arXiv subjects

B. Cunningham

Publications and source records attributed to B. Cunningham.

8 recordsLinked to original sources

arXiv2025

Many-body theory and Gaussian-basis implementation of positron annihilation $\gamma$-ray spectra on polyatomic molecules

Doppler-broadened $\gamma$-ray spectra for positron annihilation on molecules are calculated using many-body theory. By employing Gaussian bases for the electron and positron wavefunctions, a computable expression that involves a four-centre integral over the two-annihilation-photon momenta is derived for the $\gamma$ spectra in the independent particle model approximation to the annihilation vertex, and implemented in the open-source {\tt EXCITON+} code. The influence of electron-positron correlations on the $\gamma$ spectra is examined through \textit{ab initio} treatment of the positron wavefunction, whilst corrections to the annihilation vertex are treated approximately via enhancement factors previously calculated [D. G. Green and G. F. Gribakin, Phys.~Rev.~Lett.~{\bf 114}, 093201 (2015)] exactly for atoms. Calculated $\gamma$ spectra for furan and acetonitrile are presented for annihilation from the positron bound state with electrons of individual molecular orbitals. For such annihilation from the positron-molecule bound state, it is found that the magnitude of the partial contribution to the $\gamma$ spectra from individual molecular orbitals depends not just on the orbital energies, but also on the molecular symmetry, more precisely the relative localisation of the positron and electron densities.

physics.atom-ph↗

arXiv2025

Many-body theory calculations of positron binding to parabenzoquinone

Positron binding in parabenzoquinone is studied using \textit{ab initio} many-body theory. The effects of electron-positron correlations including polarization, virtual positronium formation and positron-hole repulsion, as well as those of $\pi$ bonds, aromaticity, and lone electron pairs, are considered. The binding energy is calculated as 60$\pm$16 meV, considerably larger than the 0.0925 meV value inferred from recent scattering calculations of [G. Moreira and M. Bettega, {\emph{Eur.~Phys.~J.~D}} {\bf 78} (2024)], but substantially smaller than we find in benzene (148$\pm$26 meV). The positron contact density (lifetime) is calculated as 8.0$\times10^{-3}$ a.u. (2.48 ns), vs.~1.61$\times 10^{-2}$ a.u. (0.81 ns) in benzene. The decrease (increase) in binding (annihilation rate) in parabenzoquinone compared to benzene is ascribed to the loss of aromaticity: the electron density on the positive oxygen nuclei being relatively harder for the positron to probe compared to the aromatic rings in benzene.

physics.chem-ph↗

arXiv2023

Positron annihilation and binding in aromatic and other ring molecules

Annihilation spectra are presented for aromatic and heterocyclic ring molecules resolved as a function of incident positron energy using a trap-based positron beam. Comparisons with the vibrational mode spectra yield positron-molecule binding energies. Good to excellent agreement is found between the measured binding energies and the predictions of an \textit{ab initio} many-body theory that takes proper account of electron-positron correlations including virtual-positronium formation. The calculations elucidate the competition between permanent dipole moments and $\pi$ bonds in determining the spatial distribution of the bound-state positron density. The implications of these results and the role of multimode features in annihilation in these molecules, including Fermi resonances, are discussed.

physics.chem-ph↗

arXiv2023

Many-body theory calculations of positronic-bonded molecular dianions

The energetic stability of positron di-anion systems [A$^-;e^+;$A$^-$] is studied via many-body theory, where $A^-$ includes H$^{-}$, F$^{-}$, Cl$^{-}$ and the molecular anions (CN)$^{-}$ and (NCO)$^{-}$. Specifically, the energy of the system as a function of ionic separation is determined by solving the Dyson equation for the positron in the field of the two anions, using a positron-anion self energy as constructed in [J. Hofierka, B. Cunningham, C. M. Rawlins, C. H. Patterson and D. G. Green, \emph{Nature} {\bf 606} 688 (2022)] that accounts for correlations including polarization, screening, and virtual-positronium formation. Calculations are performed for a positron interacting with H$_{2}^{2-}$, F$_{2}^{2-}$, and Cl$_{2}^{2-}$, and are found to be in good agreement with previous theory. In particular, we confirm the presence of two minima in the potential energy of the [H$^-;e^+$;H$^-$] system with respect to ionic separation: one a positronically-bonded [H$^-;e^+$;H$^-$] local minimum at ionic separations $r\sim3.4$~\AA\phantom{}, and a global minimum at smaller ionic separations $r\lesssim1.6$~\AA\phantom{} that gives overall instability of the system with respect to dissociation into a H$_2$ molecule and a positronium negative ion, Ps$^-$. The first predictions are made for positronic bonding in dianions consisting of molecular anionic fragments, specifically for (CN)$_{2}^{2-}$, and (NCO)$_{2}^{2-}$. In all cases we find that the molecules formed by the creation of a positronic bond are stable relative to dissociation into A$^-$ and $e^+$A$^-$ (positron bound to a single anion), with bond energies on the order of 1~eV and bond lengths on the order of several \r angstroms.

physics.chem-ph↗

arXiv2023

Gaussian-basis many-body theory calculations of positron binding to negative ions and atoms

Positron binding energies in the negative ions H$^-$, F$^-$, Cl$^-$ and Br$^-$, and the closed-shell atoms Be, Mg, Zn and Ca, are calculated via a many-body theory approach developed by the authors [J.~Hofierka \emph{et al.} Nature~{\bf 608}, 688-693 (2022)]. Specifically, the Dyson equation is solved using a Gaussian basis, with the positron self energy constructed from three infinite classes of diagrams that account for the strong positron-atom correlations that characterise the system including the positron-induced polarization of the electron cloud, screening of the electron-positron Coulomb interaction, virtual-positronium formation and electron-hole and positron-hole interactions. For the negative ions, binding occurs at the static level of theory, and the correlations are found to enhance the binding energies by $\sim$25--50\%, yielding results in good agreement with ($\lesssim$5\% larger than) calculations from a number of distinct methods. For the atoms, for which binding is enabled exclusively by correlations, most notably virtual-Ps formation, the binding energies are found to be of similar order to (but $\sim$10--30\% larger than) relativistic coupled-cluster calculations of [C. Harabati, V.~A.~Dzuba and V.~V. Flambaum, Phys.~Rev.~A {\bf 89}, 022517 (2014)], both of which are systematically larger than stochastic variational calculations of [M.~Bromley and J.~Mitroy, Phys.~Rev.~A {\bf 73} (2005); J.~Mitroy, J.~At.~Mol.~Sci.~{\bf 1}, 275 (2010)].

physics.atom-ph↗

arXiv2023

Many-body Theory Calculations of Positron Binding to Halogenated Hydrocarbons

Positron binding energies in halogenated hydrocarbons are calculated \emph{ab initio} using many-body theory. For chlorinated molecules, including planars for which the interaction is highly anisotropic, very good to excellent agreement with experiment and recent DFT-based model-potential calculations is found. Predictions for fluorinated and brominated molecules are presented. The comparative effect of fluorination, chlorination and bromination is elucidated by identifying trends within molecular families including dihaloethylenes and halomethanes based on global molecular properties (dipole moment, polarizability, ionization energy). It is shown that relative to brominated and chlorinated molecules, fluorinated molecules generate a less attractive positron-molecule potential due to larger ionization energies and smaller density of molecular orbitals close to the HOMO, resulting in very weak, or in most cases loss of, positron binding. Overall, however, it is shown that the global molecular properties are not universal predictors of binding energies, exemplified by consideration of CH$_3$Cl vs.~\emph{cis.}-C$_2$H$_2$F$_2$: despite the latter having a larger dipole moment, lower ionization energy and similar polarizability its binding energy is significantly smaller (25 meV vs.~3 meV, respectively), owing to the important contribution of multiple molecular orbitals to, and the anisotropy of, the positron-molecule correlation potential.

physics.chem-ph↗

arXiv2023

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 [J. Hofierka et al., Nature, 606, 688 (2022)] is combined with the shifted pseudostates method [A. R. Swann and G. F. Gribakin, Phys. Rev. A 101, 022702 (2020)] 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.

physics.atom-ph↗

arXiv2021

Many-body theory of positron binding in polyatomic molecules

Positrons bind to molecules leading to vibrational excitation and spectacularly enhanced annihilation. Whilst positron binding energies have been measured via resonant annihilation spectra for $\sim$90 molecules in the past two decades, an accurate \emph{ab initio} theoretical description has remained elusive. Of the molecules studied experimentally, calculations exist for only 6, and for these, standard quantum chemistry approaches have proved severely deficient, agreeing with experiment to at best 25% accuracy for polar molecules, and failing to predict binding in nonpolar molecules. The mechanisms of binding are not understood. Here, we develop a many-body theory of positron-molecule interactions and uncover the role of strong many-body correlations including polarization of the electron cloud, screening of the positron-electron Coulomb interaction by molecular electrons, and crucially, the unique non-perturbative process of virtual-positronium formation (where a molecular electron temporarily tunnels to the positron): they dramatically enhance binding in polar molecules and enable binding in nonpolars. We also elucidate the role of individual molecular orbitals, highlighting the importance of electronic $\pi$ bonds. Overall, we calculate binding energies in agreement with experiment (to within 1% in cases), and we predict binding in formamide and nucleobases. As well as supporting resonant annihilation experiments and positron-based molecular spectroscopy, the approach can be extended to positron scattering and annihilation $\gamma$ spectra in molecules and condensed matter, to provide fundamental insight and predictive capability required to properly interpret materials science diagnostics, develop antimatter-based technologies (including positron traps, beams and positron emission tomography), and understand positrons in the galaxy.

physics.atom-ph↗