Fast atom diffraction during grazing scattering from a MgO(001) surface

Classical and quantum mechanical rainbow-scattering of fast He atoms from a KCl(0 0 1) surface

Catalytic conversion mechanism of guaiacol as the intermediate of lignin catalytic pyrolysis on MgO surface: Density functional theory calculation

The maximum power achieved in a wide variety of high-power devices, including electron and ion diodes, z pinches, and microwave generators, is presently limited by anode-cathode gap breakdown. A frequently discussed hypothesis for this effect is ionization of fast neutral atoms injected throughout the anode-cathode gap during the power pulse. We describe a newly developed diagnostic tool that provides a direct test of this hypothesis. Time-resolved vacuum-ultraviolet absorption spectroscopy is used to directly probe fast neutral atoms with 1-mm spatial resolution in the 10-mm anode-cathode gap of the SABRE 5 MV, 1 TW applied-B ion diode. Absorption spectra collected during Ar RF glow discharges and with CO2 gas fills confirm the reliability of the diagnostic technique. Throughout the 50-100 ns ion diode pulses no measurable neutral absorption was seen, setting upper limits of (0.12-1.5)x10(14) cm(-3) for ground-state fast neutral atom densities of H, C, N, O, and F. The absence of molecular absorption bands also sets upper limits of (0.16-1.2)x10(15) cm(-3) for common simple molecules. These limits are low enough to rule out ionization of fast neutral atoms as a breakdown mechanism. Breakdown due to ionization of molecules is also found to be unlikely. This technique can now be applied to quantify the role of neutral atoms in other high-power devices.

Fast helium atoms and hydrogen molecules with energies from 400 eV up to several keV are grazingly scattered from a Fe(110) surface covered by oxygen and sulphur atoms forming $c(2\ifmmode\times\else\texttimes\fi{}2)$ and $c(1\ifmmode\times\else\texttimes\fi{}3)$ superstructures, respectively. For scattering along low-index azimuthal directions we observe defined diffraction patterns in the angular distributions for scattered projectiles. From the evaluation of those diffraction patterns we derive the widths of low-indexed axial channels, the corrugation of the interaction potential across these channels, and the normal positions of adsorbed atoms above the Fe lattice. Our analysis is based on semiclassical models using hard-wall approximation as well as individual potentials for the interaction of projectiles with the surface. By comparing the results of different models, we discuss the robustness of the information on the geometrical structure of the surfaces.

Effect of sulfur content on precipitation behaviour of MnS-containing inclusions in X70 pipeline steel

For energetic atomic beams grazingly incident at a surface along low-index directions, the fast motion of the projectile in the surface plane and the slow motion in the direction perpendicular to the surface appear nearly decoupled. Fast-atom diffraction (FAD) experiments reveal two-dimensional (2D) diffraction patterns associated with exchange of the reciprocal vector perpendicular to the low-index direction of fast motion. These results are usually interpreted within the axial-channeling approximation, where the effective 2D potential experienced by the projectile is set as an average of the 3D surface potential along the atomic strings forming the channel. In this work, using the example of grazing scattering of He atoms at a LiF(001) surface, we address theoretically the range of validity of the axial-channeling approximation. Full quantum wave-packet-propagation calculations are used to study the transition from the 2D (fast atom) to the 3D diffraction pattern characteristic for low-energy atomic and molecular projectiles scattered from surfaces. Along with exact calculations, a semianalytical perturbative treatment based on the Lippmann-Schwinger equation allows an explanation of why the diffraction processes involving the exchange of reciprocal-lattice vectors along the fast-motion direction are exponentially small in typical FAD conditions.

Initial stage of Ag deposition on regular MgO(0 0 1) surface: A DFT study

The structural and electronic properties of Au and Au2 adsorbed on five coordinated oxygen and magnesium regular terrace sites, on the neutral Fs color center (oxygen vacancy containing two electrons), and on the charged Fs+ color center (oxygen vacancy containing one electron) of the MgO(100) surface have been investigated using a density functional theory cluster embedding approach. Two types of calculations have been performed. In one, we optimized and calculated the energetics of the systems using a generalized gradient approximation, and in the second, we used a combined approach, optimizing the geometries using a local density approximation and then performing a single point energy calculation using a generalized gradient approximation. Our studies also examine the effect of relaxation of the substrate. This was accomplished through two studies: first, both the MgO surface and the Au atoms were optimized, and second, the MgO surface atoms were kept fixed while the Au atoms were allowed to relax. We report the geometry, adsorption, and dimerization energies of the lowest energy and low energy isomers at each adsorption site, charge transfers from the MgO surface to the Au atoms in the lowest energy isomers, and compare our findings with previous theoretical studies. This comparison allowed us to establish in most of the cases limits for the adsorption energies, binding energies, and geometrical parameters at each adsorption site. For the case of Au2, for each adsorption site, our studies confirm the lowest energy isomer and most of the low energy isomers, previously reported, when both the surface and the Au2 are optimized. For most cases, the studies with frozen MgO surface atoms produced similar results as the studies when the MgO surface atoms were optimized. However, for the Mg site we found that the restrictions introduced by keeping the surface frozen favored the coexistence of an additional low energy isomer, and for the Fs+ site, the restrictions favored the dissociation of the adsorbed Au2 dimer into two surface adsorbed Au atoms. In general, both fully generalized gradient and combined calculations produced similar results, though, in the case of the Fs site the combined calculations found another low lying isomer, and in the case of the Fs+ site it predicted the dissociation of the adsorbed Au2 dimer, and inverted the energetic order of the lowest energy and low energy isomers. An increase in the charge transfer from the MgO surface to the Au atoms at the different adsorption sites according with the number of localized electrons at the adsorption sites was found. Finally, we found that the site with the largest adsorption and dimerization energies, and where an Au atom is most stable and can grow, is the neutral Fs center.

In this comment we discuss some aspects of Phys. Chem. Chem. Phys., 2022, 24, 15851, by Allison et al., an article intensely motivated by our study of grazing incidence fast atom diffraction (GIFAD) for He-KCl(001) [G. A. Bocan, H. Breiss, S. Szilasi, A. Momeni, M. E. Staicu Casagrande, M. S. Gravielle, E. A. Sánchez and H. Khemliche, Phys. Rev. Lett., 2020, 125, 096101; G. A. Bocan, H. Breiss, S. Szilasi, A. Momeni, M. E. Staicu Casagrande, E. A. Sánchez, M. S. Gravielle and H. Khemliche, Phys. Rev. B, 2021, 104, 235401]. In particular, (a) we show that, contrary to first order perturbation prediction, the surface corrugation is not proportional to the tangent of the rainbow angle and, (b) we analyze whether a Morse-like formula, like the one Allison et al. use, is able to reproduce the atom-surface potential derived from density functional theory (DFT) calculations. In addition, we give some clarifications regarding specific remarks the authors made about our articles.

The micro‐diffraction method in transmission electron microscopy allows to determine the crystalline types of phase components in alloys, that is necessary to calculate the effect on mechanical properties of the materials. The analysis of chemical composition of the selection has to be done to determine the crystalline type of phase inclusion within the micro‐diffraction method. The method of EDX‐spectrometry is usually used in case of steel researches, because this method is more sensitive to heavy elements which are defining in the formation of phase components in iron‐based alloys. When the elemental composition of the inclusion obtained from containing sample site is taken into account, decoding of the diffraction pattern can be made. This procedure helps to determine the crystallographic structure of the considered phase. This paper presents a modified program for indexing electron diffraction patterns «DiffraCalc», which allows to decode the diffraction patterns obtained from the phase inclusions, considering the chemical composition. This work is based on a well‐known method of diffraction patterns indexing ‐ a method of paired reflexes [1]. Diffraction patterns indexing manually using this method has several disadvantages. Usage of this modified program has solved most of the indexing problems and has led to significant work acceleration and has improved the quality of indexing results. The program includes: automatic indexing algorithm, the connection to the extensive database of crystalline substances, compatibility with standard CIF (Crystallografic information file) [2] files, the availability to choose three reflexes pairs at the same time and the ability to work simultaneously with several phases within one‐time setting scale. Indexing mode in the modified program allows to provide automatic calculations that are held simultaneously in several prospective phases, and moreover to analyze the phase matching for diffraction pattern without user intervention. The developed program has also implemented simulation mode of the diffraction pattern of a single crystal. This mode is intended to check the indexing results. Verification of the diffraction patterns simulation mode was carried out under the program Single Crystall [3] for a number of substances (Fe, Fe 7 C 3 , Fe 3 C, etc.) with all types of crystal systems and has shown good compatibility between the calculated diffraction patterns of the Single Crystall results and the one obtained in the modified program «DiffraCalc». The examples of diffraction patterns simulation from a variety of substances is shown on Figure 1. Analysis with the proposed program does not require multiple downloads of the experimental image of diffraction pattern, has the advantage of automatic conformity assessment estimated the experimental diffraction pattern based on an assessment of conformity of interplanar distances and angles between the reciprocal lattice vectors. Thus, the program «DiffraCalc» has broad capabilities to simulate the calculated diffraction patterns and indexing of experimental one. Also, the paper contains investigations of low‐alloyed reactor pressure vessel (RPV) steels phase composition. Studies of the chemical composition of phase components were carried out by EDX‐method, the crystal structure of the phase components investigations were carried out by the micro‐diffraction method with subsequent analysis of the experimental diffraction patterns using the «DiffraCalc» program. An example of the automatic indexing of one of the electron diffraction patterns is shown on Figure 2, 3. This phase inclusion into the RPV steel is carbide of type Cr 21 Fe 2 C 6 . Experimental reflexes positions have shown good compatibility of reflexes positions, calculated in the program, with considering small deviation of the zone axis [111] from the direction of the electron beam.

SynopsisAngle- and energy-loss- resolved distributions for He atoms axially scattered off a Ag(110) surface are calculated in order to study the influence of inelastic processes on fast atom diffraction (FAD) spectra. Final projectile distributions are evaluated within a semi-classical formalism that includes dissipative effects due to electronic excitations. For monoenergetic beams the model predicts the presence of multiple peaks in energy-loss spectra. But these structures disappear when the experimental conditions are included, giving rise to an energy-loss distribution in good agreement with available experimental data. The inelastic contribution to FAD patterns is estimated.

The question of the Sun's motion relative to the local interstellar medium (LISM) is examined. Results from Russian, US, and French satellites and space probes observing scattered solar radiation in hydrogen Lyman-alpha and helium lines are presented. Data are briefly discussed on the direct registration inside the Solar System of neutral helium atoms, picked-up helium ions, and fast neutral hydrogen atoms entering from the LISM. The fast neutral hydrogen atoms arise in the region of solar wind–LISM interaction.

Oscillating ion streamline model of Jupiter's neutral sodium nebula

Short pulse, high contrast, intense laser pulses incident onto a solid target are not known to generate fast neutral atoms. Experiments carried out to study the recombination of accelerated protons show a 200 times higher neutralization than expected. Fast neutral atoms can contribute to 80% of the fast particles at 10keV, falling rapidly for higher energy. Conventional charge transfer and electron-ion recombination in a high density plasma plume near the target is unable to explain the neutralization. We present a model based on the copropagation of electrons and ions wherein recombination far away from the target surface accounts for the experimental measurements. A novel experimental verification of the model is also presented. This study provides insights into the closely linked dynamics of ions and electrons by which neutral atom formation is enhanced.

Spatial light modulators can be used for having dynamic three dimensional images. Diffraction pattern of three dimensional objects can be used for driving those imaging devices. Diffraction patterns have to be calculated rapidly to form dynamic three dimensional images. Furthermore, pixelated structure of the spatial light modulators have to be taken into account in the diffraction pattern calculation to improve the quality of the reconstructed objects. The proposed method for calculating diffraction patterns not only takes into account the pixelated structure of the spatial light modulator to obtain more accurate results, but also provides fast calculation of the diffraction pattern. The proposed method employs a lookup table which is formed by 469 pre-computed vectors and the computation time of diffraction pattern can be decreased significantly compared to standard diffraction pattern calculation method. The proposed method can be run in parallel, hence further reduction in calculation time can be achieved and even the diffraction pattern of a three dimensional object which is formed by 3144 points can be calculated within 80msec when the algorithm is run in parallel on a graphics processing unit. © 2014 IEEE.