Positronium formation from C$_{\mathrm{60}}$ (original) (raw)
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Molecular-size effects on diffraction resonances in positronium formation from fullerenes
Physical Review A
We previously predicted [P.A. Hervieux et al. Phys. Rev. A 95, 020701 (2017)] that owing to predominant electron capture by incoming positrons from the molecular shell, C 60 acts like a spherical diffractor inducing resonances in the positronium (Ps) formation as a function of the positron impact energy. By extending the study for a larger C 240 fullerene target, we now demonstrate that the diffraction resonances compactify in energy in analogy with the shrinking fringe separation for larger slit size in classical single-slit experiment. The result brings further impetus for conducting Ps spectroscopic experiments with fullerene targets, including target-and/or captured-level differential measurements. The ground states of the fullerenes are modeled in a spherical jellium frame of the local density approximation (LDA) method with the exchange-correlation functional based on the van Leeuween and Baerends (LB94) model potential, while the positron impact and Ps formation are treated in the continuum distorted-wave final state (CDW-FS) approximation.
Ubiquitous diffraction resonances in positronium formation from fullerenes
Physical Review A
Due to the dominant electron capture by positrons from the molecular shell and the spatial dephasing across the shell-width, a powerful diffraction effect universally underlies the positronium (Ps) formation from fullerenes. This results into trains of resonances in the Ps formation cross section as a function of the positron beam energy, producing structures in recoil momenta in analogy with classical single-slit diffraction fringes in the configuration space. The work opens a hitherto unknown avenue of Ps spectroscopy with nanomaterials and motivates level-differential measurements.
Optical spectroscopy of atomic and molecular positronium
Journal of Physics: Conference Series, 2014
Positronium (Ps) is a purely leptonic hydrogen-like atom formed from an electron and a positron. Since the interactions of electrons and positrons are thought to be almost entirely electromagnetic, precision measurements of the Ps energy levels should constitute a good test of QED theory. The ultimate precision is limited by the rapid annihilation of the various Ps states and the number of Ps atoms available. Much progress in making better Ps sources has been made since the 1950's when Ps was discovered and its principle characteristics measured in by the pioneering experiments of Martin Deutsch. The most notable milestones were the first reproducible schemes for making slow positrons and Ps in vacuum by Canter and his co-workers in the 1970's and the discovery of the enabling technology for accumulating slow positrons by Surko and co-workers in 1989. These techniques have made it possible to generate high density bursts of slow Ps atoms that has led to the production of dipositronium molecules, Ps 2 , and the observation of the Lyman-alpha-like transition in Ps 2 at a wavelength of 251 nm predicted by Varga and co-workers. The possibilities for 1S-2S spectroscopy of triplet and singlet Ps with precisions relevant to the proton charge radius problem and efficient production of slow Rydberg Ps atoms useful for measuring Ps free fall are discussed. ICPEAC-2013 INVOCATION We met at Lanzhou in a setting fit for kings To learn more about how the world works, Mindful of those who could not join us there.
Aspects of positronium formation in polyatomic molecules
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2006
Using a new simple experimental method for measurement of the positronium (Ps) formation cross-sections, the cross-sections, Q Ps , for many polyatomic molecules have been studied and are presented at 2 eV above the threshold energy of Ps formation E Ps. The experimental method for the Q Ps employed here is based on use of the stronger magnetic field of 31 G (compared to the 9 G used for our total cross-sections) to increase forward scattering effects, and thereby enable determination of Q Ps. The Q Ps results are discussed in relation to the corresponding total cross sections (Q t) for each molecule, i.e. as Q Ps /Q t , for about 50 different molecular gases. As for some of the characteristics of the ratio values observed (i) the ratios for alkane molecules decrease with increasing number of valence electrons, (ii) the ratios for fluoro-alkane molecules are notably lower than those for alkane molecules and (iii) the ratios for polar molecules are very low compared to other cases above.
Differential cross sections for positronium formation in positron–hydrogen scattering
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms, 2008
Positron scattering by hydrogen atom is an interesting system to test theoretical methods due to its simplicity. Recently, theoretical calculations have reported differential cross sections (DCS) for positronium (Ps) formation for this system. The present work utilises the coupled-channel optical method (CCOM) that allows simultaneous treatment of the target channels and the Ps channels in the close-coupling method and the incorporation of the continuum effects via an optical potential to provide a comparative view of the DCS for Ps(1s) formation and Ps(2s) formation at energies ranging from 20 to 100 eV. A large 12-states and 15-states CCOM calculations have been undertaken and the results compared with other available data.
Evidence of two-channel distortion effects in positronium formation reactions
Journal of Physics B: Atomic, Molecular and Optical Physics, 2004
The formation of ground-state positronium in collisions of positrons on hydrogen-like atoms is considered. In previous theoretical works, two-centre distorted wavefunctions were employed to approximate either the initial or the final channel. Here we report results obtained by means of the eikonal final state continuum distorted wave approximation for which asymptotically correct distorted wavefunctions are used for both the initial and final states of the scattering system. Comparison of the present theoretical total cross sections with experimental data reveals that distortion effects become important in both channels as the impact energy decreases. This work also shows that distortedwave theories may be extended from their usual domain of high impact energies to lower ones.
A Self-Consistent Model for Positronium Formation from Helium Atoms
Brazilian Journal of Physics, 2012
The differential and total cross sections for electron capture by positrons from helium atoms are calculated using a first-order distorted wave theory satisfying the Coulomb boundary conditions. In this formalism a parametric potential is used to describe the electron screening in a consistent and realistic manner. The present procedure is self consistent because (i) it satisfies the correct boundary conditions and post-prior symmetry, and (ii) the potential and the electron binding energies appearing in the transition amplitude are consistent with the wave functions describing the collision system. The results are compared with the other theories and with the available experimental measurements. At the considered range of collision energies, the results agree reasonably well with recent experiments and theories.
Four-body treatment of the K-shell positronium formation from multi-electron atoms
Central European Journal of Physics, 2014
The four-body Coulomb-Born distorted-wave approximation with correct boundary conditions (CBDW-4B) is applied to the K-shell positronium formation from multi-electron atoms at intermediate and high impact energies. In the present approach, both K-shell electrons are treated as active electrons. For collisions of positrons with helium, carbon, and neon atoms, both the post and prior forms of the transition amplitude are calculated and the corresponding differential and integral cross sections are compared with the results of the three-body version of the formalism (CBDW-3B). In order to investigate the effects of the static electronic correlations on the process, initial bound states of the active electrons in helium atoms are described by Hylleraas and Silverman wave functions. Also for positronium formation from helium atoms the obtained cross sections are compared with the available experimental data and also with the results of the other theories.
Physica Scripta, 2004
A report is presented of the XIIth International Workshop on Positron and Positronium Physics (Sandbjerg, Denmark, 19-21 July 2003). This workshop covered positron and positronium interactions with atoms, molecules and condensed matter systems. One key development reported was the first creation in the laboratory of low-energy antihydrogen atoms. Facets of positronelectron many-body systems were also considered, including the positronium molecule and BEC gases of positronium atoms. Aspects of the future of the field were discussed, including the development of new theoretical and experimental capabilities.