Atoms

12 pages, 5838 KiB

Open AccessEditor’s ChoiceArticle

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

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

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

Cited by 9 | Viewed by 2920

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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14 pages, 2158 KiB

Open AccessEditor’s ChoiceArticle

The Optogalvanic Spectrum of Neutral Lanthanum between 5610 and 6110 Å

by Laurentius Windholz and Tobias Binder

Atoms 2020, 8(2), 23; https://doi.org/10.3390/atoms8020023 - 19 May 2020

Cited by 8 | Viewed by 2647

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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18 pages, 694 KiB

Open AccessEditor’s ChoiceReview

A Laboratory Astrophysics Problem: The Lifetime of Very Long-Lived Levels in Low-Charge Ions

by Elmar Träbert

Atoms 2020, 8(2), 21; https://doi.org/10.3390/atoms8020021 - 11 May 2020

Cited by 8 | Viewed by 2854

Abstract

Emission lines of singly charged ions populate many astrophysical spectra. However, the interpretation of the line intensities (usually line ratios) often depends on the transition rates of the decays of very long-lived low-lying levels. For example, the line ratio of two electric-dipole forbidden [...] Read more.

Emission lines of singly charged ions populate many astrophysical spectra. However, the interpretation of the line intensities (usually line ratios) often depends on the transition rates of the decays of very long-lived low-lying levels. For example, the line ratio of two electric-dipole forbidden transitions in the 3s

^{2}

3p

^{3}

ground configuration of singly ionized sulfur (ion S

^{+}

, spectrum S II) has been interpreted in terms of a density diagnostic for planetary nebulae, i.e., for densities in the order of 10

^{4}

cm

^{- 3}

. The predicted lifetimes of the

^{2}

D

_{3 / 2, 5 / 2}^{o}

levels are in the order of one hour. Modeling indicates that a 10% uncertainty of the lifetime determination in this case corresponds to a 50% uncertainty of the density diagnostic. The available theoretical lifetime predictions scatter by much more than 10%. Considerations about an experimental approach are presented with the goal of instigating a measurement of the actual level lifetimes. Full article

(This article belongs to the Section Atomic, Molecular and Nuclear Spectroscopy and Collisions)

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20 pages, 2354 KiB

Open AccessEditor’s ChoiceArticle

Current Status and Developments of the Atomic Database on Rare-Earths at Mons University (DREAM)

by Pascal Quinet and Patrick Palmeri

Atoms 2020, 8(2), 18; https://doi.org/10.3390/atoms8020018 - 2 May 2020

Cited by 16 | Viewed by 3007

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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12 pages, 518 KiB

Open AccessEditor’s ChoiceArticle

Referencing Sources of Molecular Spectroscopic Data in the Era of Data Science: Application to the HITRAN and AMBDAS Databases

by Frances M. Skinner, Iouli E. Gordon, Christian Hill, Robert J. Hargreaves, Kelly E. Lockhart and Laurence S. Rothman

Atoms 2020, 8(2), 16; https://doi.org/10.3390/atoms8020016 - 30 Apr 2020

Cited by 6 | Viewed by 4302

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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24 pages, 333 KiB

Open AccessFeature PaperEditor’s ChoiceReview

The Leiden Atomic and Molecular Database (LAMDA): Current Status, Recent Updates, and Future Plans

by Floris F. S. van der Tak, François Lique, Alexandre Faure, John H. Black and Ewine F. van Dishoeck

Atoms 2020, 8(2), 15; https://doi.org/10.3390/atoms8020015 - 28 Apr 2020

Cited by 74 | Viewed by 4580

Abstract

The Leiden Atomic and Molecular Database (LAMDA) collects spectroscopic information and collisional rate coefficients for molecules, atoms, and ions of astrophysical and astrochemical interest. We describe the developments of the database since its inception in 2005, and outline our plans for the near [...] Read more.

The Leiden Atomic and Molecular Database (LAMDA) collects spectroscopic information and collisional rate coefficients for molecules, atoms, and ions of astrophysical and astrochemical interest. We describe the developments of the database since its inception in 2005, and outline our plans for the near future. Such a database is constrained both by the nature of its uses and by the availability of accurate data: we suggest ways to improve the synergies among users and suppliers of data. We summarize some recent developments in computation of collisional cross sections and rate coefficients. We consider atomic and molecular data that are needed to support astrophysics and astrochemistry with upcoming instruments that operate in the mid- and far-infrared parts of the spectrum. Full article

(This article belongs to the Special Issue Development and Perspectives of Atomic and Molecular Databases)

13 pages, 1007 KiB

Open AccessEditor’s ChoiceArticle

Elastic Photon Scattering on Hydrogenic Atoms near Resonances

by Dmitrii Samoilenko, Andrey V. Volotka and Stephan Fritzsche

Atoms 2020, 8(2), 12; https://doi.org/10.3390/atoms8020012 - 18 Apr 2020

Cited by 6 | Viewed by 2651

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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9 pages, 1036 KiB

Open AccessEditor’s ChoiceArticle

Differential Study of Projectile Coherence Effects on Double Capture Processes in p + Ar Collisions

by Trevor Voss, Basu R. Lamichhane, Madhav Dhital, Ramaz Lomsadze and Michael Schulz

Atoms 2020, 8(2), 10; https://doi.org/10.3390/atoms8020010 - 28 Mar 2020

Cited by 2 | Viewed by 3149

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

(This article belongs to the Special Issue Accelerator-Based Atomic Physics: Fast Ion–Atom and Ion–Molecule Collisions)

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11 pages, 350 KiB

Open AccessEditor’s ChoiceArticle

Atomic Data for Calculation of the Intensities of Stark Components of Excited Hydrogen Atoms in Fusion Plasmas

by Oleksandr Marchuk, David R. Schultz and Yuri Ralchenko

Atoms 2020, 8(1), 8; https://doi.org/10.3390/atoms8010008 - 18 Feb 2020

Cited by 2 | Viewed by 2718

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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20 pages, 3992 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Empirical Line Lists in the ExoMol Database

by Yixin Wang, Jonathan Tennyson and Sergei N. Yurchenko

Atoms 2020, 8(1), 7; https://doi.org/10.3390/atoms8010007 - 17 Feb 2020

Cited by 18 | Viewed by 4072

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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11 pages, 1631 KiB

Open AccessEditor’s ChoiceArticle

The Belgian Repository of Fundamental Atomic Data and Stellar Spectra (BRASS)

by Alex Lobel, Pierre Royer, Christophe Martayan, Michael Laverick, Thibault Merle, Mathieu Van der Swaelmen, Peter A. M. van Hoof, Marc David, Herman Hensberge and Emmanuel Thienpont

Atoms 2019, 7(4), 105; https://doi.org/10.3390/atoms7040105 - 22 Nov 2019

Cited by 2 | Viewed by 2949

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

(This article belongs to the Special Issue 13th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas)

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11 pages, 973 KiB

Open AccessEditor’s ChoiceArticle

Quantemol Electron Collisions (QEC): An Enhanced Expert System for Performing Electron Molecule Collision Calculations Using the R-Matrix Method

by Bridgette Cooper, Maria Tudorovskaya, Sebastian Mohr, Aran O’Hare, Martin Hanicinec, Anna Dzarasova, Jimena D. Gorfinkiel, Jakub Benda, Zdeněk Mašín, Ahmed F. Al-Refaie, Peter J. Knowles and Jonathan Tennyson

Atoms 2019, 7(4), 97; https://doi.org/10.3390/atoms7040097 - 17 Oct 2019

Cited by 49 | Viewed by 6709

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

(This article belongs to the Special Issue Eleventh International Conference on Atomic and Molecular Data and Their Applications)

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8 pages, 608 KiB

Open AccessEditor’s ChoiceArticle

Charge Exchange Cross Sections for Noble Gas Ions and N₂ between 0.2 and 5.0 keV

by Steven Bromley, Corey Ahl, Chad Sosolik and Joan Marler

Atoms 2019, 7(4), 96; https://doi.org/10.3390/atoms7040096 - 14 Oct 2019

Cited by 2 | Viewed by 3504

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

(This article belongs to the Special Issue Eleventh International Conference on Atomic and Molecular Data and Their Applications)

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11 pages, 307 KiB

Open AccessEditor’s ChoiceArticle

Line Shapes in a Magnetic Field: Trajectory Modifictions II: Full Collision-Time Statistics

by Spiros Alexiou

Atoms 2019, 7(4), 94; https://doi.org/10.3390/atoms7040094 - 5 Oct 2019

Cited by 13 | Viewed by 2483

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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8 pages, 538 KiB

Open AccessEditor’s ChoiceArticle

Mixed Basis Sets for Atomic Calculations

by Mikhail Kozlov and Ilya Tupitsyn

Atoms 2019, 7(3), 92; https://doi.org/10.3390/atoms7030092 - 19 Sep 2019

Cited by 6 | Viewed by 3300

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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18 pages, 1319 KiB

Open AccessEditor’s ChoiceArticle

Resonant Charge-Transfer in Grazing Collisions of H⁻ with Vicinal Nanosurfaces on Cu(111), Au(100) and Pd(111) Substrates: A Comparative Study

by John Shaw, David Monismith, Yixiao Zhang, Danielle Doerr and Himadri S. Chakraborty

Atoms 2019, 7(3), 89; https://doi.org/10.3390/atoms7030089 - 9 Sep 2019

Cited by 6 | Viewed by 2849

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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16 pages, 493 KiB

Open AccessEditor’s ChoiceArticle

Emission of Fast Hydrogen Atoms in a Low Density Gas Discharge—The Most “Natural” Mirror Laboratory

by Oleksandr Marchuk, Sven Dickheuer, Stephan Ertmer, Yuri Krasikov, Philippe Mertens, Christian Brandt, Sebastijan Brezinsek, Andrei Goriaev, Mykola Ialovega, Beatrix Göths, Arkadi Kreter and Christian Linsmeier

Atoms 2019, 7(3), 81; https://doi.org/10.3390/atoms7030081 - 19 Aug 2019

Cited by 2 | Viewed by 5135

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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10 pages, 390 KiB

Open AccessEditor’s ChoiceArticle

Electron-Impact Dissociation of Vibrationally-Excited Molecular Hydrogen into Neutral Fragments

by Liam H. Scarlett, Jeremy S. Savage, Dmitry V. Fursa, Mark C. Zammit and Igor Bray

Atoms 2019, 7(3), 75; https://doi.org/10.3390/atoms7030075 - 6 Aug 2019

Cited by 7 | Viewed by 4106

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

(This article belongs to the Special Issue Eleventh International Conference on Atomic and Molecular Data and Their Applications)

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11 pages, 1370 KiB

Open AccessEditor’s ChoiceArticle

Cross Sections and Rate Coefficients for Rovibrational Excitation of HeH⁺ Isotopologues by Electron Impact

by Mehdi Ayouz and Viatcheslav Kokoouline

Atoms 2019, 7(3), 67; https://doi.org/10.3390/atoms7030067 - 5 Jul 2019

Cited by 10 | Viewed by 3219

Abstract

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 [...] 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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13 pages, 1715 KiB

Open AccessEditor’s ChoiceArticle

Geometric Phase Effects in Ultracold Chemical Reactions

by Brian K. Kendrick and N. Balakrishnan

Atoms 2019, 7(3), 65; https://doi.org/10.3390/atoms7030065 - 3 Jul 2019

Cited by 8 | Viewed by 3938

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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14 pages, 2502 KiB

Open AccessEditor’s ChoiceArticle

Cowan Code: 50 Years of Growing Impact on Atomic Physics

by Alexander Kramida

Atoms 2019, 7(3), 64; https://doi.org/10.3390/atoms7030064 - 2 Jul 2019

Cited by 37 | Viewed by 7040

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

(This article belongs to the Special Issue Eleventh International Conference on Atomic and Molecular Data and Their Applications)

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8 pages, 349 KiB

Open AccessEditor’s ChoiceArticle

Towards the Search for Thallium Nuclear Schiff Moment in Polyatomic Molecules: Molecular Properties of Thallium Monocyanide (TlCN)

by A. V. Kudrin, A. Zaitsevskii, T. A. Isaev, D. E. Maison and L. V. Skripnikov

Atoms 2019, 7(3), 62; https://doi.org/10.3390/atoms7030062 - 29 Jun 2019

Cited by 7 | Viewed by 3182

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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6 pages, 267 KiB

Open AccessEditor’s ChoiceArticle

Transition Rates for 3s3p² ⁴P–3s3p4s ⁴P^o Transitions in Al i

by Charlotte Froese Fischer and James F. Babb

Atoms 2019, 7(2), 54; https://doi.org/10.3390/atoms7020054 - 4 Jun 2019

Viewed by 3076

Abstract

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 [...] 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)

10 pages, 936 KiB

Open AccessEditor’s ChoiceArticle

Measurement of the Magnetic Field in a Linear Magnetized Plasma by Tunable Diode Laser Absorption Spectroscopy

by Sven Dickheuer, Oleksandr Marchuk, Tsanko Vaskov Tsankov, Dirk Luggenhölscher, Uwe Czarnetzki, Wojciech Gromelski, Stephan Ertmer and Arkadi Kreter

Atoms 2019, 7(2), 48; https://doi.org/10.3390/atoms7020048 - 15 May 2019

Cited by 7 | Viewed by 5107

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} \to 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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19 pages, 1488 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Towards an Improved Test of the Standard Model’s Most Precise Prediction

by G. Gabrielse, S. E. Fayer, T. G. Myers and X. Fan

Atoms 2019, 7(2), 45; https://doi.org/10.3390/atoms7020045 - 25 Apr 2019

Cited by 27 | Viewed by 6089

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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9 pages, 1034 KiB

Open AccessEditor’s ChoiceArticle

Testing Quantum Coherence in Stochastic Electrodynamics with Squeezed Schrödinger Cat States

by Wayne Cheng-Wei Huang and Herman Batelaan

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

Cited by 11 | Viewed by 4130

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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17 pages, 2099 KiB

Open AccessEditor’s ChoiceArticle

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

by Sébastien Gamrath, Patrick Palmeri and Pascal Quinet

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

Cited by 9 | Viewed by 3221

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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23 pages, 327 KiB

Open AccessEditor’s ChoiceReview

High-Precision Atomic Mass Measurements for Fundamental Constants

by Edmund G. Myers

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

Cited by 30 | Viewed by 5727

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 article

(This article belongs to the Special Issue High Precision Measurements of Fundamental Constants)

27 pages, 564 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Theory of the Anomalous Magnetic Moment of the Electron

by Tatsumi Aoyama, Toichiro Kinoshita and Makiko Nio

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

Cited by 327 | Viewed by 14214

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

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) \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:

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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12 pages, 409 KiB

Open AccessEditor’s ChoiceArticle

Capture Cross Sections and Radiative Emission-Line Strengths for Slow Ne⁸⁺ Collisions with He and H₂

by Anthony C. K. Leung and Tom Kirchner

Atoms 2019, 7(1), 15; https://doi.org/10.3390/atoms7010015 - 23 Jan 2019

Cited by 2 | Viewed by 3288

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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19 pages, 1063 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Two-Photon Vibrational Transitions in ¹⁶O₂⁺ as Probes of Variation of the Proton-to-Electron Mass Ratio

by Ryan Carollo, Alexander Frenett and David Hanneke

Atoms 2019, 7(1), 1; https://doi.org/10.3390/atoms7010001 - 20 Dec 2018

Cited by 13 | Viewed by 4043

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

^{16} O_{2}^{+}

, 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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19 pages, 5766 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Measurements of the Neutron Lifetime

by F. E. Wietfeldt

Atoms 2018, 6(4), 70; https://doi.org/10.3390/atoms6040070 - 10 Dec 2018

Cited by 22 | Viewed by 7233

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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