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.Comment: 7 pages, 7 figures, submitted to J. Phys.

Positronium collisions with rare-gas atoms: Free-electron gas plus orthogonalizing pseudopotential model

Positronium collisions with rare-gas atoms are treated using the free-electron-gas approximation for exchange and correlation potential. The results confirm the absence of the Ramsauer-Townsend minimum in elastic scattering cross sections, but show lower cross sections in the lower-energy region when compared to previous pseudopotential calculations. This is explained by a more attractive ab initio correlation potential as compared to the previously used empirical potential. The results in the thermal-energy region agree very well with most swarm measurements for all rare-gas atoms. At higher energies, the results are compared with beam experiments and agreement for heavier rare-gas atoms Ar, Kr, and Xe is found to be very good. For He and Ne, some discrepancies with beam measurements are observed. This is explained by a poorer performance of the free-electron-gas potentials, based on the statistical Thomas-Fermi model, for systems with fewer electrons

Empirical study of cartograms

We report on an empirical study investigating the effectiveness and efficiency of spatial inference making with contiguous (value-by-area) cartograms, compared to informational equivalent choropleth maps, combined with graduated circles. We find significant differences in people's inference-making performance dependent on the map type. Overall, results suggest that the choropleth map with graduated circles is more effective and more efficient than the cartogram for the analysis of population census data. However, map effectiveness and efficiency also significantly depends on the inference task complexity, and more surprisingly, on the shape characteristics of the depicted enumeration units. For simple tasks, cartograms seem as effective and efficient as the more traditional mapping method. For complex inference questions, inference performance with cartograms is significantly dependent on whether regular or irregular zones are distorted. As we know still very little about the perception and cognition of cartograms, we hope to shed new light for this intriguing mapping method with this empirical study

Semiempirical \u3ci\u3eR\u3c/i\u3e-matrix theory of low energy electron–CF\u3csub\u3e3\u3c/sub\u3eCl inelastic scattering

We apply a semiempirical R-matrix theory to calculations of vibrational excitation and dissociative attachment in the CF3Cl molecule for electron energies below about 3 eV. We employ two sets of model parameters corresponding to two different forms of the CF3Cl− potential curve. We find that our present, ab initio calculated anion curve gives vibrational excitation and dissociative attachment cross sections in good agreement with experimental measurements. We also compare the results of our theory with those of a recently published classical theory

Near-threshold behavior of positronium-antiproton scattering

Using the convergent close-coupling theory we study the threshold behavior of cross sections for positronium (Ps) of energy E scattering on antiprotons. In the case of Ps(1s) elastic scattering, simple power laws are observed for all partial waves studied. The partial-wave summed cross section is nearly constant, and dominates the antihydrogen formation cross section at all considered energies, even though the latter is exothermic and behaves as 1/E1/2. For Ps(2s), oscillations spanning orders of magnitude on top of the 1/E behavior are found in the elastic and quasielastic cross sections. The antihydrogen formation is influenced by dipole-supported resonances below the threshold of inelastic processes. Resonance energies form a geometric progression relative to the threshold. The exothermic antihydrogen formation cross sections behave as 1/E at low energies, but are oscillation free. We demonstrate that all these rich features are reproduced by the threshold theory developed by Gailiti

Positronium collisions with polar molecules

We calculate elastic and positronium (Ps) break-up cross sections for collisions of Ps with the polar molecules CO, HCl, and LiF in the fixed-nuclei approximation. We incorporate electron exchange and correlation for these processes by using the free-electron-gas model developed earlier for Ps scattering by rare-gas atoms, N2, O2, and CO2 molecules. The present target molecules provide a range of dipole moments from the weakly polar CO to the strongly polar LiF. We find that Ps scattering is similar to electron scattering when the cross sections are plotted as a function of projectile velocity for the targets with smaller dipole moments (CO, HCl). However, we do not see such a similarity for LiF which has a large dipole moment. Below the Ps break-up threshold we observe resonance structures similar to those obtained earlier for the other molecular targets that we have studied

Modifying the photodetachment near a metal surface by a weak electric field

We show the photodetachment cross sections of H near a metal surface can be modified using a weak static electric field. The modification is possible because the oscillatory part of the cross section near a metal surface is directly connected with the transit-time and the action of the detached-electron closed-orbit which can be changed systematically by varying the static electric field strength. Photodetachment cross sections for various photon energies and electric field values are calculated and displayed.Comment: 16 pages, 7 figure

Thermal electron attachment to F2

Rate constants have been measured from 300 to 700 K for thermal electron attachment to F2 using two flowing afterglow–Langmuir probe apparatuses. Dissociative attachment yielding F− is observed with a rate constant of 5.0 ± 1.3 × 10−9 cm3 s−1 at 300 K, rising to 9.6 ± 2.4 × 10−9 cm3 s−1 at 700 K, well below the previously accepted values of McCorkle et al. [D. L.McCorkle, L. G. Christophorou, A. A. Christodoulides, and L. Pichiarella, J. Chem. Phys. 85, 1966 (1986)]. The absolute concentration of F2 reaching the afterglowis verified by measuring the near-collisional rate constant (4.5 ± 1.5 × 10−10 cm3 s−1) for Ar+ + F2→ArF+ + F. Prior attempts to apply R-matrix calculations to the F2 + e− system have failed to explain previously reported thermal and nonthermal attachment rate constants along with high-resolution, low-energy attachment cross sections. The present results are reproduced exceptionally well by R-matrix calculations employing previously calculated resonance widths without adjustment

Coupled-Channel Theory of Photoionization Microscopy

We develop a quantum-mechanical coupled-channel theory to simulate spatial distributions of electron current densities, produced in photoionization for nonhydrogenic atoms in the presence of a uniform external electric field. The coupled Schrodinger equations are numerically solved using the renormalized Numerov method. The expression for the outgoing wave function for photoelectrons ejected from the nonhydrogenic atomic source is derived. The theory is applied to investigations of photoionization for ground-state Li atoms. The distributions of electron current densities are computed and compared to the corresponding experimental images. Excellent agreement is obtained. It is, furthermore, found that the presence of the nonhydrogenic residual ion significantly changes the differential cross sections and/or electron current densities with respect to the hydrogenic case. Finally, the implications of the presence of the atomic core for quantum resonance tunneling are also analyzed