Positron scattering from noble gases

Electron and Positron Scattering Cross Sections from CO2: A Comparative Study over a Broad Energy Range (0.1-5000 eV)

In this Review, we present a comparative study between electron and positron scattering cross sections from CO₂ molecules over a broad impact energy range (0.1–5000 eV). For electron scattering, new total electron scattering cross sections (e-TCS) have been measured with a high resolution magnetically confined electron beam transmission system from 1 to 200 eV. Dissociative electron attachment processes for electron energies from 3 to 52 eV have been analyzed by measuring the relative O^– anion production yield. In addition, elastic, inelastic, and total scattering cross section calculations have been carried out in the framework of the Independent Atom Model by using the Screening Corrected Additive Rule, including interference effects (IAM-SCARI). Based on the previous cross section compilation from Itikawa (J. Phys. Chem. Ref. Data, 2002, 31, 749−767) and the present measurements and calculations, an updated recommended e-TCS data set has been used as reference values to obtain a self-consistent integral cross section data set for the elastic and inelastic (vibrational excitation, electronic excitation, and ionization) scattering channels. A similar calculation has been carried out for positrons, which shows important differences between the electron scattering behavior: e.g., more relevance of the target polarization at the lower energies, more efficient excitation of the target at intermediate energies, but a lower total scattering cross section for increasing energies, even at 5000 eV. This result does not agree with the charge independence of the scattering cross section predicted by the first Born approximation (FBA). However, we have shown that the inelastic channels follow the FBA’s predictions for energies above 500 eV while the elastic part, due to the different signs of the scattering potential constituent terms, remains lower for positrons even at the maximum impact energy considered here (5000 eV). As in the case of electrons, a self-consistent set of integral positron scattering cross sections, including elastic and inelastic (vibrational excitation, electronic excitation, positronium formation, and ionization) channels is provided. Again, to derive these data, positron scattering total cross sections based on a previous compilation from Brunger et al. (J. Phys. Chem. Ref. Data, 2017, 46, 023102) and the present calculation have been used as reference values. Data for the main inelastic channels, i.e. direct ionization and positronium formation, derived with this procedure, show excellent agreement with the experimental results available in the literature. Inconsistencies found between different model potential calculations, both for the elastic and inelastic collision processes, suggest that new calculations using more sophisticated methods are required.

Ground-state energy of quasi-free positrons in non-polar fluids

We have calculated the background energy (V₀) for positrons in noble gases with an ab initio potential and the Wigner-Seitz (WS) ansatz. In contrast to the general pseudo-potential approach, we have used accurate ab initio potentials for the positron-atom interaction. The ansatz includes an assumed form of the potential, resulting from an average over fluid atoms, and we propose four different options for this. By comparing the different options to literature data for an effective electron number (Z_eff), we find that agreement can be obtained for light elements but fails for heavy elements. We suspect that the strong polarizability of the heavy elements makes the simple potential averaging, as assumed in the WS model, insufficient to fit the measurements without also making use of pseudo-potentials. We also raise our suspicion that the comparison of annihilation rates between ground-state calculations and experimental values is not appropriate. Furthermore, the congruence of V₀ to Z_eff values predicted by a contact potential approximation appears to be invalidated by our results.

A Fully Relativistic Approach to Photon Scattering and Photoionization of Alkali Atoms

A fully relativistic approach to calculating photoionization and photon-atom scattering cross sections for quasi one-electron atoms is presented. An extensive set of photoionization cross sections have been calculated for alkali atoms: lithium, sodium, potassium, rubidium and cesium. The importance of relativistic effects and core polarization on the depth and position of the Cooper minimum in the photoionization cross section is investigated. Good agreement was found with previous Dirac-based B-spline R-matrix calculations of Zatsarinny and Tayal and recent experimental results.

A comparison of experimental and theoretical low energy positron scattering from furan

This paper presents a joint experimental and theoretical study of positron scattering from furan. Experimental data were measured using the low energy positron beamline located at the Australian National University and cover an energy range from 1 eV to 30 eV. Cross sections were measured for total scattering, total elastic and inelastic scattering, positronium formation, and differential elastic scattering. Two theoretical approaches are presented: the Schwinger multichannel method and the independent atom method with screening corrected additivity rule. In addition, our data are compared to corresponding electron scattering results from the same target with a number of significant differences observed and discussed.

Positron Scattering from Atoms and Molecules

A review on the positron scattering from atoms and molecules is presented in this article. The focus on positron scattering studies is on the rise due to their presence in various fields and application of cross section data in such environments. Positron scattering is usually investigated using theoretical approaches that are similar to those for electron scattering, being its anti-particle. However, most experimental or theoretical studies are limited to the investigation of electron and positron scattering from inert gases, single electron systems and simple or symmetric molecules. Optical potential and polarized orbital approaches are the widely used methods for investigating positron scattering from atoms. Close coupling approach has also been used for scattering from atoms, but for lighter targets with low energy projectiles. The theoretical approaches have been quite successful in predicting cross sections and agree reasonably well with experimental measurements. The comparison is generally good for electrons for both elastic and inelastic scatterings cross sections, while spin polarization has been critical due to its sensitive perturbing interaction. Positron scattering cross sections show relatively less features than that of electron scattering. The features of positron impact elastic scattering have been consistent with experiment, while total cross section requires significant improvement. For scattering from molecules, utilization of both spherical complex optical potential and R-matrix methods have proved to be efficient in predicting cross sections in their respective energy ranges. The results obtained shows reasonable comparison with most of the existing data, wherever available. In the present article we illustrate these findings with a list of comprehensive references to data sources, albeit not exhaustive.

Quantum chemical approach for positron annihilation spectra of atoms and molecules beyond plane-wave approximation

We report theoretical calculations of positron-electron annihilation spectra of noble gas atoms and small molecules using the nuclear orbital plus molecular orbital method. Instead of a nuclear wavefunction, the positronic wavefunction is obtained as the solution of the coupled Hartree-Fock or Kohn-Sham equation for a positron and the electrons. The molecular field is included in the positronic Fock operator, which allows an appropriate treatment of the positron-molecule repulsion. The present treatment succeeds in reproducing the Doppler shift, i.e., full width at half maximum (FWHM) of experimentally measured annihilation (γ-ray) spectra for molecules with a mean absolute error less than 10%. The numerical results indicate that the interpretation of the FWHM in terms of a specific molecular orbital is not appropriate.

Positron Cooling and Annihilation in Noble Gases

Positron cooling and annihilation in room temperature noble gases is simulated using accurate scattering and annihilation cross sections calculated with many-body theory, enabling the first simultaneous probing of the energy dependence of the scattering and annihilation cross sections. A strikingly small fraction of positrons is shown to survive to thermalization: ∼0.1 in He, ∼0 in Ne, ∼0.15 in Ar, ∼0.05 in Kr, and ∼0.01 in Xe. For Xe, the time-varying annihilation rate Z[over ¯]_{eff}(τ) is shown to be highly sensitive to the depletion of the momentum distribution due to annihilation, conclusively explaining the long-standing discrepancy between gas-cell and trap-based measurements. Overall, the use of the accurate atomic data gives Z[over ¯]_{eff}(τ) in close agreement with experiment for all noble gases except Ne, the experiment for which is proffered to have suffered from incomplete knowledge of the fraction of positrons surviving to thermalization and/or the presence of impurities.

Experimental and theoretical cross sections for positron scattering from the pentane isomers

Isomerism is ubiquitous in chemistry, physics, and biology. In atomic and molecular physics, in particular, isomer effects are well known in electron-impact phenomena; however, very little is known for positron collisions. Here we report on a set of experimental and theoretical cross sections for low-energy positron scattering from the three structural isomers of pentane: normal-pentane, isopentane, and neopentane. Total cross sections for positron scattering from normal-pentane and isopentane were measured at the University of Trento at incident energies between 0.1 and 50 eV. Calculations of the total cross sections, integral cross sections for elastic scattering, positronium formation, and electronic excitations plus direct ionization, as well as elastic differential cross sections were computed for all three isomers between 1 and 1000 eV using the independent atom model with screening corrected additivity rule. No definitive evidence of a significant isomer effect in positron scattering from the pentane isomers appears to be present.

Electron scattering and transport in liquid argon

The transport of excess electrons in liquid argon driven out of equilibrium by an applied electric field is revisited using a multi-term solution of Boltzmann's equation together with ab initio liquid phase cross-sections calculated using the Dirac-Fock scattering equations. The calculation of liquid phase cross-sections extends previous treatments to consider multipole polarisabilities and a non-local treatment of exchange, while the accuracy of the electron-argon potential is validated through comparison of the calculated gas phase cross-sections with experiment. The results presented highlight the inadequacy of local treatments of exchange that are commonly used in liquid and cluster phase cross-section calculations. The multi-term Boltzmann equation framework accounting for coherent scattering enables the inclusion of the full anisotropy in the differential cross-section arising from the interaction and the structure factor, without an a priori assumption of quasi-isotropy in the velocity distribution function. The model, which contains no free parameters and accounts for both coherent scattering and liquid phase screening effects, was found to reproduce well the experimental drift velocities and characteristic energies.

Low energy positron interactions with uracil--total scattering, positronium formation, and differential elastic scattering cross sections

Measurements of the grand total and total positronium formation cross sections for positron scattering from uracil have been performed for energies between 1 and 180 eV, using a trap-based beam apparatus. Angular, quasi-elastic differential cross section measurements at 1, 3, 5, 10, and 20 eV are also presented and discussed. These measurements are compared to existing experimental results and theoretical calculations, including our own calculations using a variant of the independent atom approach.

Positron interactions with water-total elastic, total inelastic, and elastic differential cross section measurements

Utilising a high-resolution, trap-based positron beam, we have measured both elastic and inelastic scattering of positrons from water vapour. The measurements comprise differential elastic, total elastic, and total inelastic (not including positronium formation) absolute cross sections. The energy range investigated is from 1 eV to 60 eV. Comparison with theory is made with both R-Matrix and distorted wave calculations, and with our own application of the Independent Atom Model for positron interactions.