Research

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5 pages, 432 KiB

Open AccessArticle

A Note on the Opacity of the Sun’s Atmosphere

by Anand. K. Bhatia and William. D. Pesnell

Atoms 2020, 8(3), 37; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8030037 - 21 Jul 2020

Cited by 2 | Viewed by 2827

The opacity of the atmosphere of the Sun is due to processes such as Thomson scattering, bound–bound transitions and photodetachment (bound–free) of hydrogen and positronium ions. The well-studied free–free transitions involving photons, electrons, and hydrogen atoms are re-examined, while free–free transitions involving positrons [...] Read more.

The opacity of the atmosphere of the Sun is due to processes such as Thomson scattering, bound–bound transitions and photodetachment (bound–free) of hydrogen and positronium ions. The well-studied free–free transitions involving photons, electrons, and hydrogen atoms are re-examined, while free–free transitions involving positrons are considered for the first time. Cross sections, averaged over a Maxwellian velocity distribution, involving positrons are comparable to those involving electrons. This indicates that positrons do contribute to the opacity of the atmosphere of the Sun. Accurate results are obtained because definitive phase shifts are known for electron–hydrogen and positron–hydrogen scattering. Full article

(This article belongs to the Special Issue Interactions of Positrons with Matter and Radiation)

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10 pages, 1934 KiB

Open AccessArticle

Nature’s Pick-Up Tool, the Stark Effect Induced Gailitis Resonances and Applications

by Chi-Yu Hu and David Caballero

Atoms 2020, 8(3), 32; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8030032 - 02 Jul 2020

Cited by 1 | Viewed by 1952

Abstract

A simple universal physical mechanism hidden for more than half a century is unexpectedly discovered from a calculation of low excitation antihydrogen. For ease of reference, this mechanism is named Gailitis resonance. We demonstrate, in great detail, that Gailitis resonances are capable of [...] Read more.

A simple universal physical mechanism hidden for more than half a century is unexpectedly discovered from a calculation of low excitation antihydrogen. For ease of reference, this mechanism is named Gailitis resonance. We demonstrate, in great detail, that Gailitis resonances are capable of explaining p+⁷Li low energy nuclear fusion, d-d fusion on a Pd lattice and the initial transient fusion peak in muon catalyzed fusion. Hopefully, these examples will help to identify Gailitis resonances in other systems. Full article

(This article belongs to the Special Issue Interactions of Positrons with Matter and Radiation)

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12 pages, 5838 KiB

Open AccessEditor’s ChoiceArticle

Deep Minima in the Triply Differential Cross Section for Ionization of Atomic Hydrogen by Electron and Positron Impact

by C. M. DeMars, S. J. Ward, J. Colgan, S. Amami and D. H. Madison

Atoms 2020, 8(2), 26; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8020026 - 29 May 2020

Cited by 9 | Viewed by 2597

Abstract

We investigate ionization of atomic hydrogen by electron- and positron-impact. We apply the Coulomb–Born (CB1) approximation, various modified CB1 approximations, the three body distorted wave (3DW) approximation, and the time-dependent close-coupling (TDCC) method to electron-impact ionization of hydrogen. For electron-impact ionization of hydrogen [...] Read more.

We investigate ionization of atomic hydrogen by electron- and positron-impact. We apply the Coulomb–Born (CB1) approximation, various modified CB1 approximations, the three body distorted wave (3DW) approximation, and the time-dependent close-coupling (TDCC) method to electron-impact ionization of hydrogen. For electron-impact ionization of hydrogen for an incident energy of approximately 76.45 eV, we obtain a deep minimum in the CB1 triply differential cross section (TDCS). However, the TDCC for 74.45 eV and the 3DW for 74.46 eV gave a dip in the TDCS. For positron-hydrogen ionization (breakup) we apply the CB1 approximation and a modified CB1 approximation. We obtain a deep minimum in the TDCS and a zero in the CB1 transition matrix element for an incident energy of 100 eV with a gun angle of

{56.13}^{°}

. Corresponding to a zero in the CB1 transition matrix element, there is a vortex in the velocity field associated with this element. For both electron- and positron-impact ionization of hydrogen the velocity field rotates in the same direction, which is anticlockwise. All calculations are performed for a doubly symmetric geometry; the electron-impact ionization is in-plane and the positron-impact ionization is out-of-plane. Full article

(This article belongs to the Special Issue Interactions of Positrons with Matter and Radiation)

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7 pages, 204 KiB

Open AccessArticle

Resonances in Systems Involving Positrons

by Anand K. Bhatia

Atoms 2020, 8(2), 20; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8020020 - 07 May 2020

Cited by 2 | Viewed by 1947

Abstract

When an incident particle on a target gets attached to the target, the cross-section at that energy could be much larger compared to those at other energies. This is a short-lived state and decays by emitting an electron. Such states can also be [...] Read more.

When an incident particle on a target gets attached to the target, the cross-section at that energy could be much larger compared to those at other energies. This is a short-lived state and decays by emitting an electron. Such states can also be formed by the absorption of a photon. Such states are below the higher thresholds and are called autoionization states, doubly excited states, or Feshbach resonances. There is also a possibility of such states to form above the thresholds. Then they are called shape resonances. Resonances are important in the diagnostic of solar and astrophysical plasmas. Some methods of calculating the resonance parameters are described and resonance parameters occurring in various systems are given. Full article

(This article belongs to the Special Issue Interactions of Positrons with Matter and Radiation)

13 pages, 549 KiB

Open AccessArticle

Analytical Results for the Three-Body Radiative Attachment Rate Coefficient, with Application to the Positive Antihydrogen Ion H⁺

by Jack C. Straton

Atoms 2020, 8(2), 13; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8020013 - 20 Apr 2020

Cited by 1 | Viewed by 2038

Abstract

To overcome the numerical difficulties inherent in the Maxwell–Boltzmann integral of the velocity-weighted cross section that gives the radiative attachment rate coefficient

α_{R A}

for producing the negative hydrogen ion H

^{-}

or its antimatter equivalent, the positive antihydrogen ion [...] Read more.

To overcome the numerical difficulties inherent in the Maxwell–Boltzmann integral of the velocity-weighted cross section that gives the radiative attachment rate coefficient

α_{R A}

for producing the negative hydrogen ion H

^{-}

or its antimatter equivalent, the positive antihydrogen ion

{\bar{H}}^{+}

, we found the analytic form for this integral. This procedure is useful for temperatures below 700 K, the region for which the production of

{\bar{H}}^{+}

has potential use as an intermediate stage in the cooling of antihydrogen to ultra-cold (sub-mK) temperatures for spectroscopic studies and probing the gravitational interaction of the anti-atom. Our results, utilizing a 50-term explicitly correlated exponential wave function, confirm our prior numerical results. Full article

(This article belongs to the Special Issue Interactions of Positrons with Matter and Radiation)

7 pages, 669 KiB

Open AccessArticle

Positron Impact Excitation of the nS States of Atomic Hydrogen

by Anand K. Bhatia

Atoms 2020, 8(1), 9; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8010009 - 03 Mar 2020

Cited by 3 | Viewed by 2130

Abstract

The excitation cross sections of the nS states, n = 2 to 6, of atomic hydrogen at various incident positron energies (10.23 to 300 eV) were calculated using the variational polarized-orbital method. Nine partial waves were used to obtain converged cross sections. The [...] Read more.

The excitation cross sections of the nS states, n = 2 to 6, of atomic hydrogen at various incident positron energies (10.23 to 300 eV) were calculated using the variational polarized-orbital method. Nine partial waves were used to obtain converged cross sections. The present results should be useful for comparison with results obtained from other theories and approximations. The positron-impact cross section was found to be higher than the electron-impact cross sections. Experimental and other theoretical results are discussed. The threshold law of excitation is discussed and the cross sections in this region were seen to obey the threshold law proportional to

{(\ln k_{f})}^{- 2}

. Cross sections were calculated in the Born approximation also and compared to those obtained using the variational polarized orbital method. Full article

(This article belongs to the Special Issue Interactions of Positrons with Matter and Radiation)

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10 pages, 1020 KiB

Open AccessArticle

Calculations of Resonance Parameters for the Doubly Excited ¹P° States in Ps⁻ Using Exponentially Correlated Wave Functions

by Sabyasachi Kar and Yew Kam Ho

Atoms 2020, 8(1), 1; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8010001 - 31 Dec 2019

Cited by 4 | Viewed by 2214

Abstract

Recent observations on resonance states of the positronium negative ion (Ps⁻) in the laboratory created huge interest in terms of the calculation of the resonance parameters of the simple three-lepton system. We calculate the resonance parameters for the doubly excited ¹ [...] Read more.

Recent observations on resonance states of the positronium negative ion (Ps⁻) in the laboratory created huge interest in terms of the calculation of the resonance parameters of the simple three-lepton system. We calculate the resonance parameters for the doubly excited ¹P° states in Ps⁻ using correlated exponential wave functions based on the complex-coordinate rotation method. The resonance energies and widths for the ¹P° Feshbach resonance states in Ps⁻ below the N = 2, 3, 4, 5 Ps thresholds are reported. The ¹P° shape resonance above the N = 2, 4 Ps thresholds are also reported. Our predications are in agreement with the available results. Few Feshbach resonance parameters below the N = 4 and 5 Ps thresholds have been reported in the literature. Our predictions will provide useful information for future resonance experiments in Ps⁻. Full article

(This article belongs to the Special Issue Interactions of Positrons with Matter and Radiation)

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4 pages, 227 KiB

Open AccessArticle

Photodetachment of the Positronium Negative Ion with Excitation in the Positronium Atom

by Anand K. Bhatia

Atoms 2019, 7(1), 2; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7010002 - 26 Dec 2018

Cited by 4 | Viewed by 2281

Abstract

Lyman-α radiation

(2 P \to 1 S)

has been seen from astrophysical sources and the sun. The line shape of this transition has been measured recently in Ps atoms both inside and outside a porous silica target. In the photodetachment of [...] Read more.

Lyman-α radiation

(2 P \to 1 S)

has been seen from astrophysical sources and the sun. The line shape of this transition has been measured recently in Ps atoms both inside and outside a porous silica target. In the photodetachment of Ps⁻, the residual Ps atom can be left in the 2P state instead of the 1S state giving rise to positronium Lyman radiation at 2432 A⁰. Photodetachment cross sections of Ps⁻ have been calculated when the Ps atom is left in nP states, n being 2, 3, 4, 5, 6 and 7, using the asymptotic form of the bound-state wave function and a plane wave for the final state wave function, following the approach of Ohmura and Ohmura [Phys. Rev. 1960, 118, 154] in the photodetachment of H⁻. Full article

(This article belongs to the Special Issue Interactions of Positrons with Matter and Radiation)

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Review

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30 pages, 1376 KiB

Open AccessReview

Positron Scattering from Atoms and Molecules

by Sultana N. Nahar and Bobby Antony

Atoms 2020, 8(2), 29; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8020029 - 15 Jun 2020

Cited by 6 | Viewed by 3402

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Interactions of Positrons with Matter and Radiation)

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12 pages, 2551 KiB

Open AccessReview

Positron Processes in the Sun

by Nat Gopalswamy

Atoms 2020, 8(2), 14; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8020014 - 22 Apr 2020

Cited by 5 | Viewed by 4030

Abstract

Positrons play a major role in the emission of solar gamma-rays at energies from a few hundred keV to >1 GeV. Although the processes leading to positron production in the solar atmosphere are well known, the origin of the underlying energetic particles that [...] Read more.

Positrons play a major role in the emission of solar gamma-rays at energies from a few hundred keV to >1 GeV. Although the processes leading to positron production in the solar atmosphere are well known, the origin of the underlying energetic particles that interact with the ambient particles is poorly understood. With the aim of understanding the full gamma-ray spectrum of the Sun, I review the key emission mechanisms that contribute to the observed gamma-ray spectrum, focusing on the ones involving positrons. In particular, I review the processes involved in the 0.511 MeV positron annihilation line and the positronium continuum emissions at low energies, and the pion continuum emission at high energies in solar eruptions. It is thought that particles accelerated at the flare reconnection and at the shock driven by coronal mass ejections are responsible for the observed gamma-ray features. Based on some recent developments I suggest that energetic particles from both mechanisms may contribute to the observed gamma-ray spectrum in the impulsive phase, while the shock mechanism is responsible for the extended phase. Full article

(This article belongs to the Special Issue Interactions of Positrons with Matter and Radiation)

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4 pages, 178 KiB

Open AccessReview

A Precis of Threshold Laws for Positron vs. Electron Impact Ionization of Atoms

by A. Temkin

Atoms 2020, 8(2), 11; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8020011 - 10 Apr 2020

Viewed by 1880

Abstract

The Coulomb-dipole theory of positron vs. electron impact ionization of hydrogen (as a proxy for neutral atoms) is reviewed, emphasizing how the analytic form of the threshold law (but not the magnitude) can be the same, whereas the physics of each is entirely [...] Read more.

The Coulomb-dipole theory of positron vs. electron impact ionization of hydrogen (as a proxy for neutral atoms) is reviewed, emphasizing how the analytic form of the threshold law (but not the magnitude) can be the same, whereas the physics of each is entirely different. Full article

(This article belongs to the Special Issue Interactions of Positrons with Matter and Radiation)

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