arXiv subjects

A. R. Swann

Publications and source records attributed to A. R. Swann.

17 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↗

arXiv2021

Self annihilation of confined positronium

The effect of confinement on the self-annihilation rate of positronium is studied in three levels of approximation. Artificial restriction of the electron-positron separation leads to an increase in the annihilation rate over its vacuum value; this increase is found to diminish exponentially as the maximum separation is increased. Confinement in a hard-wall spherical cavity with the center of mass free to move throughout the cavity also increases the annihilation rate over its vacuum value; the increase depends weakly on the position of the center of mass, being larger when the center of mass is near the cavity wall. Finally, to model confinement in a pore of a microporous material, the hard wall is replaced by physically motivated electron- and positron-wall potentials; it is found that the annihilation rate is larger than its vacuum value, in contradiction to calculations of Marlotti Tanzi et al. [Phys. Rev. Lett. 116, 033401 (2016)] that assumed hard-wall confinement for the electrons, and experimental data.

physics.atom-ph↗

arXiv2021

Effect of chlorination on positron binding to hydrocarbons: experiment and theory

Measured and calculated positron binding energies are presented for a range of hydrocarbons with up to six carbon atoms (viz., methane, acetylene, ethylene, ethane, propane, butane, and hexane) and their chlorinated counterparts. Both experiment and theory confirm the large effect that the chlorine atoms have on the positron binding energy and the strong sensitivity of the binding energy to the exact position of the chlorine atoms. The experimental binding energies have been obtained by measuring positron resonant annihilation using a trap-based positron beam. The calculations are performed using the previously developed model-correlation-potential method [A. R. Swann and G. F. Gribakin, J. Chem. Phys. 149, 244305 (2018)]. The overall trends are discussed with regard to the molecular polarizability, dipole moment, and geometry. Good agreement between theory and experiment is found, with the exception of the chlorinated ethylenes and chlorinated hexane. Calculations of the electron-positron annihilation rate in the bound state are also presented.

physics.atom-ph↗

arXiv2021

Many-body theory for positronium scattering and pickoff annihilation in noble-gas atoms

The many-body-theory approach to positronium-atom interactions developed in [Phys. Rev. Lett. \textbf{120}, 183402 (2018)] is applied to the sequence of noble-gas atoms He-Xe. The Dyson equation is solved separately for an electron and positron moving in the field of the atom, with the entire system enclosed in a hard-wall spherical cavity. The two-particle Dyson equation is solved to give the energies and wave functions of the Ps eigenstates in the cavity. From these, we determine the scattering phase shifts and cross sections, and values of the pickoff annihilation parameter $^1Z_\text{eff}$ including short-range electron-positron correlations via vertex enhancement factors. Comparisons are made with available experimental data for elastic and momentum-transfer cross sections and $^1Z_\text{eff}$. Values of $^1Z_\text{eff}$ for He and Ne, previously reported in [Phys. Rev. Lett. \textbf{120}, 183402 (2018)], are found to be in near-perfect agreement with experiment, and for Ar, Kr, and Xe within a factor of 1.2.

physics.atom-ph↗

arXiv2021

Positron cooling via inelastic collisions in CF$_4$ and N$_2$ gases

Positron cooling via inelastic collisions in CF$_4$ and N$_2$ gases is simulated, including positron-positron interactions. Owing to the molecular symmetries, cooling is assumed to be chiefly due to energy loss via vibrational (rotational) excitations for CF$_4$ (N$_2$). For CF$_4$, it is found that the inclusion of the dipole-inactive $ν_1$ mode, in addition to the dipole-active modes $ν_3$ and $ν_4$, can provide room-temperature thermalization and an accurate cooling timescale. Combination cooling enabled by the $ν_1$ mode, and positron-positron interactions both contribute to the Maxwellianization of the positron momentum distribution. For both gases the evolution of the positron temperature is found to be in excellent agreement with experiment.

physics.atom-ph↗

arXiv2020

Effect of molecular constitution and conformation on positron binding and annihilation in alkanes

The model-potential approach previously developed by the authors to study positron interactions with molecules is used to calculate the positron binding energy for $n$-alkanes (C$_n$H$_{2n+2}$) and the corresponding cycloalkanes (C$_n$H$_{2n}$). For $n$-alkanes, the dependence of the binding energy on the conformation of the molecule is investigated, with more compact structures showing greater binding energies. As a result, thermally averaged binding energies for larger alkanes ($n\gtrsim 9$) show a strong temperature dependence in the range of 100-600 K. This suggests that positron resonant annihilation can be used as a probe of rotational (trans-gauche) isomerization of $n$-alkanes. In particular, the presence of different conformers leads to shifts and broadening of vibrational Feshbach resonances in the annihilation rate, as observed with a trap-based low-energy positron beam.

physics.atom-ph↗

arXiv2020

Model-potential calculations of positron binding, scattering, and annihilation for atoms and small molecules, using a Gaussian basis

A model-potential method is employed to calculate binding, elastic scattering, and annihilation of positrons for a number of atoms and small nonpolar molecules, namely, Be, Mg, He, Ar, H$_2$, N$_2$, Cl$_2$, and CH$_4$. The model potential contains one free parameter for each type of atom within the target. Its values are chosen to reproduce existing ab initio positron-atom binding energies or scattering phase shifts. The calculations are performed using a Gaussian basis for the positron states, and we show how to obtain values of the scattering phase shifts and normalized annihilation rate $Z_{\rm eff}$ from discrete positive-energy pseudostates. Good agreement between the present results and existing calculations and experimental data, where available, is obtained, including the $Z_{\rm eff}$ value for CH$_4$, which is strongly enhanced by a low-lying virtual positron state. An exception is the room-temperature value of $Z_{\rm eff}$ for Cl$_2$, for which the present value is much smaller than the experimental value obtained over 50 years ago. Our calculations predict that among the molecular targets studied, only Cl$_2$ might support a bound state for the positron, with a binding energy of a few meV.

physics.atom-ph↗

arXiv2019

Positron Binding and Annihilation in Alkane Molecules

A model-potential approach has been developed to study positron interactions with molecules. Binding energies and annihilation rates are calculated for positron bound states with a range of alkane molecules, including rings and isomers. The calculated binding energies are in good agreement with experimental data, and the existence of a second bound state for $n$-alkanes (C$_n$H$_{2n+2}$) with $n\geq12$ is predicted in accord with experiment. The annihilation rate for the ground positron bound state scales linearly with the square root of the binding energy.

physics.atom-ph↗

arXiv2018

Calculations of positron binding and annihilation in polyatomic molecules

A model-potential approach to calculating positron-molecule binding energies and annihilation rates is developed. Unlike existing ab initio calculations, which have mostly been applied to strongly polar molecules, the present methodology can be applied to both strongly polar and weakly polar or nonpolar systems. The electrostatic potential of the molecule is calculated at the Hartree-Fock level, and a model potential that describes short-range correlations and long-range polarization of the electron cloud by the positron is then added. The Schrodinger equation for a positron moving in this effective potential is solved to obtain the binding energy. The model potential contains a single adjustable parameter for each type of atom present in the molecule. The wave function of the positron bound state may be used to compute the rate of electron-positron annihilation from the bound state. As a first application, we investigate positron binding and annihilation for the hydrogen cyanide (HCN) molecule. Results for the binding energy are found to be in accord with existing calculations, and we predict the rate of annihilation from the bound state to be $Γ=0.1$--$0.2 \times 10^9~\text{s}^{-1}$.

physics.atom-ph↗

arXiv2018

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 $^1Z_\text{eff}$ for Ps collisions with He and Ne. The cross section for He is in agreement with previous coupled-state calculations, and the momentum-transfer cross section for Ne agrees with available experimental data. $^1Z_\text{eff}$ is found to be 0.13 and 0.26 for He and Ne, respectively, in excellent agreement with the measured values.

physics.atom-ph↗

arXiv2017

Calculations of positronium-atom scattering using a spherical cavity

Positronium (Ps) scattering by noble-gas atoms (He, Ne, Ar, Kr, and Xe) is studied in the frozen-target approximation and with inclusion of the van der Waals interaction. Single-particle electron and positron states in the field of the target atom are calculated, with the system enclosed by a hard spherical wall. The two-particle Ps wave function is expanded in these states, and the Hamiltonian matrix is diagonalized, giving the Ps energy levels in the cavity. Scattering phase shifts, scattering lengths, and cross sections are extracted from these energies and compared with existing calculations and experimental data. Analysis of the effect of the van der Waals interaction shows that it cannot explain the recent experimental data of Brawley et al. for Ar and Xe [Phys. Rev. Lett. 115, 223201 (2015)].

physics.atom-ph↗

arXiv2017

Vibrational Feshbach Resonances Mediated by Nondipole Positron-Molecule Interactions

Measurements of energy-resolved positron-molecule annihilation show the existence of positron binding and vibrational Feshbach resonances. The existing theory describes this phenomenon successfully for the case of infrared-active vibrational modes which allow dipole coupling between the incident positron and the vibrational motion. Presented here are measurements of positron-molecule annihilation made using a recently developed cryogenic positron beam capable of significantly improved energy resolution. The results provide evidence of resonances associated with infrared-inactive vibrational modes, indicating that positron-molecule bound states may be populated by nondipole interactions. The anticipated ingredients for a theoretical description of such interactions are discussed.

physics.atom-ph↗

arXiv2017

Effective radius of ground- and excited-state positronium in collisions with hard walls

We determine effective collisional radii of positronium (Ps) by considering Ps states in hard-wall spherical cavities. $B$-spline basis sets of electron and positron states inside the cavity are used to construct the states of Ps. Accurate Ps energy eigenvalues are obtained by extrapolation with respect to the numbers of partial waves and radial states included in the bases. Comparison of the extrapolated energies with those of a pointlike particle provides values of the effective radius $ρ_{nl}$ of Ps($nl$) in collisions with a hard wall. We show that for $1s$, $2s$, and $2p$ states of Ps, the effective radius decreases with the increasing Ps center-of-mass momentum, and find $ρ_{1s}=1.65$ a.u., $ρ_{2s}=7.00$ a.u., and $ρ_{2p}=5.35$ a.u. in the zero-momentum limit.

physics.atom-ph↗

arXiv2016

Formation of positron-atom bound states in collisions between Rydberg Ps and neutral atoms

Predicted twenty years ago, positron binding to neutral atoms has not yet been observed experimentally. A new scheme is proposed to detect positron-atom bound states by colliding Rydberg positronium (Ps) with neutral atoms. Estimates of the charge-transfer reaction cross section are obtained using the first Born approximation for a selection of neutral atom targets and a wide range of incident Ps energies and principal quantum numbers. We also estimate the corresponding Ps ionization cross section. The accuracy of the calculations is tested by comparison with earlier predictions for Ps charge transfer in collisions with hydrogen and antihydrogen. We describe an existing Rydberg Ps beam suitable for producing positron-atom bound states and estimate signal rates based on the calculated cross sections and realistic experimental parameters. We conclude that the proposed methodology is capable of producing such states and of testing theoretical predictions of their binding energies.

physics.atom-ph↗

arXiv2015

Positronium collisions with rare-gas atoms

We calculate elastic scattering of positronium (Ps) by the Xe atom using the recently developed pseudopotential method [I. I. Fabrikant and G. F. Gribakin, Phys. Rev. A 90, 052717 (2014)] and review general features of Ps scattering from heavier rare-gas atoms: Ar, Kr, and Xe. The total scattering cross section is dominated by two contributions: elastic scattering and Ps ionization (breakup). To calculate the Ps ionization cross sections we use the binary-encounter method for Ps collisions with an atomic target. Our results for the ionization cross section agree well with previous calculations carried out in the impulse approximation. Our total Ps-Xe cross section, when plotted as a function of the projectile velocity, exhibits similarity with the electron-Xe cross section for the collision velocities higher than 0.8 a.u., and agrees very well with the measurements at Ps velocities above 0.5 a.u.

physics.atom-ph↗

arXiv2015

Effect of dipole polarizability on positron binding by strongly polar molecules

A model for positron binding to polar molecules is considered by combining the dipole potential outside the molecule with a strongly repulsive core of a given radius. Using existing experimental data on binding energies leads to unphysically small core radii for all of the molecules studied. This suggests that electron-positron correlations neglected in the simple model play a large role in determining the binding energy. We account for these by including polarization potential via perturbation theory and non-perturbatively. The perturbative model makes reliable predictions of binding energies for a range of polar organic molecules and hydrogen cyanide. The model also agrees with the linear dependence of the binding energies on the polarizability inferred from the experimental data [Danielson et al 2009 J. Phys. B: At. Mol. Opt. Phys. 42 235203]. The effective core radii, however, remain unphysically small for most molecules. Treating molecular polarization non-perturbatively leads to physically meaningful core radii for all of the molecules studied and enables even more accurate predictions of binding energies to be made for nearly all of the molecules considered.

physics.atom-ph↗

arXiv2015

van der Waals coefficients for positronium interactions with atoms

The random-phase approximation with exchange (RPAE) is used with a $B$-spline basis to compute dynamic dipole polarizabilities of noble-gas atoms and several other closed-shell atoms (Be, Mg, Ca, Zn, Sr, Cd, and Ba). From these, values of the van der Waals $C_6$ constants for positronium interactions with these atoms are determined and compared with existing data. Our best predictions of $C_6$ for Ps--noble-gas pairs are expected to be accurate to within 1%, and to within a few per cent for the alkaline earths. We also used accurate dynamic dipole polarizabilities from the literature to compute the $C_6$ coefficients for the alkali-metal atoms. Implications of increased $C_6$ values for Ps scattering from more polarizable atoms are discussed.

physics.atom-ph↗