Elastic scattering of slow positrons by helium

Many-Body Theory Calculations of Positron Scattering and Annihilation in H_{2}, N_{2}, and CH_{4}

The recently developed ab initio many-body theory of positron molecule binding [22J. Hofierka et al., Many-body theory of positron binding to polyatomic molecules, Nature (London) 606, 688 (2022)NATUAS0028-083610.1038/s41586-022-04703-3] is combined with the shifted pseudostates method [A. R. Swann and G. F. Gribakin, Model-potential calculations of positron binding, scattering, and annihilation for atoms and small molecules using a Gaussian basis, Phys. Rev. A 101, 022702 (2020)PLRAAN2469-992610.1103/PhysRevA.101.022702] to calculate positron scattering and annihilation rates on small molecules, namely H_{2}, N_{2}, and CH_{4}. The important effects of positron-molecule correlations are delineated. The method provides uniformly good results for annihilation rates on all the targets, from the simplest (H_{2}, for which only a sole previous calculation agrees with experiment), to larger targets, where high-quality calculations have not been available.

Calculation of Low-Energy Positron-Atom Scattering with Square-Integrable Wavefunctions

The variational method is applied to the low-energy positron scattering and annihilation problem. The ultimate aim of the investigation is to find a computationally economical way of accounting for strong electron–positron correlations, including the effect of virtual positronium formation. The method is applied to the study of elastic s-wave positron scattering from a hydrogen atom. A generalized eigenvalue problem is set up and solved to obtain s-wave positron–hydrogen scattering phase shifts within 8×10−3 rad of accepted values. This is achieved using a small number of terms in the variational wavefunction; in particular, only nine terms that depend on the electron–positron distance are included. The annihilation parameter Zeff is also calculated and is found to be in good agreement with benchmark calculations.

Many-body theory of positron binding to polyatomic molecules

Positron binding to molecules is key to extremely enhanced positron annihilation and positron-based molecular spectroscopy¹. Although positron binding energies have been measured for about 90 polyatomic molecules^1-6, an accurate ab initio theoretical description of positron-molecule binding has remained elusive. Of the molecules studied experimentally, ab initio calculations exist for only six; these calculations agree with experiments on polar molecules to at best 25 per cent accuracy and fail to predict binding in nonpolar molecules. The theoretical challenge stems from the need to accurately describe the strong many-body correlations including polarization of the electron cloud, screening of the electron-positron Coulomb interaction and the unique process of virtual-positronium formation (in which a molecular electron temporarily tunnels to the positron)¹. Here we develop a many-body theory of positron-molecule interactions that achieves excellent agreement with experiment (to within 1 per cent in cases) and predicts binding in formamide and nucleobases. Our framework quantitatively captures the role of many-body correlations and shows their crucial effect on enhancing binding in polar molecules, enabling binding in nonpolar molecules, and increasing annihilation rates by 2 to 3 orders of magnitude. Our many-body approach can be extended to positron scattering and annihilation γ-ray spectra in molecules and condensed matter, to provide the fundamental insight and predictive capability required to improve materials science diagnostics^7,8, develop antimatter-based technologies (including positron traps, beams and positron emission tomography)^8-10, and understand positrons in the Galaxy¹¹.

ATOM Program System and Computational Experiment

The article is devoted to a brief description of the ATOM computer program system, designed to study the structure, transition probabilities and cross sections of various processes in multielectron atoms. The theoretical study was based on the concept of a computational experiment, the main provisions of which are discussed in the article. The main approximate methods used in the system of programs for taking many-electron correlations into account and determining their role in photoionization processes, elastic and inelastic electron scattering, the decay of vacancies, and many others are presented. The most significant results obtained with this software are listed.

Many-Body Theory for Positronium-Atom Interactions

A many-body-theory approach has been developed to study positronium-atom interactions. As first applications, we calculate the elastic scattering and momentum-transfer cross sections and the pickoff annihilation rate ^{1}Z_{eff} for Ps collisions with He and Ne. For He the cross section is in agreement with previous coupled-state calculations, while comparison with experiment for both atoms highlights discrepancies between various sets of measured data. In contrast, the calculated ^{1}Z_{eff} (0.13 and 0.26 for He and Ne, respectively) are in excellent agreement with the measured values.

Detecting positron-atom bound states through resonant annihilation

A method is proposed for detecting positron-atom bound states by observing enhanced positron annihilation due to electronic Feshbach resonances at electron-volt energies. The method is applicable to a range of open-shell transition-metal atoms which are likely to bind the positron: Fe, Co, Ni, Tc, Ru, Rh, Sn, Sb, Ta, W, Os, Ir, and Pt. Estimates of their binding energies are provided.