from the bound state. As a first application, we investigate positron binding and annihilation for the hydrogen cyanide (HCN) molecule. Results for the binding energy are found to be in accord with existing calculations, and we predict the rate of annihilation from the bound state to be Γ = 0.1-0.2 × 10 9 s −1 .

“…Similar enhancement factors are used in calculations of positron annihilation in solids [44,45]. Recent many-body-theory calculations for atoms have shown that the enhancement factors are, to a good approximation, functions of the spin-orbital energy ε i [27,43]:…”

mentioning

confidence: 96%

Positron Binding and Annihilation in Alkane Molecules

2019

Phys. Rev. Lett.

Self Cite

A model-potential approach has been developed to study positron interactions with molecules. Binding energies and annihilation rates are calculated for positron bound states with a range of alkane molecules, including rings and isomers. The calculated binding energies are in good agreement with experimental data, and the existence of a second bound state for n-alkanes (C n H 2n+2 ) with n ≥ 12 is predicted in accord with experiment. The annihilation rate for the ground positron bound state scales linearly with the square root of the binding energy.