Editor's Choice Articles

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to authors, or important in this field. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

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Article
Long-Range Atom–Ion Rydberg Molecule: A Novel Molecular Binding Mechanism
Atoms 2021, 9(2), 34; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms9020034 - 21 Jun 2021
Cited by 5
Abstract
We present a novel binding mechanism where a neutral Rydberg atom and an atomic ion form a molecular bound state at a large internuclear distance. The binding mechanism is based on Stark shifts and level crossings that are induced in the Rydberg atom [...] Read more.
We present a novel binding mechanism where a neutral Rydberg atom and an atomic ion form a molecular bound state at a large internuclear distance. The binding mechanism is based on Stark shifts and level crossings that are induced in the Rydberg atom due to the electric field of the ion. At particular internuclear distances between the Rydberg atom and the ion, potential wells occur that can hold atom–ion molecular bound states. Apart from the binding mechanism, we describe important properties of the long-range atom–ion Rydberg molecule, such as its lifetime and decay paths, its vibrational and rotational structure, and its large dipole moment. Furthermore, we discuss methods of how to produce and detect it. The unusual properties of the long-range atom–ion Rydberg molecule give rise to interesting prospects for studies of wave packet dynamics in engineered potential energy landscapes. Full article
(This article belongs to the Special Issue Low Energy Interactions between Ions and Ultracold Alkali Atoms)
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Article
Few Body Effects in the Electron and Positron Impact Ionization of Atoms
Atoms 2021, 9(2), 33; https://doi.org/10.3390/atoms9020033 - 09 Jun 2021
Cited by 3
Abstract
Triple differential cross sections (TDCS) are presented for the electron and positron impact ionization of inert gas atoms in a range of energy sharing geometries where a number of significant few body effects compete to define the shape of the TDCS. Using both [...] Read more.
Triple differential cross sections (TDCS) are presented for the electron and positron impact ionization of inert gas atoms in a range of energy sharing geometries where a number of significant few body effects compete to define the shape of the TDCS. Using both positrons and electrons as projectiles has opened up the possibility of performing complementary studies which could effectively isolate competing interactions that cannot be separately detected in an experiment with a single projectile. Results will be presented in kinematics where the electron impact ionization appears to be well understood and using the same kinematics positron cross sections will be presented. The kinematics are then varied in order to focus on the role of distortion, post collision interaction (pci), and interference effects. Full article
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Article
Route to Extend the Lifetime of a Discrete Time Crystal in a Finite Spin Chain without Disorder
Atoms 2021, 9(2), 25; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms9020025 - 12 Apr 2021
Cited by 3
Abstract
Periodically driven (Floquet) systems are described by time-dependent Hamiltonians that possess discrete time translation symmetry. The spontaneous breaking of this symmetry leads to the emergence of a novel non-equilibrium phase of matter—the Discrete Time Crystal (DTC). In this paper, we propose a scheme [...] Read more.
Periodically driven (Floquet) systems are described by time-dependent Hamiltonians that possess discrete time translation symmetry. The spontaneous breaking of this symmetry leads to the emergence of a novel non-equilibrium phase of matter—the Discrete Time Crystal (DTC). In this paper, we propose a scheme to extend the lifetime of a DTC in a paradigmatic model—a translation-invariant Ising spin chain with nearest-neighbor interaction J, subjected to a periodic kick by a transverse magnetic field with frequency 2πT. This system exhibits the hallmark signature of a DTC—persistent sub-harmonic oscillations with frequency πT—for a wide parameter regime. Employing both analytical arguments as well as exact diagonalization calculations, we demonstrate that the lifetime of the DTC is maximized, when the interaction strength is tuned to an optimal value, JT=π. Our proposal essentially relies on an interaction-induced quantum interference mechanism that suppresses the creation of excitations, and thereby enhances the DTC lifetime. Intriguingly, we find that the period doubling oscillations can last eternally in even size systems. This anomalously long lifetime can be attributed to a time reflection symmetry that emerges at JT=π. Our work provides a promising avenue for realizing a robust DTC in various quantum emulator platforms. Full article
(This article belongs to the Section Cold Atoms, Quantum Gases and Bose-Einstein Condensation)
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Article
CAl4Mg0/−: Global Minima with a Planar Tetracoordinate Carbon Atom
Atoms 2021, 9(2), 24; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms9020024 - 09 Apr 2021
Cited by 7
Abstract
Isomers of CAl4Mg and CAl4Mg have been theoretically characterized for the first time. The most stable isomer for both the neutral and anion contain a planar tetracoordinate carbon (ptC) atom. Unlike the isovalent CAl4Be case, which [...] Read more.
Isomers of CAl4Mg and CAl4Mg have been theoretically characterized for the first time. The most stable isomer for both the neutral and anion contain a planar tetracoordinate carbon (ptC) atom. Unlike the isovalent CAl4Be case, which contains a planar pentacoordinate carbon atom as the global minimum geometry, replacing beryllium with magnesium makes the ptC isomer the global minimum due to increased ionic radii of magnesium. However, it is relatively easier to conduct experimental studies for CAl4Mg0/− as beryllium is toxic. While the neutral molecule containing the ptC atom follows the 18 valence electron rule, the anion breaks the rule with 19 valence electrons. The electron affinity of CAl4Mg is in the range of 1.96–2.05 eV. Both the global minima exhibit π/σ double aromaticity. Ab initio molecular dynamics simulations were carried out for both the global minima at 298 K for 10 ps to confirm their kinetic stability. Full article
(This article belongs to the Special Issue Planar Tetracoordinate Carbon—Fifty Years and Beyond)
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Article
Polaron Problems in Ultracold Atoms: Role of a Fermi Sea across Different Spatial Dimensions and Quantum Fluctuations of a Bose Medium
Atoms 2021, 9(1), 18; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms9010018 - 09 Mar 2021
Cited by 9
Abstract
The notion of a polaron, originally introduced in the context of electrons in ionic lattices, helps us to understand how a quantum impurity behaves when being immersed in and interacting with a many-body background. We discuss the impact of the impurities on the [...] Read more.
The notion of a polaron, originally introduced in the context of electrons in ionic lattices, helps us to understand how a quantum impurity behaves when being immersed in and interacting with a many-body background. We discuss the impact of the impurities on the medium particles by considering feedback effects from polarons that can be realized in ultracold quantum gas experiments. In particular, we exemplify the modifications of the medium in the presence of either Fermi or Bose polarons. Regarding Fermi polarons we present a corresponding many-body diagrammatic approach operating at finite temperatures and discuss how mediated two- and three-body interactions are implemented within this framework. Utilizing this approach, we analyze the behavior of the spectral function of Fermi polarons at finite temperature by varying impurity-medium interactions as well as spatial dimensions from three to one. Interestingly, we reveal that the spectral function of the medium atoms could be a useful quantity for analyzing the transition/crossover from attractive polarons to molecules in three-dimensions. As for the Bose polaron, we showcase the depletion of the background Bose-Einstein condensate in the vicinity of the impurity atom. Such spatial modulations would be important for future investigations regarding the quantification of interpolaron correlations in Bose polaron problems. Full article
(This article belongs to the Special Issue Physics of Impurities in Quantum Gases)
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Article
Impact Features Induced by Single Fast Ions of Different Charge-State on Muscovite Mica
Atoms 2021, 9(1), 17; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms9010017 - 25 Feb 2021
Cited by 4
Abstract
The influence of the charge state q on surface modifications induced by the impact of individual fast, heavy ions on muscovite mica was investigated. Beams of 593 MeV 197Auq+ with well-defined initial charge states over a relatively broad range of [...] Read more.
The influence of the charge state q on surface modifications induced by the impact of individual fast, heavy ions on muscovite mica was investigated. Beams of 593 MeV 197Auq+ with well-defined initial charge states over a relatively broad range of values (30 to 51) and at different irradiation geometries were used. At normal incidence, the impact features are rounded protrusions (hillocks) with ≳20 nm in diameter. At grazing angles, besides the hillocks, craters and elongated tails (up to 350 nm-long) extending along the direction of ion penetration are produced. It is shown that the impact features at normal incidence depend strongly on the initial charge state of the projectiles. This dependence is very weak at grazing angles as the ion reaches the equilibrium charge state closer to the surface. At normal ion incidence, the hillock volume scales with q3.3 ± 0.6. This dependence stems largely from the increase in the hillock height, as a weak dependence of the diameter was observed. Full article
(This article belongs to the Special Issue Charge-State Evolution in Ion-Atom/Solid Collisions)
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Article
“Amplified Spontaneous Emission” in Micro- and Nanolasers
Atoms 2021, 9(1), 6; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms9010006 - 19 Jan 2021
Cited by 6
Abstract
Amplified Spontaneous Emission is ubiquitous in systems with optical gain and is responsible for many opportunities and shortcomings. Its role in the progression from the simplest form of thermal radiation (single emitter spontaneous emission) all the way to coherent radiation from inverted systems [...] Read more.
Amplified Spontaneous Emission is ubiquitous in systems with optical gain and is responsible for many opportunities and shortcomings. Its role in the progression from the simplest form of thermal radiation (single emitter spontaneous emission) all the way to coherent radiation from inverted systems is still an open question. We critically review observations of photon bursts in micro- and nanolasers, in the perspective of currently used measurement techniques, in relation to threshold-related questions for small devices. Corresponding stochastic predictions are analyzed, and contrasted with burst absence in differential models, in light of general phase space properties. A brief discussion on perspectives is offered in the conclusions. Full article
(This article belongs to the Special Issue Collective Atomic and Free-Electron Lasing)
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Article
Investigation of Resonance-Enhanced High-Order Harmonics by Two-Component Laser-Produced Plasmas
Atoms 2021, 9(1), 1; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms9010001 - 05 Jan 2021
Cited by 4
Abstract
Resonance-enhanced harmonics from laser-produced plasma plumes are an interesting phenomenon, whose underlying mechanism is still under debate. In particular, it is unclear whether the macroscopic dispersion properties of the plasma are the key factors for the formation of the enhancement. To shed light [...] Read more.
Resonance-enhanced harmonics from laser-produced plasma plumes are an interesting phenomenon, whose underlying mechanism is still under debate. In particular, it is unclear whether the macroscopic dispersion properties of the plasma are the key factors for the formation of the enhancement. To shed light on this problem, we perform experiments with two-component plasmas, in which one of the components (tin) is known to be able to generate enhanced harmonics and the other component (lead) is known for altering the overall dispersion properties of the plasma medium. We compare the harmonics spectra from the plasma of pure tin and the plasma of tin/lead alloy. Depending on the driving wavelength, we observe enhanced harmonics at around 47 or 44 nm in both types of plasmas. The two enhanced regions could be attributed to resonances in singly-charged and doubly-charged tin ions, respectively. Our results indicate that the co-existence of lead plasma does not destroy the presence of the enhanced harmonics of tin plasma, and it seems to suggest that the macroscopic properties of the plasma are not the origin of the resonance-enhanced harmonics in tin. Full article
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Article
Multimode Collective Atomic Recoil Lasing in Free Space
Atoms 2020, 8(4), 93; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8040093 - 10 Dec 2020
Cited by 3
Abstract
Cold atomic clouds in collective atomic recoil lasing are usually confined by an optical cavity, which forces the light-scattering to befall in the mode fixed by the resonator. Here we consider the system to be in free space, which leads into a vacuum [...] Read more.
Cold atomic clouds in collective atomic recoil lasing are usually confined by an optical cavity, which forces the light-scattering to befall in the mode fixed by the resonator. Here we consider the system to be in free space, which leads into a vacuum multimode collective scattering. We show that the presence of an optical cavity is not always necessary to achieve coherent collective emission by the atomic ensemble and that a preferred scattering path arises along the major axis of the atomic cloud. We derive a full vectorial model for multimode collective atomic recoil lasing in free space. Such a model consists of multi-particle equations capable of describing the motion of each atom in a 2D/3D cloud. These equations are numerically solved by means of molecular dynamic algorithms, usually employed in other scientific fields. The numerical results show that both atomic density and collective scattering patterns are applicable to the cloud’s orientation and shape and to the polarization of the incident light. Full article
(This article belongs to the Special Issue Collective Atomic and Free-Electron Lasing)
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Article
New Energy Levels of Neutral Lanthanum Derived from an Optogalvanic Spectrum between 5610 and 6110 Å
by and
Atoms 2020, 8(4), 88; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8040088 - 04 Dec 2020
Cited by 3
Abstract
In a previous paper, we reported on a complete optogalvanic (OG) spectrum of a discharge burning in a La–Ar gas mixture, in the spectral range 5610–6110 Å (17,851 to 16,364 cm−1). Now we are able to communicate further new energy [...] Read more.
In a previous paper, we reported on a complete optogalvanic (OG) spectrum of a discharge burning in a La–Ar gas mixture, in the spectral range 5610–6110 Å (17,851 to 16,364 cm−1). Now we are able to communicate further new energy levels, found via searching for laser-induced fluorescence lines when exciting unclassified lines from the OG spectrum. We were able to find 17 new levels, and for two further levels, the line list has extended. With the help of these 19 levels, we could classify 132 spectral lines. Full article
(This article belongs to the Section Atomic, Molecular and Nuclear Spectroscopy and Collisions)
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Article
A Decade with VAMDC: Results and Ambitions
Atoms 2020, 8(4), 76; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8040076 - 21 Oct 2020
Cited by 23
Abstract
This paper presents an overview of the current status of the Virtual Atomic and Molecular Data Centre (VAMDC) e-infrastructure, including the current status of the VAMDC-connected (or to be connected) databases, updates on the latest technological development within the infrastructure and a presentation [...] Read more.
This paper presents an overview of the current status of the Virtual Atomic and Molecular Data Centre (VAMDC) e-infrastructure, including the current status of the VAMDC-connected (or to be connected) databases, updates on the latest technological development within the infrastructure and a presentation of some application tools that make use of the VAMDC e-infrastructure. We analyse the past 10 years of VAMDC development and operation, and assess their impact both on the field of atomic and molecular (A&M) physics itself and on heterogeneous data management in international cooperation. The highly sophisticated VAMDC infrastructure and the related databases developed over this long term make them a perfect resource of sustainable data for future applications in many fields of research. However, we also discuss the current limitations that prevent VAMDC from becoming the main publishing platform and the main source of A&M data for user communities, and present possible solutions under investigation by the consortium. Several user application examples are presented, illustrating the benefits of VAMDC in current research applications, which often need the A&M data from more than one database. Finally, we present our vision for the future of VAMDC. Full article
(This article belongs to the Special Issue Development and Perspectives of Atomic and Molecular Databases)
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Article
Vibrationally and Spin-Orbit-Resolved Inner-Shell X-ray Absorption Spectroscopy of the NH+ Molecular Ion: Measurements and ab Initio Calculations
Atoms 2020, 8(4), 67; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8040067 - 04 Oct 2020
Cited by 5
Abstract
This article presents N2+ fragment yields following nitrogen K-shell photo-absorption in the NH+ molecular ion measured at the SOLEIL synchrotron radiation facility in the photon energy region 390–450 eV. The combination of the high sensitivity of the merged-beam, multi-analysis ion apparatus [...] Read more.
This article presents N2+ fragment yields following nitrogen K-shell photo-absorption in the NH+ molecular ion measured at the SOLEIL synchrotron radiation facility in the photon energy region 390–450 eV. The combination of the high sensitivity of the merged-beam, multi-analysis ion apparatus (MAIA) with the high spectral resolution of the PLEIADES beamline helped to resolve experimentally vibrational structures of highly excited [N1s−1H]*+ electronic states with closed or open-shell configurations. The assignment of the observed spectral features was achieved with the help of density functional theory (DFT) and post-Hartree Fock Multiconfiguration Self-Consistent-Field/Configuration Interaction (MCSCF/CI) ab-initio theoretical calculations of the N1s core-to-valence and core-to-Rydberg excitations, including vibrational dynamics. New resonances were identified compared to previous work, owing to detailed molecular modeling of the vibrational, spin-orbit coupling and metastable state effects on the spectra. The latter are evidenced by spectral contributions from the 4Σ electronic state which lies 0.07 eV above the NH+2Π ground state. Full article
(This article belongs to the Special Issue Interaction of Ionizing Photons with Atomic and Molecular Ions)
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Article
Spin-Polarized Photoelectron Fluxes from Fullerene Anions
Atoms 2020, 8(4), 65; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8040065 - 29 Sep 2020
Cited by 4
Abstract
Initial insights into spin-polarized photoelectron fluxes from fullerene anions are presented here. Both the angle-dependent and angle-integrated degrees of spin polarization of said photoelectron fluxes are discussed. Empty C60(2p) and endohedral [email protected] [...] Read more.
Initial insights into spin-polarized photoelectron fluxes from fullerene anions are presented here. Both the angle-dependent and angle-integrated degrees of spin polarization of said photoelectron fluxes are discussed. Empty C60(2p) and endohedral [email protected]60(2p) and [email protected]60(2p) anions, where the attached electron resides in a 2p state, are chosen as case studies. We uncover the characteristics of the phenomenon in the framework of a semi-empirical methodology where the C60 cage is modeled by a spherical annular potential, rather than aiming at a rigorous study. It is found that the spin-polarization degree of photoelectron fluxes from fullerene anions can reach large values, including a nearly complete polarization, at/in specific values/domains of the photoelectron momentum. This is shown to correlate with an inherent feature of photoionization of fullerenes, the abundance of resonances, known as confinement resonances, in their photodetachment spectra owing to a large empty space inside fullerenes. Moreover, the results obtained can serve as a touchstone for future studies of the phenomenon by more rigorous theories and/or experiments to reveal the significance of interactions omitted in the present study. Full article
(This article belongs to the Special Issue Interaction of Ionizing Photons with Atomic and Molecular Ions)
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Article
Net Electron Capture in Collisions of Multiply Charged Projectiles with Biologically Relevant Molecules
Atoms 2020, 8(3), 59; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8030059 - 17 Sep 2020
Cited by 5
Abstract
A model for the description of proton collisions from molecules composed of atoms such as hydrogen, carbon, nitrogen, oxygen and phosphorus (H, C, N, O, P) was recently extended to treat collisions with multiply charged ions with a focus on net ionization. Here [...] Read more.
A model for the description of proton collisions from molecules composed of atoms such as hydrogen, carbon, nitrogen, oxygen and phosphorus (H, C, N, O, P) was recently extended to treat collisions with multiply charged ions with a focus on net ionization. Here we complement the work by focusing on net capture. The ion–atom collisions are computed using the two-center basis generator method. The atomic net capture cross sections are then used to assemble two models for ion–molecule collisions: An independent atom model (IAM) based on the Bragg additivity rule (labeled IAM-AR), and also the so-called pixel-counting method (IAM-PCM) which introduces dependence on the orientation of the molecule during impact. The IAM-PCM leads to significantly reduced capture cross sections relative to IAM-AR at low energies, since it takes into account the overlap of effective atomic cross sectional areas. We compare our results with available experimental and other theoretical data focusing on water vapor (H2O), methane (CH4) and uracil (C4H4N2O2). For the water molecule target we also provide results from a classical-trajectory Monte Carlo approach that includes dynamical screening effects on projectile and target. For small molecules dominated by a many-electron atom, such as carbon in methane or oxygen in water, we find a saturation phenomenon for higher projectile charges (q=3) and low energies, where the net capture cross section for the molecule is dominated by the net cross section for the many-electron atom, and the net capture cross section is not proportional to the total number of valence electrons. Full article
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Article
Development of NIST Atomic Databases and Online Tools
Atoms 2020, 8(3), 56; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8030056 - 05 Sep 2020
Cited by 87
Abstract
Over the last 25 years, the atomic standard reference databases and online tools developed at the National Institute of Standards and Technology (NIST) have provided users around the world with the highest-quality data on various atomic parameters (e.g., level energies, transition wavelengths, and [...] Read more.
Over the last 25 years, the atomic standard reference databases and online tools developed at the National Institute of Standards and Technology (NIST) have provided users around the world with the highest-quality data on various atomic parameters (e.g., level energies, transition wavelengths, and oscillator strengths) and online capabilities for fast and reliable collisional-radiative modeling of diverse plasmas. Here we present an overview of the recent developments regarding NIST numerical and bibliographic atomic databases and outline the prospects and vision of their evolution. Full article
(This article belongs to the Special Issue Development and Perspectives of Atomic and Molecular Databases)
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Article
PyAtomDB: Extending the AtomDB Atomic Database to Model New Plasma Processes and Uncertainties
Atoms 2020, 8(3), 49; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8030049 - 24 Aug 2020
Cited by 10
Abstract
The AtomDB project provides models of X-ray and extreme ultraviolet emitting astrophysical spectra for optically thin, hot plasma. We present the new software package, PyAtomDB, which now underpins the entire project, providing access to the underlying database, collisional radiative model calculations, and spectrum [...] Read more.
The AtomDB project provides models of X-ray and extreme ultraviolet emitting astrophysical spectra for optically thin, hot plasma. We present the new software package, PyAtomDB, which now underpins the entire project, providing access to the underlying database, collisional radiative model calculations, and spectrum generation for a range of models. PyAtomDB is easily extensible, allowing users to build new tools and models for use in analysis packages such as XSPEC. We present two of these, the kappa and ACX models for non-Maxwellian and Charge-Exchange plasmas respectively. In addition, PyAtomDB allows for full open access to the apec code, which underlies all of the AtomDB spectra and has enabled the development of a module for estimating the sensitivity of emission lines and diagnostic line ratios to uncertainties in the underlying atomic data. We present these publicly available tools and results for several X-ray diagnostics of Fe L-shell ions and He-like ions as examples. Full article
(This article belongs to the Special Issue Development and Perspectives of Atomic and Molecular Databases)
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Article
Atomic Data for Plasma Spectroscopy: The CHIANTI Database, Improvements and Challenges
Atoms 2020, 8(3), 46; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8030046 - 20 Aug 2020
Cited by 8
Abstract
CHIANTI is an atomic database and software package for modeling emission lines and continua from hot astrophysical plasmas. It is freely available to all researchers and has been widely used in the Heliophysics and Astrophysics communities for almost 25 years. In this review, [...] Read more.
CHIANTI is an atomic database and software package for modeling emission lines and continua from hot astrophysical plasmas. It is freely available to all researchers and has been widely used in the Heliophysics and Astrophysics communities for almost 25 years. In this review, we summarize the properties of the current version of the database and give an overview of the relevant atomic processes. We also discuss progress towards a complete implementation of collisional-radiative modeling, simultaneously solving for atomic level and ion populations for individual elements. Full article
(This article belongs to the Special Issue Development and Perspectives of Atomic and Molecular Databases)
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Article
Deep Minima in the Triply Differential Cross Section for Ionization of Atomic Hydrogen by Electron and Positron Impact
Atoms 2020, 8(2), 26; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8020026 - 29 May 2020
Cited by 7
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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Article
The Optogalvanic Spectrum of Neutral Lanthanum between 5610 and 6110 Å
by and
Atoms 2020, 8(2), 23; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8020023 - 19 May 2020
Cited by 7
Abstract
We report on a complete optogalvanic spectrum of a discharge burning in a La-Ar gas mixture, in the spectral range 5610–6110 Å (17,851 to 16,364 cm−1). About 1900 overlapping laser scans, each between 1 and 1.5 cm−1 wide, were necessary [...] Read more.
We report on a complete optogalvanic spectrum of a discharge burning in a La-Ar gas mixture, in the spectral range 5610–6110 Å (17,851 to 16,364 cm−1). About 1900 overlapping laser scans, each between 1 and 1.5 cm−1 wide, were necessary to cover this range. The resolution of the spectra is limited by the Doppler width of the spectral features to about 0.03 cm−1 (or ca. 0.01 Å) and is comparable with a Fourier-transform spectrum, but the sensitivity is much higher. Indeed, we could find more than 1800 lines, from which about 800 could be classified as transitions between known energy levels. The main focus of the investigations was to discover previously unknown energy levels by means of excitation of unclassified spectral features. Full article
(This article belongs to the Section Atomic, Molecular and Nuclear Spectroscopy and Collisions)
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Article
Current Status and Developments of the Atomic Database on Rare-Earths at Mons University (DREAM)
Atoms 2020, 8(2), 18; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8020018 - 02 May 2020
Cited by 6
Abstract
The main purpose of the Database on Rare Earths At Mons University (DREAM) is to provide the scientific community with updated spectroscopic parameters related to lanthanide atoms (Z = 57–71) in their lowest ionization stages. The radiative parameters (oscillator strengths and transitions [...] Read more.
The main purpose of the Database on Rare Earths At Mons University (DREAM) is to provide the scientific community with updated spectroscopic parameters related to lanthanide atoms (Z = 57–71) in their lowest ionization stages. The radiative parameters (oscillator strengths and transitions probabilities) listed in the database have been obtained over the past 20 years by the Atomic Physics and Astrophysics group of Mons University, Belgium, thanks to a systematic and extensive use of the pseudo-relativistic Hartree-Fock (HFR) method modified for taking core-polarization and core-penetration effects into account. Most of these theoretical results have been validated by the good agreement obtained when comparing computed radiative lifetimes and accurate experimental values measured by the time-resolved laser-induced fluorescence technique. In the present paper, we report on the current status and developments of the database that gathers radiative parameters for more than 72,000 spectral lines in neutral, singly-, doubly-, and triply-ionized lanthanides. Full article
(This article belongs to the Special Issue Development and Perspectives of Atomic and Molecular Databases)
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Article
Referencing Sources of Molecular Spectroscopic Data in the Era of Data Science: Application to the HITRAN and AMBDAS Databases
Atoms 2020, 8(2), 16; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8020016 - 30 Apr 2020
Cited by 4
Abstract
The application described has been designed to create bibliographic entries in large databases with diverse sources automatically, which reduces both the frequency of mistakes and the workload for the administrators. This new system uniquely identifies each reference from its digital object identifier (DOI) [...] Read more.
The application described has been designed to create bibliographic entries in large databases with diverse sources automatically, which reduces both the frequency of mistakes and the workload for the administrators. This new system uniquely identifies each reference from its digital object identifier (DOI) and retrieves the corresponding bibliographic information from any of several online services, including the SAO/NASA Astrophysics Data Systems (ADS) and CrossRef APIs. Once parsed into a relational database, the software is able to produce bibliographies in any of several formats, including HTML and BibTeX, for use on websites or printed articles. The application is provided free-of-charge for general use by any scientific database. The power of this application is demonstrated when used to populate reference data for the HITRAN and AMBDAS databases as test cases. HITRAN contains data that is provided by researchers and collaborators throughout the spectroscopic community. These contributors are accredited for their contributions through the bibliography produced alongside the data returned by an online search in HITRAN. Prior to the work presented here, HITRAN and AMBDAS created these bibliographies manually, which is a tedious, time-consuming and error-prone process. The complete code for the new referencing system can be found on the HITRANonline GitHub website. Full article
(This article belongs to the Special Issue Development and Perspectives of Atomic and Molecular Databases)
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Article
Elastic Photon Scattering on Hydrogenic Atoms near Resonances
Atoms 2020, 8(2), 12; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8020012 - 18 Apr 2020
Cited by 5
Abstract
Scattering of light on relativistic heavy ion beams is widely used for characterizing and tuning the properties of both the light and the ion beam. Its elastic component—Rayleigh scattering—is investigated in this work for photon energies close to certain electronic transitions because of [...] Read more.
Scattering of light on relativistic heavy ion beams is widely used for characterizing and tuning the properties of both the light and the ion beam. Its elastic component—Rayleigh scattering—is investigated in this work for photon energies close to certain electronic transitions because of its potential usage in the Gamma Factory initiative at CERN. The angle-differential cross-section, as well as the degree of polarization of the scattered light are investigated for the cases of 1 s 2 p 1 / 2 and 1 s 2 p 3 / 2 resonance transitions in H-like lead ions. In order to gauge the validity and uncertainty of frequently used approximations, we compare different methods. In particular, rigorous quantum electrodynamics calculations are compared with the resonant electric-dipole approximation evaluated within the relativistic and nonrelativistic formalisms. For better understanding of the origin of the approximation, the commonly used theoretical approach is explained here in detail. We find that in most cases, the nonrelativistic resonant electric-dipole approximation fails to describe the properties of the scattered light. At the same time, its relativistic variant agrees with the rigorous treatment within a level of 10% to 20%. These findings are essential for the design of an experimental setup exploiting the scattering process, as well as for the determination of the scattered light properties. Full article
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Article
Differential Study of Projectile Coherence Effects on Double Capture Processes in p + Ar Collisions
Atoms 2020, 8(2), 10; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8020010 - 28 Mar 2020
Abstract
We have measured differential yields for double capture and double capture accompanied by ionization in 75 keV p + Ar collisions. Data were taken for two different transverse projectile coherence lengths. A small effect of the projectile coherence properties on the yields were [...] Read more.
We have measured differential yields for double capture and double capture accompanied by ionization in 75 keV p + Ar collisions. Data were taken for two different transverse projectile coherence lengths. A small effect of the projectile coherence properties on the yields were found for double capture, but not for double capture plus ionization. The results suggest that multiple projectile–target interactions can lead to a significant weakening of projectile coherence effects. Full article
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Article
Atomic Data for Calculation of the Intensities of Stark Components of Excited Hydrogen Atoms in Fusion Plasmas
Atoms 2020, 8(1), 8; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8010008 - 18 Feb 2020
Abstract
Motional Stark effect (MSE) spectroscopy represents a unique diagnostic tool capable of determining the magnitude of the magnetic field and its direction in the core of fusion plasmas. The primary excitation channel for fast hydrogen atoms in injected neutral beams, with energy in [...] Read more.
Motional Stark effect (MSE) spectroscopy represents a unique diagnostic tool capable of determining the magnitude of the magnetic field and its direction in the core of fusion plasmas. The primary excitation channel for fast hydrogen atoms in injected neutral beams, with energy in the range of 25–1000 keV, is due to collisions with protons and impurity ions (e.g., He 2 + and heavier impurities). As a result of such excitation, at the particle density of 10 13 –10 14 cm 3 , the line intensities of the Stark multiplets do not follow statistical expectations (i.e., the populations of fine-structure levels within the same principal quantum number n are not proportional to their statistical weights). Hence, any realistic modeling of MSE spectra has to include the relevant collisional atomic data. In this paper we provide a general expression for the excitation cross sections in parabolic states within n = 3 for an arbitrary orientation between the direction of the motion-induced electric field and the proton-atom collisional axis. The calculations make use of the density matrix obtained with the atomic orbital close coupling method and the method can be applied to other collisional systems (e.g., He 2 + , Be 4 + , C 6 + , etc.). The resulting cross sections are given as simple fits that can be directly applied to spectral modeling. For illustration we note that the asymmetry detected in the first classical cathode ray experiments between the red- and blue-shifted spectral components can be quantitatively studied using the proposed approach. Full article
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Article
Empirical Line Lists in the ExoMol Database
Atoms 2020, 8(1), 7; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms8010007 - 17 Feb 2020
Cited by 12
Abstract
The ExoMol database aims to provide comprehensive molecular line lists for exoplanetary and other hot atmospheres. The data are expanded by inclusion of empirically derived line lists taken from the literature for a series of diatomic molecules, namely CH, NH, OH, AlCl, AlF, [...] Read more.
The ExoMol database aims to provide comprehensive molecular line lists for exoplanetary and other hot atmospheres. The data are expanded by inclusion of empirically derived line lists taken from the literature for a series of diatomic molecules, namely CH, NH, OH, AlCl, AlF, OH + , CaF, MgF, KF, NaF, LiCl, LiF, MgH, TiH, CrH, FeH, C 2 , CP, CN, CaH, and triplet N 2 . Generally, these line lists are constructed from measured spectra using a combination of effective rotational Hamiltonian models for the line positions and ab initio (transition) dipole moments to provide intensities. This work results in the inclusion of 22 new molecules (36 new isotopologues) in the ExoMol database. Full article
(This article belongs to the Special Issue Development and Perspectives of Atomic and Molecular Databases)
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Article
The Belgian Repository of Fundamental Atomic Data and Stellar Spectra (BRASS)
Atoms 2019, 7(4), 105; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7040105 - 22 Nov 2019
Cited by 1
Abstract
Background: BRASS (Belgian Repository of Fundamental Atomic Data and Stellar Spectra) is an international networking project for the development of a new public database providing accurate fundamental atomic data of vital importance for stellar spectroscopic research. We present an overview of research results [...] Read more.
Background: BRASS (Belgian Repository of Fundamental Atomic Data and Stellar Spectra) is an international networking project for the development of a new public database providing accurate fundamental atomic data of vital importance for stellar spectroscopic research. We present an overview of research results obtained in the past four years. Methods: The BRASS database offers atomic line data we thoroughly tested by comparing theoretical and observed stellar spectra. We perform extensive quality assessments of selected atomic input data using advanced radiative transfer spectrum synthesis calculations, which we compare to high-resolution Mercator-HERMES and ESO-VLT-UVES spectra of F-, G-, and K-type benchmark stars observed with very high signal-to-noise ratios. We have retrieved about half a million atomic lines required for our detailed spectrum synthesis calculations from the literature and online databases such as VAMDC, NIST, VALD, CHIANTI, Spectr-W 3 , TIPbase, TOPbase, SpectroWeb. Results: The atomic datasets have been cross-matched based on line electronic configuration information and organized in a new online repository called BRASS. The validated atomic data, combined with the observed and theoretical spectra are also interactively offered in BRASS. The combination of these datasets is a novel approach for its development providing a universal reference for advanced stellar spectroscopic research. Conclusion: We present an overview of the BRASS Data Interface developments allowing online user interaction for the combined spectrum and atomic data display, line identification, atomic data accuracy assessments including line log(gf)-values, and line equivalent width measurements. Full article
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Article
Quantemol Electron Collisions (QEC): An Enhanced Expert System for Performing Electron Molecule Collision Calculations Using the R-Matrix Method
Atoms 2019, 7(4), 97; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7040097 - 17 Oct 2019
Cited by 18
Abstract
Collisions of low energy electrons with molecules are important for understanding many aspects of the environment and technologies. Understanding the processes that occur in these types of collisions can give insights into plasma etching processes, edge effects in fusion plasmas, radiation damage to [...] Read more.
Collisions of low energy electrons with molecules are important for understanding many aspects of the environment and technologies. Understanding the processes that occur in these types of collisions can give insights into plasma etching processes, edge effects in fusion plasmas, radiation damage to biological tissues and more. A radical update of the previous expert system for computing observables relevant to these processes, Quantemol-N, is presented. The new Quantemol Electron Collision (QEC) expert system simplifyies the user experience, improving reliability and implements new features. The QEC graphical user interface (GUI) interfaces the Molpro quantum chemistry package for molecular target setups, and the sophisticated UKRmol+ codes to generate accurate and reliable cross-sections. These include elastic cross-sections, super elastic cross-sections between excited states, electron impact dissociation, scattering reaction rates, dissociative electron attachment, differential cross-sections, momentum transfer cross-sections, ionization cross sections, and high energy electron scattering cross-sections. With this new interface we will be implementing dissociative recombination estimations, vibrational excitations for neutrals and ions, and effective core potentials in the near future. Full article
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Article
Charge Exchange Cross Sections for Noble Gas Ions and N2 between 0.2 and 5.0 keV
Atoms 2019, 7(4), 96; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7040096 - 14 Oct 2019
Cited by 1
Abstract
Charge transfer of an electron from a neutral atom to an ion is a fundamental interaction that plays a dominant role in the energy balance of atmospheric and astrophysical plasmas. The present investigation measured the charge exchange cross sections of noble gas ions [...] Read more.
Charge transfer of an electron from a neutral atom to an ion is a fundamental interaction that plays a dominant role in the energy balance of atmospheric and astrophysical plasmas. The present investigation measured the charge exchange cross sections of noble gas ions (He + , Ne + , Ar + , Kr + ) with N 2 in the intermediate energy range 0.2–5.0 keV. The systems were chosen because there remains a lack of consensus amongst previous measurements and regions where there were no previous measurements. A description of the mechanical design for an electrically floated gas cell is described herein. Full article
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Article
Line Shapes in a Magnetic Field: Trajectory Modifictions II: Full Collision-Time Statistics
Atoms 2019, 7(4), 94; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7040094 - 05 Oct 2019
Cited by 6
Abstract
In a previous paper, a variation of the Collision-time Statistics method was applied to identify the relevant perturbers for line broadening under the action of a constant magnetic field. As discussed, that version was simplified and inadequate for low magnetic field and/or large [...] Read more.
In a previous paper, a variation of the Collision-time Statistics method was applied to identify the relevant perturbers for line broadening under the action of a constant magnetic field. As discussed, that version was simplified and inadequate for low magnetic field and/or large perturber mass (ions). The purpose of the present work is to augment the previous work, so that such cases can be handed efficiently. The results may also be used to construct analytic, i.e., impact/unified models under the usual assumptions in these models. Full article
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Article
Mixed Basis Sets for Atomic Calculations
Atoms 2019, 7(3), 92; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7030092 - 19 Sep 2019
Cited by 4
Abstract
Many numerical methods of atomic calculations use one-electron basis sets. These basis sets must meet rather contradictory requirements. On the one hand, they must include physically justified orbitals, such as Dirac–Fock ones, for the one-electron states with high occupation numbers. On the other [...] Read more.
Many numerical methods of atomic calculations use one-electron basis sets. These basis sets must meet rather contradictory requirements. On the one hand, they must include physically justified orbitals, such as Dirac–Fock ones, for the one-electron states with high occupation numbers. On the other hand, they must ensure rapid convergence of the calculations in respect to the size of the basis set. It is difficult to meet these requirements using a single set of orbitals, while merging different subsets may lead to linear dependence and other problems. We suggest a simple unitary operator that allows such merging without aforementioned complications. We demonstrated robustness of the method on the examples of Fr and Au. Full article
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Article
Resonant Charge-Transfer in Grazing Collisions of H with Vicinal Nanosurfaces on Cu(111), Au(100) and Pd(111) Substrates: A Comparative Study
Atoms 2019, 7(3), 89; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7030089 - 09 Sep 2019
Abstract
We compare the electron dynamics at monocrystalline Cu(111), Au(100) and Pd(111) precursor substrates with vicinal nanosteps. The unoccupied bands of a surface superlattice are populated via the resonant charge transfer (RCT) between the surface and a H ion that flies by at [...] Read more.
We compare the electron dynamics at monocrystalline Cu(111), Au(100) and Pd(111) precursor substrates with vicinal nanosteps. The unoccupied bands of a surface superlattice are populated via the resonant charge transfer (RCT) between the surface and a H ion that flies by at grazing angles. A quantum mechanical wave packet propagation approach is used to simulate the motion of the active electron, and time-evolved wave packet densities are used to visualize the dynamics through the superlattice. The survived ion fraction in the reflected beam generally exhibits modulations as a function of the vicinal terrace size and shows peaks at those energies that access the image state subband dispersions. Differences in magnitudes of the ion-survival as a function of the particular substrate selection and the ion-surface interaction time, based on the choice of two ion-trajectories, are examined. A square well model, producing standing waves between the steps on the surface, explains the energies of the maxima in the ion survival probability for all the metals considered. This indicates that the primary process of confinement induced subband formation is robust. The work may motivate measurements and applications of shallow-angle ion-scattering spectroscopy to access electronic substructures in periodically nanostructured surfaces. Full article
(This article belongs to the Section Atomic, Molecular and Nuclear Spectroscopy and Collisions)
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Article
Emission of Fast Hydrogen Atoms in a Low Density Gas Discharge—The Most “Natural” Mirror Laboratory
Atoms 2019, 7(3), 81; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7030081 - 19 Aug 2019
Cited by 2
Abstract
In this work, we present a new application for the line shapes of emission induced by reflected hydrogen atoms. Optical properties of the solids in contact with the plasma could be effectively measured at the wavelength of Balmer lines: time-resolved measurements of reflectance [...] Read more.
In this work, we present a new application for the line shapes of emission induced by reflected hydrogen atoms. Optical properties of the solids in contact with the plasma could be effectively measured at the wavelength of Balmer lines: time-resolved measurements of reflectance and polarization properties of mirrors are performed using the wavelength separation of the direct and reflected signals. One uses the Doppler effect of emission of atoms excited by collisions with noble gases, primarily with Ar or with Kr. In spite of a new application of line shapes, the question of the source of the strong signal in the case of Ar exists: the emission observed in the case of the excitation of H or D atoms by Ar exceeds the signal induced by collisions with Kr atoms by a factor of five, and the only available experimental data for the ground state excitation show practically equal cross-sections for both gases in the energy range of 80–200 eV. Full article
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Article
Electron-Impact Dissociation of Vibrationally-Excited Molecular Hydrogen into Neutral Fragments
Atoms 2019, 7(3), 75; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7030075 - 06 Aug 2019
Cited by 4
Abstract
We present convergent close-coupling (CCC) calculations of electron-impact dissociation of vibrationally-excited molecular hydrogen into neutral fragments. This work follows from our previous results for dissociation of molecular hydrogen in the ground vibrational level [Scarlett et al., Eur. Phys. J. D 72, 34 [...] Read more.
We present convergent close-coupling (CCC) calculations of electron-impact dissociation of vibrationally-excited molecular hydrogen into neutral fragments. This work follows from our previous results for dissociation of molecular hydrogen in the ground vibrational level [Scarlett et al., Eur. Phys. J. D 72, 34 (2018)], which were obtained from calculations performed in a spherical coordinate system. The present calculations, performed utilizing a spheroidal formulation of the molecular CCC method, reproduce the previous dissociation cross sections for the ground vibrational level, while allowing the extension to scattering on excited levels. Full article
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Article
Cross Sections and Rate Coefficients for Rovibrational Excitation of HeH+ Isotopologues by Electron Impact
Atoms 2019, 7(3), 67; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7030067 - 05 Jul 2019
Cited by 6
Abstract
Cross sections and thermal rate coefficients for rotational and vibration excitation of the four stable isotopologues of the 4HeH+ ion by electron impact are presented. The data are calculated using a previously developed theoretical approach. The obtained rate coefficients are fitted [...] Read more.
Cross sections and thermal rate coefficients for rotational and vibration excitation of the four stable isotopologues of the 4 HeH + ion by electron impact are presented. The data are calculated using a previously developed theoretical approach. The obtained rate coefficients are fitted to analytical formulas with the 10–10,000 K interval of applicability. These present results could be useful in tokamak plasma and astrophysical modeling and can help in the detection of these species in the interstellar medium. Full article
(This article belongs to the Section Atomic, Molecular and Nuclear Spectroscopy and Collisions)
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Article
Geometric Phase Effects in Ultracold Chemical Reactions
Atoms 2019, 7(3), 65; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7030065 - 03 Jul 2019
Cited by 4
Abstract
The role of the geometric phase effect in chemical reaction dynamics has long been a topic of active experimental and theoretical investigations. The topic has received renewed interest in recent years in cold and ultracold chemistry where it was shown to play a [...] Read more.
The role of the geometric phase effect in chemical reaction dynamics has long been a topic of active experimental and theoretical investigations. The topic has received renewed interest in recent years in cold and ultracold chemistry where it was shown to play a decisive role in state-to-state chemical dynamics. We provide a brief review of these developments focusing on recent studies of O + OH and hydrogen exchange in the H + H 2 and D + HD reactions at cold and ultracold temperatures. Non-adiabatic effects in ultracold chemical dynamics arising from the conical intersection between two electronic potential energy surfaces are also briefly discussed. By taking the hydrogen exchange reaction as an illustrative example it is shown that the inclusion of the geometric phase effect captures the essential features of non-adiabatic dynamics at collision energies below the conical intersection. Full article
(This article belongs to the Special Issue Few-body Physics in Ultracold Quantum Gases)
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Article
Cowan Code: 50 Years of Growing Impact on Atomic Physics
Atoms 2019, 7(3), 64; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7030064 - 02 Jul 2019
Cited by 19
Abstract
The famous Cowan’s book, “The Theory of Atomic Structure and Spectra”, published in 1981, and his suite of computer codes based on it, continue to be highly influential in atomic physics and many other research areas. As of September 2018, there have been [...] Read more.
The famous Cowan’s book, “The Theory of Atomic Structure and Spectra”, published in 1981, and his suite of computer codes based on it, continue to be highly influential in atomic physics and many other research areas. As of September 2018, there have been more than 5000 citations to Cowan’s book and codes, and each year adds about 150 citations to this list. The present work briefly describes what these codes do and why they are responsible for most of the current progress in the analyses of atomic spectra. Various modifications of these codes, including my own, will also be described. Full article
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Article
Towards the Search for Thallium Nuclear Schiff Moment in Polyatomic Molecules: Molecular Properties of Thallium Monocyanide (TlCN)
Atoms 2019, 7(3), 62; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7030062 - 29 Jun 2019
Cited by 6
Abstract
Molecular properties of the thallium monocyanide (Tl·CN) system in its ground electronic state are studied using high-precision ab initio relativistic two-component pseudopotential replacing 60 inner-core electrons of Tl. A relativistic coupled-cluster method with single, double and perturbative triple amplitudes is employed to account [...] Read more.
Molecular properties of the thallium monocyanide (Tl·CN) system in its ground electronic state are studied using high-precision ab initio relativistic two-component pseudopotential replacing 60 inner-core electrons of Tl. A relativistic coupled-cluster method with single, double and perturbative triple amplitudes is employed to account for electronic correlations. Extrapolation of results to the complete basis set limit is used for all studied properties. The global potential energy minimum of Tl·CN corresponds to the linear cyanide (TlCN) isomer, while the non-rigid isocyanide-like (TlNC) structure lies by approximately 11 kJ/mol higher in energy. The procedure of restoration of the wavefunction in the “core” region of Tl atom was applied to calculate the interaction of the Tl nuclear Schiff moment with electrons. The parameter X of the interaction of the Tl nuclear Schiff moment with electrons in the linear TlCN molecule equals 7150 a.u. The prospects of using the TlCN molecule for the experimental detection of the nuclear Schiff moment are discussed. Full article
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Article
Transition Rates for 3s3p2 4P–3s3p4s 4Po Transitions in Al i
Atoms 2019, 7(2), 54; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7020054 - 04 Jun 2019
Abstract
Fully relativistic calculations have been performed for two multiplets, 3s3p24P and 3s3p4s4Po, in Al i. Wave functions were obtained for all levels of these multiplets using the [...] Read more.
Fully relativistic calculations have been performed for two multiplets, 3 s 3 p 2 4 P and 3 s 3 p 4 s 4 P o , in Al i. Wave functions were obtained for all levels of these multiplets using the grasp programs. Reported are the E1 transitions rates for all transitions between levels of these multiplets. Transition energies and transition rates are compared with observed values and other theory. Our calculated transition rates are smaller by about 10% than observed rates, reducing a large discrepancy between earlier calculations and experiments. Full article
(This article belongs to the Section Atomic, Molecular and Nuclear Spectroscopy and Collisions)
Article
Measurement of the Magnetic Field in a Linear Magnetized Plasma by Tunable Diode Laser Absorption Spectroscopy
Atoms 2019, 7(2), 48; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7020048 - 15 May 2019
Cited by 6
Abstract
Tunable diode laser absorption spectroscopy (TDLAS) is a commonly used technique to measure the temperature and density of atoms or molecules in a gas. In this work, we demonstrate that the TDLAS diagnostics could be effectively applied to measure the magnetic field in [...] Read more.
Tunable diode laser absorption spectroscopy (TDLAS) is a commonly used technique to measure the temperature and density of atoms or molecules in a gas. In this work, we demonstrate that the TDLAS diagnostics could be effectively applied to measure the magnetic field in a low-density weakly magnetized plasma using the Zeeman splitting of the absorption spectrum of lines from noble gases. The laser wavelength is tailored to fit the 1 s 5 2 p 6 transition of atomic Ar with the wavelength λ = 763.51 nm . Two mechanisms of line broadening and splitting are observed: Doppler broadening and Zeeman effect. The latter is especially pronounced by applying polarization-selective observation of the absorption to the TDLAS measurements. By fitting the σ and π components of the absorption spectrum, the line-integrated magnetic field on the order of 30–50 mT is determined. The agreement between the measured values and the vacuum field (neglecting the impact of the plasma) calculations on the axis of the PSI-2 is found to be about 15–20%. Full article
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Article
Towards an Improved Test of the Standard Model’s Most Precise Prediction
Atoms 2019, 7(2), 45; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7020045 - 25 Apr 2019
Cited by 20
Abstract
The electron and positron magnetic moments are the most precise prediction of the standard model of particle physics. The most accurate measurement of a property of an elementary particle has been made to test this result. A new experimental method is now being [...] Read more.
The electron and positron magnetic moments are the most precise prediction of the standard model of particle physics. The most accurate measurement of a property of an elementary particle has been made to test this result. A new experimental method is now being employed in an attempt to improve the measurement accuracy by an order of magnitude. Positrons from a “student source” now suffice for the experiment. Progress toward a new measurement is summarized. Full article
(This article belongs to the Special Issue High Precision Measurements of Fundamental Constants)
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Article
Testing Quantum Coherence in Stochastic Electrodynamics with Squeezed Schrödinger Cat States
Atoms 2019, 7(2), 42; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7020042 - 05 Apr 2019
Cited by 8
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
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Article
Radiative Transition Parameters in Atomic Lanthanum from Pseudo-Relativistic Hartree–Fock and Fully Relativistic Dirac–Hartree–Fock Calculations
Atoms 2019, 7(1), 38; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7010038 - 20 Mar 2019
Cited by 6
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 article
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Article
Theory of the Anomalous Magnetic Moment of the Electron
Atoms 2019, 7(1), 28; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7010028 - 22 Feb 2019
Cited by 143
Abstract
The anomalous magnetic moment of the electron ae 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 α , with an effective parameter α / π . Both numerical and analytic evaluations of a e up to ( α / π ) 4 are firmly established. The coefficient of ( α / π ) 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 ( theory ) = 1 159 652 181.606 ( 11 ) ( 12 ) ( 229 ) × 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: α 1 ( Cs ) = 137.035 999 046 ( 27 ) . The discrepancy between a e ( theory ) and a e ( ( experiment ) ) is 2.4 σ . Assuming that the standard model is valid so that a e (theory) = a e (experiment) holds, we obtain α 1 ( a e ) = 137.035 999 1496 ( 13 ) ( 14 ) ( 330 ) , which is nearly as accurate as α 1 ( Cs ) . The uncertainties are from the tenth-order QED term, hadronic term, and the best measurement of a e , in this order. Full article
(This article belongs to the Special Issue High Precision Measurements of Fundamental Constants)
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Article
Capture Cross Sections and Radiative Emission-Line Strengths for Slow Ne8+ Collisions with He and H2
Atoms 2019, 7(1), 15; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7010015 - 23 Jan 2019
Cited by 1
Abstract
The Ne8+–He and –H2 collision systems are examined at impact speeds ranging between 0.17 and 0.4 a.u. Transition probabilities for electron capture are obtained using the two-center basis generator method performed within the independent-electron model. The aim of calculating capture [...] Read more.
The Ne8+–He and –H2 collision systems are examined at impact speeds ranging between 0.17 and 0.4 a.u. Transition probabilities for electron capture are obtained using the two-center basis generator method performed within the independent-electron model. The aim of calculating capture cross sections for these collision systems is to provide new theoretical verification of previously reported experimental data and to provide aid for astrophysical X-ray studies. This study also examines the applicability of the independent-electron model with effective potentials to describe two-electron capture for these two systems. Comparisons of capture cross sections and radiative-emission counts with the available experimental and theoretical data show an overall good agreement. Full article
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Article
Two-Photon Vibrational Transitions in 16O2+ as Probes of Variation of the Proton-to-Electron Mass Ratio
Atoms 2019, 7(1), 1; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms7010001 - 20 Dec 2018
Cited by 7
Abstract
Vibrational overtones in deeply-bound molecules are sensitive probes for variation of the proton-to-electron mass ratio μ. In nonpolar molecules, these overtones may be driven as two-photon transitions. Here, we present procedures for experiments with 16O2+, including state-preparation through [...] Read more.
Vibrational overtones in deeply-bound molecules are sensitive probes for variation of the proton-to-electron mass ratio μ . In nonpolar molecules, these overtones may be driven as two-photon transitions. Here, we present procedures for experiments with 16 O 2 + , including state-preparation through photoionization, a two-photon probe, and detection. We calculate transition dipole moments between all X 2 Π g vibrational levels and those of the A 2 Π u excited electronic state. Using these dipole moments, we calculate two-photon transition rates and AC-Stark-shift systematics for the overtones. We estimate other systematic effects and statistical precision. Two-photon vibrational transitions in 16 O 2 + provide multiple routes to improved searches for μ variation. Full article
(This article belongs to the Special Issue High Precision Measurements of Fundamental Constants)
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Article
Measurements of the Neutron Lifetime
Atoms 2018, 6(4), 70; https://0-doi-org.brum.beds.ac.uk/10.3390/atoms6040070 - 10 Dec 2018
Cited by 14
Abstract
Free neutron decay is a fundamental process in particle and nuclear physics. It is the prototype for nuclear beta decay and other semileptonic weak particle decays. Neutron decay played a key role in the formation of light elements in the early universe. The [...] Read more.
Free neutron decay is a fundamental process in particle and nuclear physics. It is the prototype for nuclear beta decay and other semileptonic weak particle decays. Neutron decay played a key role in the formation of light elements in the early universe. The precise value of the neutron mean lifetime, about 15 min, has been the subject of many experiments over the past 70 years. The two main experimental methods, the beam method and the ultracold neutron storage method, give average values of the neutron lifetime that currently differ by 8.7 s (4 standard deviations), a serious discrepancy. The physics of neutron decay, implications of the neutron lifetime, previous and recent experimental measurements, and prospects for the future are reviewed. Full article
(This article belongs to the Special Issue High Precision Measurements of Fundamental Constants)
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Figure 1