Revisiting the Stark Width and Shift of He II P*α**Atoms* **2018**, *6*(2), 23; doi:10.3390/atoms6020023 - 24 April 2018**Abstract **

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We report experimental determination of plasma-induced Stark widths and shifts of the He II P $\alpha $ line and a comparison of the results with calculations performed by several computational approaches. The measurements were carried out in a small compressing plasma channel device, reaching

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We report experimental determination of plasma-induced Stark widths and shifts of the He II P $\alpha $ line and a comparison of the results with calculations performed by several computational approaches. The measurements were carried out in a small compressing plasma channel device, reaching electron densities in excess of ${10}^{18}\phantom{\rule{0.166667em}{0ex}}{\mathrm{cm}}^{-3}$ and temperatures of a few eV. The experimental data are in a good agreement with some previously published studies. However, the measured relation between the Stark shift and width could not be reproduced by either of the codes, and this disagreement is not yet resolved. This suggests the existence of an additional effect that is not accounted for in the present models and leads to a larger than expected Stark shift of the He II P $\alpha $ line.
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Matrix Methods for Solving Hartree-Fock Equations in Atomic Structure Calculations and Line Broadening*Atoms* **2018**, *6*(2), 22; doi:10.3390/atoms6020022 - 23 April 2018**Abstract **

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Atomic structure of N-electron atoms is often determined by solving the Hartree-Fock equations, which are a set of integro-differential equations. The integral part of the Hartree-Fock equations treats electron exchange, but the Hartree-Fock equations are not often treated as an integro-differential equation. The

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Atomic structure of N-electron atoms is often determined by solving the Hartree-Fock equations, which are a set of integro-differential equations. The integral part of the Hartree-Fock equations treats electron exchange, but the Hartree-Fock equations are not often treated as an integro-differential equation. The exchange term is often approximated as an inhomogeneous or an effective potential so that the Hartree-Fock equations become a set of ordinary differential equations (which can be solved using the usual shooting methods). Because the Hartree-Fock equations are an iterative-refinement method, the inhomogeneous term relies on the previous guess of the wavefunction. In addition, there are numerical complications associated with solving inhomogeneous differential equations. This work uses matrix methods to solve the Hartree-Fock equations as an integro-differential equation. It is well known that a derivative operator can be expressed as a matrix made of finite-difference coefficients; energy eigenvalues and eigenvectors can be obtained by using linear-algebra packages. The integral (exchange) part of the Hartree-Fock equation can be approximated as a sum and written as a matrix. The Hartree-Fock equations can be solved as a matrix that is the sum of the differential and integral matrices. We compare calculations using this method against experiment and standard atomic structure calculations. This matrix method can also be used to solve for free-electron wavefunctions, thus improving how the atoms and free electrons interact. This technique is important for spectral line broadening in two ways: it improves the atomic structure calculations, and it improves the motion of the plasma electrons that collide with the atom.
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Improving the Method of Measuring the Electron Density via the Asymmetry of Hydrogenic Spectral Lines in Plasmas by Allowing for Penetrating Ions*Atoms* **2018**, *6*(2), 21; doi:10.3390/atoms6020021 - 18 April 2018**Abstract **

There was previously proposed and experimentally implemented a new diagnostic method for measuring the electron density N_{e} using the asymmetry of hydrogenic spectral lines in dense plasmas. Compared to the traditional method of deducing N_{e} from the experimental widths of spectral

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There was previously proposed and experimentally implemented a new diagnostic method for measuring the electron density N_{e} using the asymmetry of hydrogenic spectral lines in dense plasmas. Compared to the traditional method of deducing N_{e} from the experimental widths of spectral lines, the new method has the following advantages. First, the traditional method requires measuring widths of at least two spectral lines (to isolate the Stark broadening from competing broadening mechanisms), while for the new diagnostic method it is sufficient to obtain the experimental profile of just one spectral line. Second, the traditional method would be difficult to implement if the center of the spectral lines was optically thick, while the new diagnostic method could still be used even in this case. In the theory underlying this new diagnostic method, the contribution of plasma ions to the spectral line asymmetry was calculated only for configurations where the perturbing ions were outside the bound electron cloud of the radiating atom/ion (non-penetrating configurations). In the present paper, we take into account the contribution to the spectral line asymmetry from *penetrating configurations*, where the perturbing ion is inside the bound electron cloud of the radiating atom/ion. We show that in high-density plasmas, the allowance for penetrating ions can result in significant corrections to the electron density deduced from the spectral line asymmetry.
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Quantum and Semiclassical Stark Widths for Ar VII Spectral Lines*Atoms* **2018**, *6*(2), 20; doi:10.3390/atoms6020020 - 16 April 2018**Abstract **

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We present in this paper the results of a theoretical study of electron impact broadening for several lines of the Ar VII ion. The results have been obtained using our quantum mechanical method and the semiclassical perturbation one. Results are presented for electron

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We present in this paper the results of a theoretical study of electron impact broadening for several lines of the Ar VII ion. The results have been obtained using our quantum mechanical method and the semiclassical perturbation one. Results are presented for electron density 10^{18} cm^{−3} and for electron temperatures ranging from $2\times {10}^{4}$ to $5\times {10}^{5}$ K required for plasma modeling. Our results have been compared to other semiclassical ones obtained using different sources of atomic data. A study of the strong collisions contributions to line broadening has been performed. The atomic structure and collision data used for the calculations of line broadening are also calculated by our codes and compared to available theoretical results. The agreement found between the two calculations ensures that our line broadening procedure uses adequate structure and collision data.
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Broadening of the Neutral Helium 492 nm Line in a Corona Discharge: Code Comparisons and Data Fitting*Atoms* **2018**, *6*(2), 19; doi:10.3390/atoms6020019 - 16 April 2018**Abstract **

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Passive plasma spectroscopy is a well-established non-intrusive diagnostic technique. Depending on the emitter and its environment which determine the dominant interactions and effects governing emission line shapes, passive spectroscopy allows the determination of electron densities, emitter and perturber temperatures, as well as other

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Passive plasma spectroscopy is a well-established non-intrusive diagnostic technique. Depending on the emitter and its environment which determine the dominant interactions and effects governing emission line shapes, passive spectroscopy allows the determination of electron densities, emitter and perturber temperatures, as well as other quantities like relative abundances. However, using spectroscopy requires appropriate line shape codes retaining all the physical effects governing the emission line profiles. This is required for line shape code developers to continuously correct or improve them to increase their accuracy when applied for diagnostics. This is exactly the aim expected from code–code and code–data comparisons. In this context, the He i 492 nm line emitted in a helium corona discharge at room temperature represents an ideal case since its profile results from several broadening mechanisms: Stark, Doppler, resonance, and van der Waals. The importance of each broadening mechanism depends on the plasma parameters. Here the profiles of the He i 492 nm in a helium plasma computed by various codes are compared for a selected set of plasma parameters. In addition, preliminary results related to plasma parameter determination using an experimental spectrum from a helium corona discharge at atmospheric pressure, are presented.
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Influence of the p ¯ -p Nuclear Interaction on the Rate of the Low-Energy p ¯ + H μ → ( p ¯ p ) α + μ − Reaction*Atoms* **2018**, *6*(2), 18; doi:10.3390/atoms6020018 - 9 April 2018**Abstract **

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The influence of an additional strong $\overline{\mathrm{p}}$ -p nuclear interaction in a three-charge-particle system with arbitrary masses is investigated. Specifically, the system of $\overline{\mathrm{p}},\phantom{\rule{4pt}{0ex}}{\mu}^{-}$ , and p is considered in this paper, where $\overline{\mathrm{p}}$ is an antiproton,

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The influence of an additional strong $\overline{\mathrm{p}}$ -p nuclear interaction in a three-charge-particle system with arbitrary masses is investigated. Specifically, the system of $\overline{\mathrm{p}},\phantom{\rule{4pt}{0ex}}{\mu}^{-}$ , and p is considered in this paper, where $\overline{\mathrm{p}}$ is an antiproton, ${\mu}^{-}$ is a muon and p is a proton. A numerical computation in the framework of a detailed few-body approach is carried out for the following protonium (antiprotonic hydrogen) formation three-body reaction: $\overline{\mathrm{p}}+{\mathrm{H}}_{\mu}\left(1s\right)\to {\left(\overline{\mathrm{p}}\mathrm{p}\right)}_{\alpha}+{\mu}^{-}$ . Here, ${\mathrm{H}}_{\mu}\left(1s\right)$ is a ground state muonic hydrogen, i.e., a bound state of p and ${\mu}^{-}$ . A bound state of *p* and its antimatter counterpart $\overline{\mathrm{p}}$ is a protonium atom in a quantum atomic state $\alpha $ , i.e., $Pn={\left(\overline{\mathrm{p}}\mathrm{p}\right)}_{\alpha}$ . The low-energy cross sections and rates of the $Pn$ formation reaction are computed in the framework of coupled Faddeev-Hahn-type equations. The strong $\overline{\mathrm{p}}$ -p interaction is included in these calculations within a first order approximation. It was found, that the inclusion of the nuclear interaction results in a quite significant correction to the rate of the three-body reaction.
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Studying Antimatter Gravity with Muonium*Atoms* **2018**, *6*(2), 17; doi:10.3390/atoms6020017 - 9 April 2018**Abstract **

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The gravitational acceleration of antimatter, $\overline{g}$ , has yet to be directly measured; an unexpected outcome of its measurement could change our understanding of gravity, the universe, and the possibility of a fifth force. Three avenues are apparent for such a measurement:

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The gravitational acceleration of antimatter, $\overline{g}$ , has yet to be directly measured; an unexpected outcome of its measurement could change our understanding of gravity, the universe, and the possibility of a fifth force. Three avenues are apparent for such a measurement: antihydrogen, positronium, and muonium, the last requiring a precision atom interferometer and novel muonium beam under development. The interferometer and its few-picometer alignment and calibration systems appear feasible. With 100 nm grating pitch, measurements of $\overline{g}$ to 10%, 1%, or better can be envisioned. These could constitute the first gravitational measurements of leptonic matter, of 2nd-generation matter, and possibly, of antimatter.
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Doppler Broadening of Spectral Line Shapes in Relativistic Plasmas*Atoms* **2018**, *6*(2), 16; doi:10.3390/atoms6020016 - 4 April 2018**Abstract **

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In this work, we report some relativistic effects on the spectral line broadening. In particular, we give a new Doppler broadening in extra hot plasmas that takes into account the possible high velocity of the emitters. This suggests the use of an appropriate

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In this work, we report some relativistic effects on the spectral line broadening. In particular, we give a new Doppler broadening in extra hot plasmas that takes into account the possible high velocity of the emitters. This suggests the use of an appropriate distribution of the velocities for the emitters. Indeed, the Juttner-Maxwell distribution of the velocities is more adequate for relativistic velocities of the emitters when the latter are in plasma with an extra high temperature. We find an asymmetry in the Doppler line shapes unlike the case of the traditional Doppler effect.
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Stark Broadening of Cr III Spectral Lines: DO White Dwarfs*Atoms* **2018**, *6*(2), 15; doi:10.3390/atoms6020015 - 3 April 2018**Abstract **

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Using the modified semiempirical method of Dimitrijević and Konjević, Stark widths have been calculated for six Cr III transitions, for an electron density of 10${}^{17}$ cm${}^{\u20123}$ and for temperatures from 5000–80,000 K. Results have been used for the investigation of

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Using the modified semiempirical method of Dimitrijević and Konjević, Stark widths have been calculated for six Cr III transitions, for an electron density of 10${}^{17}$ cm${}^{\u20123}$ and for temperatures from 5000–80,000 K. Results have been used for the investigation of the influence of Stark broadening on spectral lines in cool DO white dwarf atmospheres. Calculated Stark widths will be implemented in the STARK-B database, which is also a part of the Virtual Atomic and Molecular Data Center (VAMDC).
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The Possibility of Measuring Nuclear Shapes by Using Spectral Lines of Muonic Ions*Atoms* **2018**, *6*(2), 14; doi:10.3390/atoms6020014 - 1 April 2018**Abstract **

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We analytically calculated the shift of spectral lines of hydrogenlike ions for non-spherical nuclear shapes, such as the oblate or prolate ellipsoid of revolution. We show that the allowance for the ellipsoidal nuclear shape can change the shift of spectral lines of muonic

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We analytically calculated the shift of spectral lines of hydrogenlike ions for non-spherical nuclear shapes, such as the oblate or prolate ellipsoid of revolution. We show that the allowance for the ellipsoidal nuclear shape can change the shift of spectral lines of muonic hydrogenlike ions by several times compared to the corresponding shift for spherical nuclei. This can serve as an additional method for the experimental determination of the quadrupole moment of nuclei and of the standard beta-parameter related to the quadrupole moment.
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The Fourth Workshop on Lineshape Code Comparison: Line Merging*Atoms* **2018**, *6*(2), 13; doi:10.3390/atoms6020013 - 31 March 2018**Abstract **

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For a given set of plasma parameters, along a single series (Lyman, Balmer, etc.) the lines with higher principal quantum number (*n*) lines get progressively wider, closer to each other, and start merging for a certain critical *n*. In the

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For a given set of plasma parameters, along a single series (Lyman, Balmer, etc.) the lines with higher principal quantum number (*n*) lines get progressively wider, closer to each other, and start merging for a certain critical *n*. In the present work, four different codes (with further options) are used to calculate the entire Balmer series for moderate and high electron densities. Particular attention is paid to the relevant physics, such as the cutoff criteria, strong and penetrating electron collisions.
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Influence of Helical Trajectories of Perturbers on Stark Line Shapes in Magnetized Plasmas*Atoms* **2018**, *6*(1), 12; doi:10.3390/atoms6010012 - 13 March 2018**Abstract **

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In plasmas subject to a strong magnetic field, the dynamical properties of the microfield are affected by the cyclotron motion, which can alter Stark-broadened lines. We illustrate this effect through calculations of the hydrogen Lyman $\alpha $ line in an ideal one-component plasma. A

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In plasmas subject to a strong magnetic field, the dynamical properties of the microfield are affected by the cyclotron motion, which can alter Stark-broadened lines. We illustrate this effect through calculations of the hydrogen Lyman $\alpha $ line in an ideal one-component plasma. A focus is put on the central Zeeman component. It is shown that the atomic dipole autocorrelation function decreases more slowly if the cyclotron motion is retained. In the frequency domain, this denotes a reduction of the line broadening. A discussion based on numerical simulations and analytical estimates is done.
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ZEST: A Fast Code for Simulating Zeeman-Stark Line-Shape Functions*Atoms* **2018**, *6*(1), 11; doi:10.3390/atoms6010011 - 12 March 2018**Abstract **

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We present the ZEST code, dedicated to the calculation of line shapes broadened by Zeeman and Stark effects. As concerns the Stark effect, the model is based on the *Standard Lineshape Theory* in which ions are treated in the quasi-static approximation, whereas the

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We present the ZEST code, dedicated to the calculation of line shapes broadened by Zeeman and Stark effects. As concerns the Stark effect, the model is based on the *Standard Lineshape Theory* in which ions are treated in the quasi-static approximation, whereas the effects of electrons are represented by weak collisions in the framework of a binary collision relaxation theory. A static magnetic field may be taken into account in the radiator Hamiltonian in the dipole approximation, which leads to additional Zeeman splitting patterns. Ion dynamics effects are implemented using the fast Frequency-Fluctuation Model. For fast calculations, the static ion microfield distribution in the plasma is evaluated using analytic fits of Monte-Carlo simulations, which depend only on the ion-ion coupling parameter and the electron-ion screening factor.
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Stark Broadening of Se IV, Sn IV, Sb IV and Te IV Spectral Lines*Atoms* **2018**, *6*(1), 10; doi:10.3390/atoms6010010 - 7 March 2018**Abstract **

Stark broadening parameters, line width and shift, are needed for investigations, analysis and modelling of astrophysical, laboratory, laser produced and technological plasmas. Especially in astrophysics, due to constantly increasing resolution of satellite borne spectrographs, and large terrestrial telescopes, data on trace elements, which

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Stark broadening parameters, line width and shift, are needed for investigations, analysis and modelling of astrophysical, laboratory, laser produced and technological plasmas. Especially in astrophysics, due to constantly increasing resolution of satellite borne spectrographs, and large terrestrial telescopes, data on trace elements, which were previously insignificant, now have increasing importance. Using the modified semiempirical method of Dimitrijević and Konjević, here, Stark widths have been calculated for 2 Se IV, 6 Sn IV, 2 Sb IV and 1 Te IV transitions. Results have been compared with existing theoretical data for Sn IV. Obtained results will be implemented in the STARK-B database, which is also a part of Virtual atomic and molecular data center (VAMDC).
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Correcting the Input Data for Calculating the Asymmetry of Hydrogenic Spectral Lines in Plasmas*Atoms* **2018**, *6*(1), 9; doi:10.3390/atoms6010009 - 6 March 2018**Abstract **

We provide corrections to the data in Sholin’s tables from his paper in *Optics and Spectroscopy***26** (1969) 27. Since his data was used numerous times by various authors to calculate the asymmetry of hydrogenic spectral lines in plasmas, our corrections should motivate

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We provide corrections to the data in Sholin’s tables from his paper in *Optics and Spectroscopy***26** (1969) 27. Since his data was used numerous times by various authors to calculate the asymmetry of hydrogenic spectral lines in plasmas, our corrections should motivate revisions of the previous calculations of the asymmetry and its comparison with the experimental asymmetry, and thus should have a practical importance.
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The Effect of Correlation on Spectra of the Lanthanides: Pr^{3+}*Atoms* **2018**, *6*(1), 8; doi:10.3390/atoms6010008 - 25 February 2018**Abstract **

The effect of correlation on the spectra of lanthanide ions has been investigated using variational methods based on multiconfiguration Dirac–Hartree–Fock (MCDHF) theory. Results from several computational models are reported for Pr^{3+}. The first assumes an inactive Cd-like $4{d}^{10}$ core

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The effect of correlation on the spectra of lanthanide ions has been investigated using variational methods based on multiconfiguration Dirac–Hartree–Fock (MCDHF) theory. Results from several computational models are reported for Pr^{3+}. The first assumes an inactive Cd-like $4{d}^{10}$ core with valence electrons in $4{f}^{2}5{s}^{2}5{p}^{6}$ subshells. Additional models extend correlation to include core effects. It is shown that, with such models, the difference between computed energy levels and those from observed data increases with the energy of the level, suggesting that correlation among outer electrons should also be based on the correlated core of excited configuration state functions (CSFs). Some M1 transition probabilities are reported for the most accurate model and compared with predictions obtained from semi-empirical methods.
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Excitation of the 2*S* State of Atomic Hydrogen by Electron Impact*Atoms* **2018**, *6*(1), 7; doi:10.3390/atoms6010007 - 12 February 2018**Abstract **

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The excitation cross sections of the 2*S* state of atomic hydrogen at 10 low incident electron energies (10.30 and 54.5 eV) have been calculated using the variational polarized method. Nine partial waves are used to get convergence of cross sections in the

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The excitation cross sections of the 2*S* state of atomic hydrogen at 10 low incident electron energies (10.30 and 54.5 eV) have been calculated using the variational polarized method. Nine partial waves are used to get convergence of cross sections in the above energy range. The maximum of the cross section is 0.137 $\pi {a}_{0}^{2}$ at 11.14 eV which is close to the experimental result 0.163 $\pm 0.2\pi {a}_{0}^{2}$ at 11.6 $\pm $ 0.2 eV. The present results are compared with other calculations, many of them are based on the close-coupling approximation, including the *R*-matrix method. Differential cross sections at 13.6 eV incident energy have also been calculated. Spin-flip cross sections have been calculated and compared with those obtained using the close-coupling approximation.
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Contribution of Lienard-Wiechert Potential to the Electron Broadening of Spectral Lines in Plasmas*Atoms* **2018**, *6*(1), 6; doi:10.3390/atoms6010006 - 8 February 2018**Abstract **

Lienard-Wiechert or retarded electric and magnetic fields are produced by moving electric charges with respect to a rest frame. In hot plasmas, such fields may be created by high velocity free electrons. The resulting electric field has a relativistic expression that depends on

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Lienard-Wiechert or retarded electric and magnetic fields are produced by moving electric charges with respect to a rest frame. In hot plasmas, such fields may be created by high velocity free electrons. The resulting electric field has a relativistic expression that depends on the ratio of the free electron velocity to the speed of light in vacuum c. In this work, we consider the semi-classical dipole interaction between the emitter ions and the Lienard-Wiechert electric field of the free electrons and compute its contribution to the broadening of the spectral line shape in hot and dense plasmas.
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Acknowledgement to Reviewers of *Atoms* in 2017*Atoms* **2018**, *6*(1), 5; doi:10.3390/atoms6010005 - 24 January 2018**Abstract **

Peer review is an essential part in the publication process, ensuring that *Atoms* maintains high quality standards for its published papers [...]
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The Application of the Cut-Off Coulomb Model Potential for the Calculation of Bound-Bound State Transitions*Atoms* **2018**, *6*(1), 4; doi:10.3390/atoms6010004 - 12 January 2018**Abstract **

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In this contribution, we present results of bound state transition modeling using the cut-off Coulomb model potential. The cut-off Coulomb potential has proven itself as a model potential for the dense hydrogen plasma. The main aim of our investigation include further steps of

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In this contribution, we present results of bound state transition modeling using the cut-off Coulomb model potential. The cut-off Coulomb potential has proven itself as a model potential for the dense hydrogen plasma. The main aim of our investigation include further steps of improvement of the usage of model potential. The results deal with partially ionized dense hydrogen plasma. The presented results covers ${N}_{e}=6.5\times {10}^{18}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{-3}$ , $T=\mathrm{18,000}\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and ${N}_{e}=1.5\times {10}^{19}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{-3}$ , $T=\mathrm{23,000}\phantom{\rule{4pt}{0ex}}\mathrm{K},$ where the comparison with the experimental data should take place, and the theoretical values for comparison. Since the model was successfully applied on continuous photoabsorption of dense hydrogen plasma in the broad area of temperatures and densities, it is expected to combine both continuous and bound-bound photoabsorption within single quantum mechanical model with the same success.
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