arXiv subjects

J. P. Cassidy

Publications and source records attributed to J. P. Cassidy.

4 recordsLinked to original sources

arXiv2025

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

Doppler-broadened $γ$-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 $γ$ 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 $γ$ 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 $γ$ 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 $γ$ 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↗

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

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 $π$ 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 $r\sim3.4$~Å\phantom{}, and a global minimum at smaller ionic separations $r\lesssim1.6$~Å\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↗