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*Atoms*
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Open Access Journal

*Atoms*(ISSN 2218-2004; CODEN: ATOMC5) is an international, peer-reviewed and cross-disciplinary scholarly journal of scientific studies related to all aspects of the atom published quarterly online by MDPI.

**Open Access**free for readers, with article processing charges (APC) paid by authors or their institutions.**High visibility:**Indexed in the Emerging Sources Citation Index (**ESCI**- Web of Science),**Scopus**(from Vol. 5, 2017),**ADS - Astrophysics Data System**and INSPEC (IET).**Rapid publication:**manuscripts are peer-reviewed and a first decision provided to authors approximately 25.3 days after submission; acceptance to publication is undertaken in 7.2 days (median values for papers published in this journal in the second half of 2018).**Recognition of reviewers:**reviewers who provide timely, thorough peer-review reports receive vouchers entitling them to a discount on the APC of their next publication in any MDPI journal, in appreciation of the work done.

## Latest Articles

Open AccessArticle

Frozen Core Approximation and Nuclear Screening Effects in Single Electron Capture Collisions

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*Atoms*

**2019**,

*7*(2), 44; https://doi.org/10.3390/atoms7020044 - 12 April 2019

**Abstract**

Differential cross sections (DCS) for single electron capture from helium by heavy ion impact are calculated using a frozen core 3-body model and an active electron 4-body model within the first Born approximation. DCS are presented for H

^{+}, He^{2+}, [...] Read more.
Differential cross sections (DCS) for single electron capture from helium by heavy ion impact are calculated using a frozen core 3-body model and an active electron 4-body model within the first Born approximation. DCS are presented for H

^{+}, He^{2+}, Li^{3+}, and C^{6+}projectiles with velocities of 1 MeV/amu and 10 MeV/amu. In general, the DCS from the two models are found to differ by about one to two orders of magnitude with the active electron 4-body model showing better agreement with experiment. Comparison of the models reveals two possible sources of the magnitude difference: the inactive electron’s change of state and the projectile–target Coulomb interaction used in the different models. Detailed analysis indicates that the uncaptured electron’s change of state can safely be neglected in the frozen core approximation, but that care must be used in modeling the projectile–target interaction. Full articleFigure 1

Open AccessArticle

Turbulent Intermittency in a Random Fiber Laser

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by Antônio M. S. Macêdo, Iván R. Roa González, Ernesto P. Raposo, Leonardo de S. Menezes and Anderson S. L. Gomes

*Atoms*

**2019**,

*7*(2), 43; https://doi.org/10.3390/atoms7020043 - 9 April 2019

**Abstract**

In fluid turbulence, intermittency is the emergence of non-Gaussian tails in the distribution of velocity increments in small space and/or time scales. Intermittence is thus expected to gradually disappear as one moves from small to large scales. Here we study the turbulent-like intermittency
[...] Read more.

In fluid turbulence, intermittency is the emergence of non-Gaussian tails in the distribution of velocity increments in small space and/or time scales. Intermittence is thus expected to gradually disappear as one moves from small to large scales. Here we study the turbulent-like intermittency effect experimentally observed in the distribution of intensity fluctuations in a disordered continuous-wave-pumped erbium-doped-based random fiber laser with specially-designed random fiber Bragg gratings. The intermittency effect is investigated as a crossover in the distribution of intensity increments from a heavy-tailed distribution (for short time scales), to a Gaussian distribution (for large time scales). The results are theoretically supported by a hierarchical stochastic model that incorporates Kolmogorov’s theory of turbulence. In particular, the discrete version of the hierachical model allows a general direct interpretation of the number of relevant scales in the photonic hierarchy as the order of the transitions induced by the non-linearities in the medium. Our results thus provide further statistical evidence for the interpretation of the turbulence-like emission previously observed in this system.
Full article

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Open AccessArticle

Testing Quantum Coherence in Stochastic Electrodynamics with Squeezed Schrödinger Cat States

*Atoms*

**2019**,

*7*(2), 42; https://doi.org/10.3390/atoms7020042 - 5 April 2019

**Abstract**

The interference pattern in electron double-slit diffraction is a hallmark of quantum mechanics. A long-standing question for stochastic electrodynamics (SED) is whether or not it is capable of reproducing such effects, as interference is a manifestation of quantum coherence. In this study, we
[...] Read more.

The interference pattern in electron double-slit diffraction is a hallmark of quantum mechanics. A long-standing question for stochastic electrodynamics (SED) is whether or not it is capable of reproducing such effects, as interference is a manifestation of quantum coherence. In this study, we used excited harmonic oscillators to directly test this quantum feature in SED. We used two counter-propagating dichromatic laser pulses to promote a ground-state harmonic oscillator to a squeezed Schrödinger cat state. Upon recombination of the two well-separated wavepackets, an interference pattern emerges in the quantum probability distribution but is absent in the SED probability distribution. We thus give a counterexample that rejects SED as a valid alternative to quantum mechanics.
Full article

Open AccessCommunication

New Solar Metallicity Measurements

*Atoms*

**2019**,

*7*(2), 41; https://doi.org/10.3390/atoms7020041 - 4 April 2019

**Abstract**

In the past years, a systematic downward revision of the metallicity of the Sun has led to the “solar modeling problem”, namely the disagreement between predictions of standard solar models and inferences from helioseismology. Recent solar wind measurements of the metallicity of the
[...] Read more.

In the past years, a systematic downward revision of the metallicity of the Sun has led to the “solar modeling problem”, namely the disagreement between predictions of standard solar models and inferences from helioseismology. Recent solar wind measurements of the metallicity of the Sun, however, provide once more an indication of a high-metallicity Sun. Because of the effects of possible residual fractionation, the derived value of the metallicity ${Z}_{\odot}=0.0196\pm 0.0014$ actually represents a lower limit to the true metallicity of the Sun. However, when compared with helioseismological measurements, solar models computed using these new abundances fail to restore agreement, owing to the implausibly high abundance of refractory (Mg, Si, S, Fe) elements, which correlates with a higher core temperature and hence an overproduction of solar neutrinos. Moreover, the robustness of these measurements is challenged by possible first ionization potential fractionation processes. I will discuss these solar wind measurements, which leave the “solar modeling problem” unsolved.
Full article

Open AccessArticle

Laser-Assisted (e, 2e) Collisions in the Symmetric/Asymmetric Coplanar Geometry

*Atoms*

**2019**,

*7*(2), 40; https://doi.org/10.3390/atoms7020040 - 2 April 2019

**Abstract**

In this review, we present a comprehensive survey of laser-assisted (e, 2e) reactions. The influence of a laser field on the dynamics of (e, 2e) collisions in atomic hydrogen is analyzed in the symmetric and asymmetric coplanar geometries. Particular attention is devoted to
[...] Read more.

In this review, we present a comprehensive survey of laser-assisted (e, 2e) reactions. The influence of a laser field on the dynamics of (e, 2e) collisions in atomic hydrogen is analyzed in the symmetric and asymmetric coplanar geometries. Particular attention is devoted to the construction of the dressed (laser-modified) target wave functions, in both the initial and final states. The calculation is performed in the framework of Coulomb-Volkov-Born approximation, where the initial and final electrons are described by Volkov wave functions, while the interaction of the incident electron with the target atom is treated in the first and the second Born approximation. The state of the ejected electron is described by a Volkov/Coulomb-Volkov wave function. A detailed account is also given of the techniques we have used to evaluate the scattering amplitudes. The influence of the laser parameters (frequency, intensity, and direction of polarization) on the angular distribution of the ejected electron is discussed, and a number of illustrative examples are given. The structure of the triple differential cross section in the vicinity of resonances is also analyzed.
Full article

Open AccessArticle

The Role of Vacuum Fluctuations and Symmetry in the Hydrogen Atom in Quantum Mechanics and Stochastic Electrodynamics

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*Atoms*

**2019**,

*7*(2), 39; https://doi.org/10.3390/atoms7020039 - 31 March 2019

**Abstract**

Stochastic Electrodynamics (SED) has had success modeling black body radiation, the harmonic oscillator, the Casimir effect, van der Waals forces, diamagnetism, and uniform acceleration of electrodynamic systems using the stochastic zero-point fluctuations of the electromagnetic field with classical mechanics. However the hydrogen atom,
[...] Read more.

Stochastic Electrodynamics (SED) has had success modeling black body radiation, the harmonic oscillator, the Casimir effect, van der Waals forces, diamagnetism, and uniform acceleration of electrodynamic systems using the stochastic zero-point fluctuations of the electromagnetic field with classical mechanics. However the hydrogen atom, with its 1/r potential remains a critical challenge. Numerical calculations have shown that the SED field prevents the electron orbit from collapsing into the proton, but, eventually the atom becames ionized. We look at the issues of the H atom and SED from the perspective of symmetry of the quantum mechanical Hamiltonian, used to obtain the quantum mechanical results, and the Abraham-Lorentz equation, which is a force equation that includes the effects of radiation reaction, and is used to obtain the SED simulations. We contrast the physical computed effects of the quantized electromagnetic vacuum fluctuations with the role of the real stochastic electromagnetic field.
Full article

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Open AccessArticle

Radiative Transition Parameters in Atomic Lanthanum from Pseudo-Relativistic Hartree–Fock and Fully Relativistic Dirac–Hartree–Fock Calculations

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*Atoms*

**2019**,

*7*(1), 38; https://doi.org/10.3390/atoms7010038 - 20 March 2019

**Abstract**

Calculated radiative transition probabilities and oscillator strengths are reported for 392 lines of neutral lanthanum (La I) atom in the spectral range from the near ultraviolet to the mid infrared. They were obtained using two different theoretical methods based on the pseudo-relativistic Hartree–Fock
[...] Read more.

Calculated radiative transition probabilities and oscillator strengths are reported for 392 lines of neutral lanthanum (La I) atom in the spectral range from the near ultraviolet to the mid infrared. They were obtained using two different theoretical methods based on the pseudo-relativistic Hartree–Fock (HFR) and the fully relativistic multiconfiguration Dirac–Hartree–Fock (MCDHF) approaches, both including the most important intravalence and core-valence electron correlations. The quality of these radiative parameters was assessed through detailed comparisons between the results obtained using different physical models and between our theoretical results and the experimental data, where available. Of the total number of La I lines listed in the present work, about 60% have

*gf*- and*gA*-values determined for the first time. Full articleFigure 1

Open AccessReview

High-Precision Atomic Mass Measurements for Fundamental Constants

*Atoms*

**2019**,

*7*(1), 37; https://doi.org/10.3390/atoms7010037 - 18 March 2019

**Abstract**

Atomic mass measurements are essential for obtaining several of the fundamental constants. The most precise atomic mass measurements, at the 10

^{−10}level of precision or better, employ measurements of cyclotron frequencies of single ions in Penning traps. We discuss the relation of [...] Read more.
Atomic mass measurements are essential for obtaining several of the fundamental constants. The most precise atomic mass measurements, at the 10

^{−10}level of precision or better, employ measurements of cyclotron frequencies of single ions in Penning traps. We discuss the relation of atomic masses to fundamental constants in the context of the revised SI. We then review experimental methods, and the current status of measurements of the masses of the electron, proton, neutron, deuteron, tritium, helium-3, helium-4, oxygen-16, silicon-28, rubidium-87, and cesium-133. We conclude with directions for future work. Full articleOpen AccessArticle

Universal Scattering of Ultracold Atoms and Molecules in Optical Potentials

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*Atoms*

**2019**,

*7*(1), 36; https://doi.org/10.3390/atoms7010036 - 15 March 2019

**Abstract**

Universal collisions describe the reaction of molecules and atoms as dominated by long-range interparticle interactions. Here, we calculate the universal inelastic rate coefficients for a large group of ultracold polar molecules in their lower ro-vibrational states colliding with one of their constituent atoms.
[...] Read more.

Universal collisions describe the reaction of molecules and atoms as dominated by long-range interparticle interactions. Here, we calculate the universal inelastic rate coefficients for a large group of ultracold polar molecules in their lower ro-vibrational states colliding with one of their constituent atoms. The rate coefficients are solely determined by values of the dispersion coefficient and reduced mass of the collisional system. We use the

*ab initio*coupled-cluster linear response method to compute dynamic molecular polarizabilities and obtain the dispersion coefficients for some of the collisional partners and use values from the literature for others. Our polarizability calculations agree well with available experimental measurements. Comparison of our inelastic rate coefficients with results of numerically exact quantum-mechanical calculations leads us to conjecture that collisions with heavier atoms can be expected to be more universal. Full articleFigure 1

Open AccessReview

Interrelationship between Lab, Space, Astrophysical, Magnetic Fusion, and Inertial Fusion Plasma Experiments

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*Atoms*

**2019**,

*7*(1), 35; https://doi.org/10.3390/atoms7010035 - 11 March 2019

**Abstract**

The objectives of this review are to articulate geospace, heliospheric, and astrophysical plasma physics issues that are addressable by laboratory experiments, to convey the wide range of laboratory experiments involved in this interdisciplinary alliance, and to illustrate how lab experiments on the centimeter
[...] Read more.

The objectives of this review are to articulate geospace, heliospheric, and astrophysical plasma physics issues that are addressable by laboratory experiments, to convey the wide range of laboratory experiments involved in this interdisciplinary alliance, and to illustrate how lab experiments on the centimeter or meter scale can develop, through the intermediary of a computer simulation, physically credible scaling of physical processes taking place in a distant part of the universe over enormous length scales. The space physics motivation of laboratory investigations and the scaling of laboratory plasma parameters to space plasma conditions, having expanded to magnetic fusion and inertial fusion experiments, are discussed. Examples demonstrating how laboratory experiments develop physical insight, validate or invalidate theoretical models, discover unexpected behavior, and establish observational signatures for the space community are presented. The various device configurations found in space-related laboratory investigations are outlined.
Full article

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Open AccessReview

Electron Scattering Processes in Non-Monochromatic and Relativistically Intense Laser Fields

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*Atoms*

**2019**,

*7*(1), 34; https://doi.org/10.3390/atoms7010034 - 6 March 2019

**Abstract**

The theoretical analysis of four fundamental laser-assisted non-linear scattering processes are summarized in this review. Our attention is focused on Thomson, Compton, Møller and Mott scattering in the presence of intense electromagnetic radiation. Depending on the phenomena under considerations, we model the laser
[...] Read more.

The theoretical analysis of four fundamental laser-assisted non-linear scattering processes are summarized in this review. Our attention is focused on Thomson, Compton, Møller and Mott scattering in the presence of intense electromagnetic radiation. Depending on the phenomena under considerations, we model the laser field as a single laser pulse of ultrashort duration (for Thomson and Compton scattering) or non-monochromatic trains of pulses (for Møller and Mott scattering).
Full article

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Open AccessArticle

Electron-Induced Chemistry in the Condensed Phase

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*Atoms*

**2019**,

*7*(1), 33; https://doi.org/10.3390/atoms7010033 - 4 March 2019

**Abstract**

Electron–molecule interactions have been studied for a long time. Most of these studies have in the past been limited to the gas phase. In the condensed-phase processes that have recently attracted attention from academia as well as industry, a theoretical understanding is mostly
[...] Read more.

Electron–molecule interactions have been studied for a long time. Most of these studies have in the past been limited to the gas phase. In the condensed-phase processes that have recently attracted attention from academia as well as industry, a theoretical understanding is mostly based on electron–molecule interaction data from these gas phase experiments. When transferring this knowledge to condensed-phase problems, where number densities are much higher and multi-body interactions are common, care must be taken to critically interpret data, in the light of this chemical environment. The paper presented here highlights three typical challenges, namely the shift of ionization energies, the difference in absolute cross-sections and branching ratios, and the occurrence of multi-body processes that can stabilize otherwise unstable intermediates. Examples from recent research in astrochemistry, where radiation driven chemistry is imminently important are used to illustrate these challenges.
Full article

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Open AccessArticle

Cavity-Enhanced Photodetachment of H

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^{−}as a Means to Produce Energetic Neutral Beams for Plasma Heating*Atoms*

**2019**,

*7*(1), 32; https://doi.org/10.3390/atoms7010032 - 1 March 2019

**Abstract**

Neutral beam injection, for plasma heating, will supposedly be achieved, in ITER, by collisional detachment of a pre-accelerated D${}^{-}$ beam. Collisional detachment, however, makes use of a D${}_{2}$ -filled neutralisation chamber, which has severe drawbacks, including the necessity to set the
[...] Read more.

Neutral beam injection, for plasma heating, will supposedly be achieved, in ITER, by collisional detachment of a pre-accelerated D${}^{-}$ beam. Collisional detachment, however, makes use of a D${}_{2}$ -filled neutralisation chamber, which has severe drawbacks, including the necessity to set the D${}^{-}$ -ion source at −1 MV. Photodetachment, in contradistinction, would have several advantages as a neutralisation method, including the absence of gas injection, and the possibility to set the ion source close to the earth potential. Photodetachment, however, requires a very high laser flux. The presented work has consisted in implementing an optical cavity, with a finesse greater than 3000, to make such a high illumination possible with a state-of-the-art CW (continuous-wave) laser. A 1.2 keV ${}^{1}{\mathrm{H}}^{-}$ -beam (only 20 times slower than the 1 MeV ${}^{2}{\mathrm{D}}^{-}$ ion beams to be prepared for ITER) was photodetached with more-than-50% efficiency, with only 24 W of CW laser input. This experimental demonstration paves the way for developing real-size photoneutralizers, based on the implementation of refolded optical cavities around the ion beams of neutral beam injectors. Depending on whether the specifications of the laser power or the cavity finesse will be more difficult to achieve in real scale, different architectures can be considered, with greater or smaller numbers of optical refoldings or (inclusively) optical cavities in succession, on the beam to be neutralised.
Full article

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Open AccessArticle

Distortion of the Ionization Cross Section of He by the Coherence Properties of a C

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^{6+}Beam*Atoms*

**2019**,

*7*(1), 31; https://doi.org/10.3390/atoms7010031 - 1 March 2019

**Abstract**

We analyze the influence of the coherence of the projectile’s beam in scattering phenomena. We focus our study in the ionization of He by ${\mathrm{C}}^{6+}$ projectiles at 100 MeV/amu. We assess the influence of this effect by performing a Born initial
[...] Read more.

We analyze the influence of the coherence of the projectile’s beam in scattering phenomena. We focus our study in the ionization of He by ${\mathrm{C}}^{6+}$ projectiles at 100 MeV/amu. We assess the influence of this effect by performing a Born initial state and continuum distorted wave final state (CDW-B1) calculation together with a rigorous procedure to account for the initial coherence properties of the projectile’s beam. These calculations, which had been previously performed for only the scattering and perpendicular collision planes and within the First Born approximation (FBA), were repeated for an ampler set of collision planes. Additionally, a more refined method to describe the applicability of the aforementioned procedure, is used. We achieve a better qualitative agreement with the experimental results.
Full article

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Open AccessReview

Simulation of Spectra Code (SOS) for ITER Active Beam Spectroscopy

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by Manfred von Hellermann, Maarten de Bock, Oleksandr Marchuk, Detlev Reiter, Stanislav Serov and Michael Walsh

*Atoms*

**2019**,

*7*(1), 30; https://doi.org/10.3390/atoms7010030 - 1 March 2019

**Abstract**

The concept and structure of the Simulation of Spectra (SOS) code is described starting with an introduction to the physics background of the project and the development of a simulation tool enabling the modeling of charge-exchange recombination spectroscopy (CXRS) and associated passive background
[...] Read more.

The concept and structure of the Simulation of Spectra (SOS) code is described starting with an introduction to the physics background of the project and the development of a simulation tool enabling the modeling of charge-exchange recombination spectroscopy (CXRS) and associated passive background spectra observed in hot fusion plasmas. The generic structure of the code implies its general applicability to any fusion device, the development is indeed based on over two decades of spectroscopic observations and validation of derived plasma data. Four main types of active spectra are addressed in SOS. The first type represents thermal low-Z impurity ions and the associated spectral background. The second type of spectra represent slowing-down high energy ions created from either thermo-nuclear fusion reactions or ions from injected high energy neutral beams. Two other modules are dedicated to CXRS spectra representing bulk plasma ions (H

^{+}, D^{+}, or T^{+}) and beam emission spectroscopy (BES) or Motional Stark Effect (MSE) spectrum appearing in the same spectral range. The main part of the paper describes the physics background for the underlying emission processes: active and passive CXRS emission, continuum radiation, edge line emission, halo and plume effect, or finally the charge exchange (CX) cross-section effects on line shapes. The description is summarized by modeling the fast ions emissions, e.g., either of the*α*particles of the fusion reaction or of the beam ions itself. Full articleFigure 1

Open AccessReview

Stochastic Electrodynamics: The Closest Classical Approximation to Quantum Theory

*Atoms*

**2019**,

*7*(1), 29; https://doi.org/10.3390/atoms7010029 - 1 March 2019

**Abstract**

Stochastic electrodynamics is the classical electrodynamic theory of interacting point charges which includes random classical radiation with a Lorentz-invariant spectrum whose scale is set by Planck’s constant. Here, we give a cursory overview of the basic ideas of stochastic electrodynamics, of the successes
[...] Read more.

Stochastic electrodynamics is the classical electrodynamic theory of interacting point charges which includes random classical radiation with a Lorentz-invariant spectrum whose scale is set by Planck’s constant. Here, we give a cursory overview of the basic ideas of stochastic electrodynamics, of the successes of the theory, and of its connections to quantum theory.
Full article

Open AccessFeature PaperArticle

Theory of the Anomalous Magnetic Moment of the Electron

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*Atoms*

**2019**,

*7*(1), 28; https://doi.org/10.3390/atoms7010028 - 22 February 2019

**Abstract**

The anomalous magnetic moment of the electron ${a}_{e}$ measured in a Penning trap occupies a unique position among high precision measurements of physical constants in the sense that it can be compared directly with the theoretical calculation based on the renormalized quantum
[...] Read more.

The anomalous magnetic moment of the electron ${a}_{e}$ measured in a Penning trap occupies a unique position among high precision measurements of physical constants in the sense that it can be compared directly with the theoretical calculation based on the renormalized quantum electrodynamics (QED) to high orders of perturbation expansion in the fine structure constant $\alpha $ , with an effective parameter $\alpha /\pi $ . Both numerical and analytic evaluations of ${a}_{e}$ up to ${(\alpha /\pi )}^{4}$ are firmly established. The coefficient of ${(\alpha /\pi )}^{5}$ has been obtained recently by an extensive numerical integration. The contributions of hadronic and weak interactions have also been estimated. The sum of all these terms leads to ${a}_{e}\left(\mathrm{theory}\right)$ = $1\phantom{\rule{3.33333pt}{0ex}}159\phantom{\rule{3.33333pt}{0ex}}652\phantom{\rule{3.33333pt}{0ex}}181.606\phantom{\rule{3.33333pt}{0ex}}\left(11\right)\left(12\right)\left(229\right)\times {10}^{-12}$ , where the first two uncertainties are from the tenth-order QED term and the hadronic term, respectively. The third and largest uncertainty comes from the current best value of the fine-structure constant derived from the cesium recoil measurement: ${\alpha}^{-1}\left(\mathrm{Cs}\right)=137.035\phantom{\rule{3.33333pt}{0ex}}999\phantom{\rule{3.33333pt}{0ex}}046\phantom{\rule{3.33333pt}{0ex}}\left(27\right)$ . The discrepancy between ${a}_{e}\left(\mathrm{theory}\right)$ and ${a}_{e}\left(\left(\mathrm{experiment}\right)\right)$ is 2.4$\sigma $ . Assuming that the standard model is valid so that ${a}_{e}$ (theory) = ${a}_{e}$ (experiment) holds, we obtain ${\alpha}^{-1}\left({a}_{e}\right)\phantom{\rule{3.33333pt}{0ex}}=137.035\phantom{\rule{3.33333pt}{0ex}}999\phantom{\rule{3.33333pt}{0ex}}1496\phantom{\rule{3.33333pt}{0ex}}\left(13\right)\left(14\right)\left(330\right)$ , which is nearly as accurate as ${\alpha}^{-1}\left(\mathrm{Cs}\right)$ . The uncertainties are from the tenth-order QED term, hadronic term, and the best measurement of ${a}_{e}$ , in this order.
Full article

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Open AccessArticle

Entanglement, Complementarity, and Vacuum Fields in Spontaneous Parametric Down-Conversion

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*Atoms*

**2019**,

*7*(1), 27; https://doi.org/10.3390/atoms7010027 - 19 February 2019

**Abstract**

Using two crystals for spontaneous parametric down-conversion in a parallel setup, we observe two-photon interference with high visibility. The high visibility is consistent with complementarity and the absence of which-path information. The observations are explained as the effects of entanglement or equivalently in
[...] Read more.

Using two crystals for spontaneous parametric down-conversion in a parallel setup, we observe two-photon interference with high visibility. The high visibility is consistent with complementarity and the absence of which-path information. The observations are explained as the effects of entanglement or equivalently in terms of interfering probability amplitudes and also by the calculation of a second-order field correlation function in the Heisenberg picture. The latter approach brings out explicitly the role of the vacuum fields in the down-conversion at the crystals and in the photon coincidence counting. For comparison, we show that the Hong–Ou–Mandel dip can be explained by the same approach in which the role of the vacuum signal and idler fields, as opposed to entanglement involving vacuum states, is emphasized. We discuss the fundamental limitations of a theory in which these vacuum fields are treated as classical, stochastic fields.
Full article

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Open AccessArticle

On the Time Scales of Optical Variability of AGN and the Shape of Their Optical Emission Line Profiles

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*Atoms*

**2019**,

*7*(1), 26; https://doi.org/10.3390/atoms7010026 - 14 February 2019

**Abstract**

The mechanism of the optical variability of active galactic nuclei (AGN) is still very puzzling. It is now widely accepted that the optical variability of AGN is stochastic, producing red noise-like light curves. In case they were to be periodic or quasi-periodic, one
[...] Read more.

The mechanism of the optical variability of active galactic nuclei (AGN) is still very puzzling. It is now widely accepted that the optical variability of AGN is stochastic, producing red noise-like light curves. In case they were to be periodic or quasi-periodic, one should expect that the time scales of optical AGN variability should relate to orbiting time scales of regions inside the accretion disks with temperatures mainly emitting the light in this wavelength range. Knowing the reverberation scales and masses of AGN, expected orbiting time scales are in the order of decades. Unfortunately, most of monitored AGN light curves are not long enough to investigate such time scales of periodicity. Here we investigate the AGN optical variability time scales and their possible connections with the broad emission line shapes.
Full article

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Open AccessArticle

Diagnostic of Langmuir Solitons in Plasmas Using Hydrogenic Spectral Lines

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by Eugene Oks

*Atoms*

**2019**,

*7*(1), 25; https://doi.org/10.3390/atoms7010025 - 13 February 2019

**Abstract**

The development of various spectroscopic diagnostics of relatively weak Langmuir waves in plasmas and their successful implementation have a history of over 50 years. As for spectroscopic diagnostics of Langmuir solitons (i.e., relatively strong Langmuir waves) in plasmas, there have only been very
[...] Read more.

The development of various spectroscopic diagnostics of relatively weak Langmuir waves in plasmas and their successful implementation have a history of over 50 years. As for spectroscopic diagnostics of Langmuir solitons (i.e., relatively strong Langmuir waves) in plasmas, there have only been very few theoretical papers. The most promising result so far was based on using satellites of the dipole-forbidden spectral lines of He, Li, or He-like and Li-like ions. It was shown that, in the case of Langmuir solitons, the peak intensity of the satellites of the dipole-forbidden lines can be significantly enhanced—by orders of magnitude—compared to the case of non-solitonic Langmuir waves. This distinctive feature of satellites under Langmuir solitons allows them to be distinguished from non-solitonic Langmuir waves. In the present paper, we perform a general study of the effects of Langmuir solitons on arbitrary spectral lines of hydrogen or hydrogen-like ions. Then, using the Ly-beta line as an example, we compare the main features of the profiles for the case of the Langmuir solitons with the case of the non-solitonic Langmuir waves of the same amplitude. We also show how the line profiles depend on the amplitude of the Langmuir solitons and on their separation from each other within the sequence of the solitons.
Full article

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## News

20 March 2019

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## Special Issues

Special Issue in
Atoms

Search for New Physics with Cold and Controlled Molecules
Guest Editors: Mikhail G. Kozlov, Timur A. IsaevDeadline: 1 May 2019

Special Issue in
Atoms

Atomic Processes in Plasmas: APiP-2019
Guest Editors: Yuri Ralchenko, Alexander KramidaDeadline: 1 June 2019

Special Issue in
Atoms

Plasma Spectroscopy in the Presence of Magnetic Fields
Guest Editor: Mohammed KOUBITI Deadline: 30 June 2019

Special Issue in
Atoms

Atomic and Ionic Collisions with Formation of Quasimolecules
Guest Editors: Vladimir A. Sreckovic, Milan S. Dimitrijević, Nikolai N. BezuglovDeadline: 1 August 2019