Session GT3: Antimatter

Electron-Positronium Scattering and the Photodetachment of the Positronium Negative Ion

Following the experimental observation of the positronium negative ion (Ps^-), we studied theoretically electron-Ps scattering and the photodetachment of this ion below the Ps(n=2) threshold [1-3]. We employed both the Kohn and inverse Kohn variational methods with highly correlated trial functions to compute ^1,3S and ^1,3P wave phase shifts for electron-Ps scattering. We also determined the binding energy of Ps^- using the Rayleigh-Ritz variational method and a Hylleraas trial function [2]. Using the  bound-state wave function and the ¹P electron-Ps continuum wave function determined from the Kohn variational method, we computed the photodetachment cross section in both the length and velocity forms. For the higher energies that we considered, our results of the photodetachment cross section differ from both the hyperspherical close-coupling results [4] and the B-spline close coupling results [5]. Due to this discrepancy and the  observation of the photodetachment of Ps^- [6], William Mitchell and I have computed the photodetachment cross section in the length and velocity forms.  We used a larger basis set for the bound-state wave function than previously employed [1-3]. Also, we used the Kohn, inverse Kohn and complex Kohn variational methods for the ¹P continuum wave function.  These recent cross section results are consistent with the prior variational results [1-3]. Very recently, we have computed the photodetachment cross section in the acceleration form using variationally determined wave functions. In addition to computing the ¹P phase shifts for electron-Ps scattering, we have computed the ^1,3S and ³P phase shifts and have begun a calculation to compute the ^1,3D phase shifts.

 

[1] S. J. Ward and M. R. C. McDowell, Europhys. Lett. 1, 167 (1986).

[2] S. J. Ward, Thesis, University of London, Unpublished (1986).

[3] S. J. Ward, J. W. Humberston and M. R. C. McDowell, J. Phys. B 30, 127 (1987).

[4] A. Igarashi, I. Shimamura and N. Toshima, New J. Phys. 2, 17 (2000).

[5] A. Igarashi, S. Nakazaki, and A. Ohsaki, Phys. Rev. A 61, 032710 (2000).

[6] Y. Nagashima, PlenaryIPlenary Talk at the 18^th International Conference on Positron Annihilation (ICPA-18), 2018.   

Positron Binding and Annihilation in Benzene and Other Ring Hydrocarbons

Positrons have been shown to bind to most molecules via the formation of vibtational Feshbach resonances (VFR) on fundamental vibrational modes. Positron interactions with benzene and other unsaturated ring hydrocarbons are expected to play an important role in many areas including chemistry, biology, and astrophysics. Here we discuss two related studies to explore the important role that molecular π bonds play in determining positron binding and annihilation in these molecules. First, the use of a cryogenic, trap-based beam, is used to measure the spectrum for benzene and several partially deuterated benzenes. Earlier measurements of the spectrum exhibited unusually broad components that made difficult the identification of the fundamental-mode resonances and raised questions about the resulting value of the positron binding energy, E_b. The new high-resolution experiments separate the broad spectrum into several features and allows for a more precise measurement of E_b [1]. The second study utilizes the room temperature, trap-based beam to measure the binding energy for a wide range of ring hydrocarbons with various numbers of π bonds, including several aromatic molecules such as benzene. In particular, the results explore the important role that the molecular π bonds play in determining the binding energy of these molecules [2]. These results will be compared to recent many-body calculations [3] which elucidate the importance of electron-positron correlation effects in determining the positron-molecule binding energy. These studies also elucidate the richness of the annihilation spectra in the ring molecules studied, and in particular, the observation of VFR features beyond the fundamental vibrational modes will be discussed.

 

This work was done in collaboration with S. Ghosh and C. M. Surko.

[1] S. Ghosh, J. R. Danielson and C. M. Surko, Phys. Rev. Lett. 129, 123401 (2022).

[2] J. R. Danielson, S. Ghosh and C. M. Surko, Phys. Rev. A 106, 032811 (2022).

[3] J. Hofierka, B. Cunningham, C. M. Rawlings, C. H. Patterson, and D. Green, Nature 606, 688 (2022)

 

    

Many-body theory of positron binding to polyatomic molecules

Positrons are unique probes of matter, with applications in materials science (ultra-sensitive diagnostic studies of surfaces, defects and porosity), medical imaging (PET), astrophysics, molecular spectroscopy, and fundamental physics.

Positron interactions with matter are characterised by strong many-body correlations. They significantly modify scattering, and enhance annihilation rates by orders of magnitudes. They also make the theoretical description of positron-matter interactions formidably challenging.

We have developed a diagrammatic many-body description of positron-molecule interactions that takes ab initio account of the correlations, implemented in the state-of-the-art code EXCITON. We solve the Dyson equation for the positron quasiparticle wavefunction in a Gaussian basis, constructing the positron-molecule self-energy including the GW diagram (at RPA/TDHF/BSE levels of theory), describing polarisation, screening and electron-hole attraction interactions, the ladder series of positron-electron interactions that describes virtual positronium formation, and the ladder series of positron-hole interactions. We have used it to calculate binding energies for a range of polar and non-polar molecules, focusing chiefly on the molecules for which both theory and experiment exist, but also making predictions (e.g. of positron binding to DNA nucleobases, and of the effect of fluorination vs chlorination in hydrocarbons). Delineating the effects of the correlations, we show, in particular, that virtual-positronium formation significantly enhances binding in polar molecules, and moreover, that it can be essential to support binding in non-polar molecules. 

Overall, we find the best agreement with experiment to date (to within a few percent in cases). We have recently developed and extended the method to the calculation of the positron scattering and annihilation gamma spectra in molecules providing insight that should support the development of fundamental experiments and the myriad of antimatter-based technologies and applications. 

Moreover, the positron-molecule problem provides a testbed for the development of methods to tackle the quantum many-body problem, for which our results can serve as benchmarks.   

Development of an Antimatter Chemistry Network

In this presentation we survey antimatter collisional and radiative processes with the purpose of optimizing formation of the antihydrogen atom, anion, cation and molecular species. We draw inspiration and knowledge from studies of the early Universe hydrogen chemistry, where future antimatter production capabilities will soon meet these conditions. Using modern atomic physics techniques, it should be feasible to control antimatter in the laboratory to facilitate antimatter chemistry and enhance production rates. Here we summarize what is known from hydrogen chemistry that is of relevance for antimatter production and to indicate, based upon possible reaction rates, which processes may be fruitful to pursue to create new antimatter entities, and which processes to discount noting experimental capabilities.