Session ET43: Positron Annihilation, Charge Exchange and Elastic Scattering

Many-body theory of positron binding and annihilation in polyatomic molecules

Positrons bind to molecules leading to rapid annihilation [1]. Whilst binding energies have been measured for around 90 molecules over past two decades, an accurate \emph{ab initio} theoretical approach has remained elusive [1]. The theoretical difficulty is due to the need to accurately account for strong many-body correlations that characterise the positron-molecule system. These include electronic polarisation, electron screening of the positron-molecule Coulomb interaction, and the unique process of virtual-positronium formation (where a molecular electron temporarily tunnels to the positron) [1]. Although assumed to be important, especially in non-polar molecules, their specific role in positron-molecule binding is not well understood. Standard quantum chemistry approaches, which neglect or treat the correlations perturbatively, have proved deficient, agreeing with experiment to at best 25\% accuracy for polar molecules (see [1] and references therein, and [2]).

In this talk I will discuss how many-body theory insightfully delineates the effects of correlations and gives the most accurate \emph{ab initio} calculations of positron-molecule binding energies in polyatomic molecules (agreeing with experiment to ~10% in most cases). Notably, we find that the non-perturbative process of virtual-positronium formation is essential to support binding in non-polar molecules including CS$_2$, CSe$_2$ and benzene etc, and that it significantly enhances binding in organic polar molecules. We also elucidate the contribution of individual occupied electronic molecular orbitals to binding and positron annihilation from the bound state. 

[1] G. F. Gribakin, J. A. Young and C. M. Surko, Rev. Mod. Phys. 82, 2557 (2010).

[2] J. Romero et al. J. Chem. Phys. 141, 114103 (2014);

[3] J. Horfierka, B. J. Cunningham, C. M. Rawlins, C. P. Patterson and D. G. Green, arXiv2105.06959 (2021).    

Positron Annihilation Resonances in Molecules: Going Beyond Fundamental Modes

Slow positrons ($< 0.5$ eV) bind to molecules through Feshbach resonant excitation of dipole-active fundamental vibrational modes, and this leads to annihilation rates that are much faster than any direct annihilation process. For modes with strong infrared activity, intramolecular vibrational redistribution (IVR) leads to a further enhancement of the annihilation by spreading the coupling over a large number of near degenerate combination and overtone modes. Recently, distinct enhanced annihilation resonances have been observed that involve vibrational modes beyond the fundamentals. In this talk, evidence for these resonances will be presented for ring and chain alkane molecules, an effect that appears to be generic. The energy spectra of the new resonances will be compared to infrared absorption spectra and shown to correlate to a region populated by combinations and/or overtones of the fundamental vibrations. Although the infrared amplitude is much weaker than for the fundamentals, the magnitude of these annihilation resonances show that many modes must be participating, and thus IVR appears to be strong even for these non-fundamental modes. For molecules with binding energies larger than those of the highest lying fundamental modes, such resonances are expected to dominate the annihilation spectrum. An example of this effect is seen in polycyclic aromatic hydrocarbon (PAH) molecules, a potentially important component of annihilation in the interstellar medium. A room temperature (FWHM $\sim 36$ meV), as well as a cryogenically cold (FWHM $\sim 20$ meV), positron beam is used in these studies. Implications of the energy distributions of these beams in determining the shape and magnitude of the observed resonances will also be discussed.   

Time-Dependent Close-Coupling Calculations of Charge Exchange between Bare Ne and Mg ions and atomic H and He targets at solar wind velocities

Charge exchange between highly charged ions and neutrals introduces a problematic background source in each astrophysical x-ray observation. For H-like ions produced by charge exchange between bare ions and neutrals the angular momentum states within each n shell are degenerate. While several analytical angular momentum distributions are available (low energy, statistical), the exact distribution of angular momentum states is not yet known, and the most appropriate choice for solar wind velocities (~1 - 5 keV/u) is not apparent. We present nl-resolved CX cross sections for bare Ne and Mg incident on atomic H and He targets calculated using the time-dependent close-coupling (TDCC) approach. We combined our nl-resolved CX cross sections and other atomic data in an x-ray cascade model and compared our synthetic spectra against other work in the literature. At solar wind velocities, we find some similarities between Multi-Channel Landau-Zener (MCLZ) n-resolved cross sections with an applied statistical angular momentum distribution and our TDCC results. We find poorer agreement between TDCC and MCLZ n-resolved cross sections with a low energy l-distribution applied.   

Anisotropic angular scattering models of elastic electron-neutral collisions for Monte Carlo plasma simulation

Many laboratory and industrial plasma applications require accurate modeling techniques to understand the interplay between microscopic and macroscopic processes. A prime example of this interplay is how particle and Monte Carlo simulation codes use angular scattering of electrons following elastic scattering events in order to help simulate plasma devices. The forward peaked nature of high energy electron elastic scattering is relatively trivial to accurately describe in plasma simulations. However, for lower energy collisions, which produce near isotropic or backward peaked differential cross sections, there is not a strong consensus among the plasma modeling community on how to best describe these angular scattering trends. In this study, we propose a systematic method to approximate the aforementioned non-trivial angular scattering behavior with a formula that can be readily implemented in Particle-in-Cell and/or Monte Carlo plasma simulation codes. Specific application of this method is demonstrated for fusion relevant atomic hydrogen and helium, as well as for molecular hydrogen.