Configuration-interaction calculations of positron binding to zinc and cadmium

The configuration-interaction method is applied to the study of positronic zinc (e+Zn) and positronic cadmium (e+Cd). The estimated binding energies and annihilation rates were 0.00373 hartree and 0.42×109 sec-1 for e+Zn and 0.006 10 hartree and 0.56×109 sec-1 for e+Cd. The low-energy elastic cross section and Zeff were estimated from a model potential that was tuned to the binding energies and annihilation rates. Since the scattering lengths were positive (14.5a0 for Zn and 11.6a0 for Cd) the differential cross sections are larger at backward angles than at forward angles just above threshold. The possibilities of measuring differential cross sections to confirm positron binding to these atoms is discussed

Positron and positronium interactions with Cu

The configuration-interaction (CI) method is used to investigate the interactions of positrons and positronium with copper at low energies. The calculations were performed within the framework of the fixed-core approximation with semiempirical polarization potentials used to model dynamical interactions between the active particles and the (1s-3d) core. Initially, calculations upon the e(+)Li system were used to refine the numerical procedures and highlighted the extreme difficulties of using an orthodox CI calculation to describe the e(+)Li system. The positron binding energy of e(+) Cu derived from a CI calculation which included electron and positron orbitals with l less than or equal to 18 was. 0.005 12 hartree while the spin-averaged annihilation rate was 0.507 x 10(9) s(-1). The configuration basis used for the bound-state calculation was also used as a part of the trial wave function for a Kohn variational calculation of positron-copper scattering. The positron-copper system has a scattering length of about 13.1a(0) and the annihilation parameter Z(eff) at threshold was 72.9. The dipole polarizability of the neutral copper ground state was computed and found to be 41.6a(0)(3). The structure of CuPs was also studied with the CI method and it was found to have a binding energy of 0.0143 hartree and an annihilation rate of similar to2 x 10(9) s(-1)

Momentum transfer cross sections for the heavy noble gases

We present momentum transfer cross sections for elastic electron scattering from argon, krypton and xenon atoms over the energy range from zero to 1 keV. These have been calculated using the Dirac equations with a relativistic complex optical potential which includes polarization of the target atom by the incident electron and allows for the absorption of some of the incident electron flux into channels representing excitation and ionization of the atom. In order to aid in plasma modelling calculations, we provide simple analytic fits to these cross sections as well as to the elastic scattering cross sections. Comparisons are made with previous experimental and theoretical results

A multi-term solution of the space-time Boltzmann equation for electrons in gases and liquids

In this study we have developed a full multi-term space-time solution of Boltzmann's equation for electron transport in gases and liquids. A Green's function formalism is used that enables flexible adaptation to various experimental systems. The spatio-temporal evolution of electrons in liquids in the non-hydrodynamic regime is benchmarked for a model Percus-Yevick (PY) liquid against an independent Monte Carlo simulation, and then applied to liquid argon. The temporal evolution of Franck-Hertz oscillations in configuration and energy space are observed for the model liquid with large differences apparent when compared to the dilute gas case, for both the velocity distribution function components and the transport quantities. The packing density in the PY liquid is shown to influence both the magnitude and wavelength of Franck-Hertz oscillations of the steady-state Townsend (SST) simulation. Transport properties are calculated from the non-hydrodynamic theory in the long time limit under SST conditions which are benchmarked against hydrodynamic transport coefficients. Finally, the spatio-temporal relaxation of low-energy electrons in liquid argon was investigated, with striking differences evident in the spatio-temporal development of the velocity distribution function components between the uncorrelated gas and true liquid approximations, due largely to the presence of a Ramsauer minimum in the former and not in the latter

Integral cross sections for electron-magnesium scattering over a broad energy range (0-5000 eV)

We report the results from the application of our optical potential and relativistic optical potential (ROP) methods to electron-magnesium scattering. The energy range of this study was 0-5000 eV, with the results for the integral elastic cross sections, summed discrete electronic-state excitation integral cross sections, momentum transfer cross sections, and total ionisation cross sections being reported. Where possible, we compare the present results to the available experimental data and to the earlier results from close coupling and R-matrix type computations. Typically, a quite fair level of accord is found between our ROP calculations and the earlier theoretical and experimental cross sections. Additionally, from the assembled database, we provide for the modeling community some recommended cross section sets for use in their simulations, in which magnesium is a constituent. Electron transport coefficients are subsequently calculated for reduced electric fields ranging from 0.1 to 1000 Td using a multi-term solution of Boltzmann's equation. Substantial differences in the transport coefficients between the ROP calculations and the recommended cross sections are observed over the range of fields considered, clearly illustrating the importance of the veracity of the database in the simulations

Positron Scattering from the Group IIB Metals Zinc and Cadmium: Recommended Cross Sections and Transport Simulations

15 pags., 5 figs., 3 tabs.Results from the application of our optical potential and relativistic optical potential models to positron scattering from gas-phase zinc (Zn) and cadmium (Cd) are presented. In particular, integral cross sections (ICSs) for elastic scattering, positronium formation, summed discrete electronic-state excitation, and ionization scattering processes are reported for both species and over an extended incident positron energy range. From those ICSs, the total cross section is subsequently constructed by taking their sum. We note that there are currently no experimental data available for any of these scattering processes for either species, with earlier computational results being limited to the elastic channel and restricted to relatively narrow incident positron energy regimes. Nonetheless, we construct recommended positron cross section datasets for both zinc and cadmium over the incident positron energy range of 0-10 000 eV. The recommended positron cross section data are subsequently employed in a multi-term Boltzmann equation analysis to simulate the transport of positrons, under the influence of an applied (external) electric field, through the background Zn and Cd gases. Qualitatively similar behavior in the calculated transport coefficients was observed between both species. Finally, for the case of zinc, the present positron transport coefficients are compared against corresponding results from electron transport with some significant differences now being observed.This work was financially supported, in part, by the Australian Research Council (Project Nos. DP180101655 and DP190100696), the Spanish Ministerio de Ciencia, Innovacion y Universidades (Project No. PID2019–104727RB-C21), and the CSIC (Project No. LINKA20085)

Positron scattering from gas-phase beryllium and magnesium: theory, recommended cross sections, and transport simulations

Results from the application of our optical potential and relativistic optical potential models to positron scattering from gas-phase beryllium (Be) and magnesium (Mg) are presented. Specifically, total cross sections and integral cross sections for the elastic, positronium formation, summed discrete electronic-state excitation, and ionization scattering processes are reported for both species and over an extended incident positron energy range. Where possible, these results are compared against the existing theoretical and experimental data, although it must be noted here that no current measurements are yet available for Be and those that are available for Mg are largely restricted to the total cross section. Nonetheless, on the basis of that comparison, recommended cross section datasets, for all the aforementioned cross sections, are formed. Those recommended cross section data are subsequently employed in a Boltzmann equation analysis to simulate the transport of positrons, under the influence of an applied (external) electric field, through the background Be and Mg gases. Note that relativistic optical potential results for the elastic momentum transfer cross section are also reported, to allow us to account for anisotropy effects in our transport simulations. Finally, our positron simulation results for quantities such as the ionization rate coefficients and flux and bulk drift velocities are compared with the corresponding electron transport results with significant differences being observed

Integral cross Sections for electron-zinc scattering over a broad energy range (0.01-5000 eV)

We report results from the application of our optical potential and relativistic optical potential methods to electron-zinc scattering. The energy range of this study was 0.01-5000 eV, with original results for the summed discrete electronic-state integral excitation cross sections and total ionization cross sections being presented here. When combined with our earlier elastic scattering data [Marinkovic et al., Phys. Rev. A 99, 062702 (2019)], and the quite limited experimental and theoretical results for those processes from other groups, we critically assemble a recommended integral cross section database for electron-zinc scattering. Electron transport coefficients are subsequently calculated for reduced electric fields ranging from 0.1 to 1000 Td, using a multiterm solution of Boltzmann's equation. Some differences with corresponding results from the earlier study of White et al. [J. Phys. D: Appl. Phys. 37, 3185 (2004)] were noted, indicating in part the necessity of having accurate and complete cross section data, over a wide energy regime, when undertaking such transport simulations