Target Atoms Research Articles

Effect of Radio-Frequency Power on the Composition of BiVO4 Thin-Film Photoanodes Sputtered from a Single Target

BiVO4 films were fabricated by radio frequency (RF) sputtering from a single target. The deposited BiVO4 films were found to be rich in Bi, and the reason for the Bi-richness was investigated. It was inferred from the Monte Carlo simulation that, during sputtering, the transfer process of target atoms through argon gas played a major role in this phenomenon. The transfer process resulted in an imbalanced ratio of Bi and V, arising from the difference in atom mass and interaction radius. The high RF power was found to be effective in adjusting the Bi/V ratio, influencing the sputtering yield. This type of preferential sputtering was maintained by the diffusion of target atoms from the bulk to the surface. BiVO4 films with monoclinic scheelite crystal structures were obtained at high RF power values and found to exhibit photocatalytic performances beneficial for photoanodic applications.

Analytical expression for the electronic stopping cross section of atomic gas targets for hydrogen projectiles

The author derives an analytical expression for the electronic stopping cross section of atomic gas targets at nonrelativistic velocities for hydrogen ions. The expression is based on an ab initio approach to account for the electronic excitations and charge exchange processes at low collision energies and the correct Bethe approach at high collision energies.

Photoionization of aligned excited states in neon by attosecond laser pulses

We numerically describe the ionization process induced by linearly and circularly polarized extreme ultraviolet (XUV) attosecond laser pulses on an aligned atomic target, specifically the excited state Ne*(1s 22s 22p 5[]3s[]). We compute the excited atomic state by applying the time-dependent restricted-active-space self-consistent field method to fully account for the electronic correlation. We find that correlation-assisted ionization channels can dominate channels accessible without correlation. We also observe that the rotation of the photoelectron momentum distribution by circularly polarized laser pulses compared to the case of linear polarization can be explained in terms of differences in accessible ionization channels. This study shows that it is essential to include electron correlation effects to obtain an accurate description of the photoelectron emission dynamics from aligned excited states.

Atomic form factors and inverse Primakoff scattering of axion

We reexamine the inverse Primakoff scattering of axions, whose scattering cross section depends on the distribution of electrons in target atoms. We evaluate it using a form factor computed with a relativistic Hartree-Fock wave function and compare it with the previous results obtained with those based on the screened Coulomb potential for the electrostatic field in the atom. We take xenon as an example for the target atom and show that the scattering cross section was overestimated by more than an order of magnitude for axions with ≲O(10) keV energies, like solar axions. It is also found that inelastic scattering processes, in which the final state contains an excited or ionized atom, can be comparable or even be dominant when the size of momentum transfer is ≲1 keV. For more energetic axions, on the other hand, the total scattering cross section is found to be well approximated by a simple expression and has little dependence on the atomic structure. As an application of this result, we consider supernova axions, whose energy is 10–100 MeV, and show that O(1) inverse Primakoff events are expected for axions from a nearby supernova in the future neutrino experiments, which may warrant a more detailed study on the search strategy of this process.

Simulation study of surface sputtering and distribution of ions in silicon due to low-energy high-fluence cobalt irradiation

The study of the range and distribution of low-energy heavy ions implanted into lighter substrates is paramount important in terms of three-dimensional device fabrications. In this study, we report the re-distribution of implanted ions due to the sputtering of target atoms, simulated considering dynamic changes in the target due to the high-fluence irradiation of Co and Fe ions at low energy (55 keV) in Silicon substrate and nanowires. The simulations were compared by using various ion-solid interaction codes that are capable of incorporating the targets with both one-dimensional (1D) and three-dimensional (3D) structures. In the 1D simulation codes, the target is generally defined as a stack of sharp interfaced layers that have an infinite lateral extension. Both static code like "Stopping and Range of Ions in Matter (SRIM)" and dynamic code like "Static and Dynamic Trim for Sequential and Parallel computer (SDTrimSP)" were utilized to carry out the simulations in silicon substrates. However, to simulate the distribution of ions with target layers involving nanostructures with a curved surface such as nanowires “Ion range and damage in nanostructures (Iradina)” code was used. Silicon nanowires of different diameters were chosen as the target, and Co ions were implanted at single energy along the surface normal. The fluence of the Co ions was then varied to obtain the optimal distribution of implanted ions in the silicon nanowire. The Si nanowire shows a curved depth profile for the implanted ions with different ranges towards the edge and the center of the nanowire. The surface sputtering in the case of the nanowire was more as compared to the bulk Si substrate. However, in the case of the Si substrate, the atoms are found preferentially sputtered in the forward direction between 30° - 60° angle with a peak around 45° angle to the target surface normal with variation in the angle of incidence.

Electron interference in atomic ionization by two crossing polarized ultrashort pulses

Formation of geometrically regular interference patterns in the photoelectron momentum distributions (PMDs) corresponding to the photoionization of atoms by two single-color, crossing ultrashort pulses is investigated both analytically and numerically. It is shown that, in contrast to the photoionization by monochromatic pulses, PMDs for the ionization by crossing and co-propagating broadband pulses are essentially different (unless both pulses are linearly polarized), namely, when one pulse is linearly polarized along the propagation direction, $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbf{k}}$, of the circularly polarized (CP) pulse, then interference maxima (minima) of the ionization probability have the form of three-dimensional single-arm regular spirals which are wound along $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbf{k}}$. Next, the interference maxima (minima) of the ionization probability by a pair of crossing elliptically polarized pulses have the form of either Newton's rings or two-arm Fermat's spirals, depending on the position of a detection plane. Remarkably, these regular patterns occur only for certain values of the pulse ellipticities, and they become distorted for CP pulses. For both above-mentioned pulse configurations, the features of interference patterns depend on the time delay between pulses, their relative electric field amplitude, and relative carrier-envelope phase. Our predictions, illustrated by the numerical results for the ionization of H and He atoms by two orthogonal pulses, are quite general and we expect them to be valid for the ionization of any randomly oriented atomic or molecular target.

Introducing block design in graph neural networks for molecular properties prediction

The number of states required for describing a many-body quantum system increases exponentially with the number of particles; thus, it is time- and effort-consuming to exactly calculate molecular properties. Herein, we propose a deep learning algorithm named block-based graph neural network (BGNN) as an approximate solution. The algorithm can be understood as a representation learning process to extract useful interactions between a target atom and its neighboring atomic groups. Compared to other graph model variants, BGNN achieved the smallest mean absolute errors in most tasks on two large molecular datasets, QM9 and Alchemy. Our advanced machine learning method exhibits general applicability and can be readily employed for bioactivity prediction and other tasks relevant to drug discovery and materials design.

X-ray emission for Ar11+ ions impacting on various targets in the collisions near the Bohr velocity* *Project supported by the National Key R&D Program of China (Grant No. 2017YFA0402300), the National Natural Science Foundation of China (Grant Nos. 11505248, 11775042, 11875096, and 11605147) the Scientific Research Program Funded by Shaanxi Provincial Education Department, China (Grant No. 20JK0975), the Scientific Research

X-ray emission from the collisions of 3 MeV Ar11 + ions with V, Fe, Co, Ni, Cu, and Zn is investigated. Both the x-rays of the target atom and projectile are observed simultaneously. The x-ray yield is extracted from the original count. The inner-shell ionization cross section is estimated by the binary encounter approximation model and compared with the experimental result. The remarkable result is that the Ar K-shell x-ray yield is diminished with the target atomic number increasing, which is completely opposite to the theoretical calculation. That is interpreted by the competitive consumption of the energy loss for the ionization of inner-shell electrons between the projectile and target atom.

On how to measure the probabilities of target atom ionization and target ion back-attraction in high-power impulse magnetron sputtering

High-power impulse magnetron sputtering (HiPIMS) is an ionized physical vapor deposition technique that provides a high flux of ionized target species for thin film growth. Optimization of HiPIMS processes is, however, often difficult, since the influence of external process parameters, such as working gas pressure, magnetic field strength, and pulse configuration, on the deposition process characteristics is not well understood. The reason is that these external parameters are only indirectly connected to the two key flux parameters, the deposition rate and ionized flux fraction, via two internal discharge parameters: the target atom ionization probability αt and the target ion back-attraction probability βt. Until now, it has been difficult to assess αt and βt without resorting to computational modeling, which has hampered knowledge-based optimization. Here, we present a simple method to deduce αt and βt based on measured deposition rates of neutrals and ions. The core of the method is a refined analytical model, which is described in detail. This approach is furthermore validated by independent calculations of αt and βt using the considerably more complex ionization region model, which is a plasma-chemical global discharge model.

Resilient quantum gates on periodically driven Rydberg atoms

Fault-tolerant implementation of quantum gates is one of preconditions for realizing quantum computation. The platform of Rydberg atoms is one of the most promising candidates for achieving quantum computation. We propose to implement a controlled-$Z$ gate on Rydberg atoms where an amplitude-modulated field is employed to induce Rydberg antiblockade. Gate robustness against the fluctuations in the Rydberg-Rydberg interaction can be largely enhanced by adjusting amplitude-modulated field. Furthermore, we introduce a Landau-Zener-St\"uckelberg transition on the target atom so as to improve the gate resilience to the deviation in the gate time and the drift in the pulse amplitude. With feasible experimental parameters, one can achieve the gate with low fidelity errors caused by atomic decay, interatomic dipole-dipole force, and Doppler effects. Finally, we generalize the gate scheme into multiqubit cases, where resilient multiqubit phase gates can be obtained in one step with an unchanged gate time as the number of qubits increases.

I L-shell X-rays from near Bohr-velocity I<sup>20+ </sup>ions impacting on various targets

The L-shell X-ray emissions of iodine are investigated as a function of target atomic number for 4.5-MeV I<sup>20+</sup> ions impacting on Fe, Co, Ni, Cu and Zn targets. Six distinct L-subshell X-rays are observed. The energy of the x-ray has a blue shift compared with the atomic data. The relative intensity ratio of Lβ<sub>1, 3, 4</sub> and Lβ<sub>2, 15</sub> to Lα<sub>1, 2</sub> almost increase linearly with the target atomic number increasing. The ratio of I(Lι) to I(Lα<sub>1, 2</sub>) and I (Lγ<sub>2, 3, 4, 4'</sub>) to I(Lγ<sub>1</sub>) are approximately proportional to the square of target atomic number. It is indicated that during the interaction of highly charged heavy ions with atom in the energy region near the Bohr velocity, the inner-shell process is mainly caused by the close-range collisions below the surface. There, the projectile not only has enough time to capture electrons from the target atom to be neutralized, but also has enough kinetic energy to ionize the inner-shell electron by coulomb interaction. At the balance between electron capture and ionization, the outer-shells of M, N, O etc. could be multiply ionized. The extent of multiple ionization increases with the target atomic number increasing. That leads to the energy shift, resulting in the change of the relative intensity ratio for the L-subshell X-ray. The smaller the atomic fluorescence, the larger the enhanced fluorescence caused by multiple ionization.

Ionization and fragmentation of fluorene upon 250 keV proton impact

We have investigated the ionization and fragmentation of a polycyclic aromatic hydrocarbon molecule, fluorene ( $$\hbox {C}_{{13}}\,\hbox {H}_{{10}}$$ ), upon proton impact with energy 250 keV. Various ionization and fragmentation products have been identified. Hydrogen-loss process from singly and doubly ionized parent molecules has been found to be a dominating channel. In case of singly ionized molecule, up to three H-losses have been observed. However, two H-loss ion has not been observed. For the doubly ionized molecule up to four H-losses have been observed. The ratio of doubly to singly charged parent ions are found to be quite high compared to the atomic targets, such as, He and Ne.

Study of the Dressed State of Hydrogen Atom in Electron Atom Elastic Collision in Laser Field

We discuss the various important aspects of the theory of electron-atom elastic collision in laser field. We analyze the collision accompanied by the transfer of photons. We study the free electron states i.e. Volkov states and target atomic states i.e. Dressed states. We calculate the first-Born scattering matrix for electron-atom elastic collision. The present work accounts the calculation for hydrogen as target atom in soft photon limits i.e., in weak field. The dressing effect becomes significant in the region of low momentum transfer, reaches maximum of value of 25000 a.u. at momentum transfer of 0.44. This work explains that the differential cross-section does not occur very low and at very high momentum transfer. However, it occurs at moderate momentum transfer.

Trajectory-dependent electronic excitations by light and heavy ions around and below the Bohr velocity

We present experiments demonstrating trajectory-dependent electronic excitations at low ion velocities, where ions are expected to primarily interact with delocalized valence electrons. The energy loss of H$^+$, H$_2 ^+$, He$^+$, B$^+$, N$^+$, Ne$^+$, $^{28/29}$Si$^+$ and Ar$^+$ in self-supporting silicon membranes was analysed along channelled and random trajectories in a transmission approach. For all ions, we observe a difference in electronic stopping dependent on crystal orientation. For heavier ions, the energy-loss difference between channelling and random geometry is generally found more pronounced, and, in contrast to protons, increases for decreasing ion energy. Due to the inefficiency of core-electron excitations at employed ion velocities, we explain these results by reionization events occurring in close collisions of ions with target atoms, which are heavily suppressed for channelled trajectories. These processes result in trajectory-dependent mean charge states, which strongly affects the energy loss. The strength of the effect seems to exhibit a Z$_1$ oscillation with an observed minimum for Ne. We, furthermore, demonstrate that the simplicity of our experimental geometry leads to results that can serve as excellent benchmark systems for dynamic calculations of the electronic systems of solids using time-dependent density functional theory.

Change in ion sputtering coefficients of targets due to cross-dusting during simultaneous operation of two sputters

The influence of cross-dusting of silicon and metal targets simultaneously sputtered by argon ions on partial sputtering coefficients of targets materials components is studied to take this effect into account in the technology of gradient optical coating deposition. The effect has been numerically simulated as series of cascades of binary collisions of bombarding ions and recoil atoms with target atoms. Addition of metal atoms (Ti, V, Zr, Hf, Nb and Ta) to the Si target enhances the partial sputtering of Si atoms but addition of Si atoms to the metal targets decreases the partial sputtering of metals atoms. Also the coefficients of reflection (back scattering) of the bombarding argon ions from the targets with different additives have been calculated. It is possible that enhanced sputtering of Si atoms from the coatings with a “heavy” component by reflected neutralized ions would lead to dusting the “heavy” target by silicon.

Efficient Intermolecular Energy Exchange and Soft Ionization of Water at Nanoplatelet Interfaces.

X-ray, energetic photon, and electron irradiation can ionize and electronically excite target atoms and molecules. These excitations undergo complicated relaxation and energy-transfer processes that ultimately determine the manifold system responses to the deposited excess energy. In weakly bound gas- and solution-phase samples, intermolecular Coulomb decay (ICD) and electron-transfer-mediated decay (ETMD) can occur with neighboring atoms or molecules, leading to efficient transfer of the excess energy to the surroundings. In ionic solids such as metal oxides, intra- and interatomic Auger decay produces localized final states that lead to lattice damage and typically the removal of cations from the substrate. The relative importance of Auger-stimulated damage (ASD) versus ICD and ETMD in microsolvated nanoparticle interfaces is not known. Though ASD is generally expected, essentially no lattice damage resulting from the ionization and electronic excitation of microsolvated boehmite (AlOOH) nanoplatelets has been detected. Rather efficient energy transfer and soft ionization of interfacial water molecules has been observed. This is likely a general phenomenon at gas-oxyhydroxide nanoparticle interfaces where the density of states of the ionized chemisorbed species significantly overlaps with the core hole states of the solid.

Toward Accurate Predictions of Atomic Properties via Quantum Mechanics Descriptors Augmented Graph Convolutional Neural Network: Application of This Novel Approach in NMR Chemical Shifts Predictions.

In this study, an augmented Graph Convolutional Network (GCN) with quantum mechanics (QM) descriptors was reported for its accurate predictions of NMR chemical shifts with respect to experimental values. The prediction errors of 13C/1H NMR chemical shifts can be as small as 2.14/0.11 ppm. There are two crucial characteristics for this modified GCN: in one aspect, such a novel neural network could efficiently extract the overall molecule structure information; in another aspect, it could accurately solve the chemical environment of the target atom. As there exists an imperfect linear regression between the experimental NMR chemical shifts (δ) and the density functional theory (DFT) calculated isotropic shielding constants (σ), the inclusion of QM descriptors within GCN can largely improve its performance. Moreover, few-shot learning also becomes feasible with these descriptors. The success of this novel GCN in chemical shifts predictions also indicates its potential applicability for other computational studies.

Elsepa—Dirac partial-wave calculation of elastic scattering of electrons and positrons by atoms, positive ions and molecules (New Version Announcement)

A new version of the Fortran program elsepa, which calculates differential and integrated cross sections for elastic scattering of electrons and positrons, is presented. Details of the program and its applications are given in the original paper [Comput. Phys. Commun.165 (2005) 157–190]. Dirac phase shifts are now calculated by using the recently published subroutine package radial (Salvat and Fernández-Varea, 2019), which solves the radial wave equation for real or complex central potentials by means of a robust and accurate power-series method. In addition, elastic collisions with atoms in elemental solids are described by using the muffin-tin optical model potential proposed by Bote et al., (2009), which is somewhat more elaborate and flexible than that in the original elsepa code and allows adjusting the absorption potential to give inelastic cross sections in close agreement with empirical data. With the use of the radial subroutines, the work of elsepa is reduced to (1) the definition of the interaction potential and (2) the summation of the partial-wave series of the scattering amplitudes. The structure of the new code has been simplified and stricter criteria for the convergence of the partial-wave series have been adopted. The distribution package includes gnuplot scripts for easy visualization of the calculation results. Program summaryProgram Title:elsepaCPC Library link to program files:https://doi.org/10.17632/w4hm5vymym.1Licensing provisions: CC BY NC 3.0Programming language: Fortran 90Journal reference of previous version: Comput. Phys. Commun. 165 (2005) 157–190Does the new version supersede the previous version?: YesReasons for the new version: This new version offers increased accuracy, and a more flexible modeling of elastic collisions of electrons and positrons with atoms in elemental solids.Summary of revisions: The calculation of phase shifts is now performed by the generic subroutines of the radial package [1]. A more flexible optical-model potential for scattering in solids is included [2]. Stricter criteria for convergence of the partial-wave series are applied. The distribution package has been reorganized, with the numerical database files now placed in a separate directory; scripts for direct visualization of the calculation results with the plotting program gnuplot (http://www.gnuplot.info) are also included.Nature of problem: The code calculates differential cross sections, total cross sections and transport cross sections for single elastic scattering of electrons and positrons by neutral atoms, positive ions and randomly oriented molecules. When the energy of the projectile is less than about 5 MeV, the programs also compute scattering amplitudes and spin polarization functions.Solution method: The effective interaction between the projectile and the target atom is represented by a local central potential that can optionally include an imaginary (absorptive) part to account approximately for the coupling with inelastic channels. For projectiles with kinetic energy less that about 5 MeV, the code performs a conventional relativistic Dirac partial-wave analysis. For higher kinetic energies, where the convergence of the partial-wave series is too slow, approximate factorization methods are used. The programs only admit kinetic energies higher than 5 eV, a practical lower limit that may be changed by editing the source files.Additional comments including restrictions and unusual features: The calculations are based on the static-field approximation. The optional correlation-polarization and inelastic absorption corrections are obtained from approximate, semi-empirical models. elsepa allows considering an absorption potential only for projectiles with energies less than about 1 MeV; for higher energies, the absorption potential is set to zero to prevent the occurrence of numerical instabilities. Calculations for molecules are based on a single-scattering independent-atom approximation. To ensure accuracy of the results for scattering by ions, the electron density of the ion must be supplied by the user; it can be generated, e.g., by running the program dhfs of the radial package [1].AcknowledgmentsWe are indebted to Dr John Villarrubia for pointing out various cases of incomplete convergence of the original code. Financial support from the Spanish Ministerio de Ciencia, Innovación y Universidades / Agencia Estatal de Investigación, Spain / European Regional Development Fund , European Union, (project no. RTI2018-098117-B-C22) is gratefully aknowledged.

Structure evolution, bandgap, and dielectric function in La-doped hafnium oxide thin layer subjected to swift Xe ion irradiation

High-resolution transmission electron microscopy, electron diffraction, and electron energy-loss spectroscopy provide information on the structural evolution, dielectric function, and bandgap values of nanocrystalline 10 nm thick lanthanum doped hafnia (La:HfO2) layers in TiN/La:HfO2/TiN/SiO2/Si irradiated with 24, 46, 72, and 160 MeV (0.2–1.2 MeV/u) Xe ions. Swift heavy Xe ions were expected to create significant atomic rearrangements when passed through a solid losing energy mainly through electronic excitation and ionization of the target atoms. Local heating and subsequent rapid cooling in the region around the ion track can lead to re-solidification with the formation of a new stable crystalline phase, and direct crystal-to-crystal transformations are possible. The structure evolution of hafnia nanocrystals from the orthorhombic Pbcm in the pristine layer to the tetragonal P42/nmc phase in the 160 MeV Xe ion irradiated layer was observed. The mixture of tetragonal and orthorhombic phases was found in samples irradiated with ions of intermediate energies. Textured hafnia layers were formed as a result of ion irradiation. The changes in plasmon line shape and the blueshift of the plasmon energy-loss peak from 14.9 eV in the pristine layer to 15.4 eV in 46 MeV and 15.9 eV in 160 MeV Xe ion irradiated hafnia reflect structural transformations, the increase in the Hf coordination number, and crystal orientations. Valence-electron energy-loss spectroscopy measurements showed a slight increase in the bandgap value from 6.1 eV in the pristine sample to 6.2 eV and 6.3 eV in irradiated samples with 46 and 160 MeV Xe ions, respectively, and dielectric functions changed insignificantly in irradiated hafnia layers.

Optimized geometric quantum computation with a mesoscopic ensemble of Rydberg atoms

We propose a nonadiabatic non-Abelian geometric quantum operation scheme to realize universal quantum computation with mesoscopic Rydberg atoms. A single control atom entangles a mesoscopic ensemble of target atoms through long-range interactions between Rydberg states. We demonstrate theoretically that both the single qubit and two-qubit quantum gates can achieve high fidelities around or above 99.9% in ideal situations. Besides, to address the experimental issue of Rabi frequency fluctuation (Rabi error) in Rydberg atom and ensemble, we apply the dynamical-invariant-based zero systematic-error sensitivity (ZSS) optimal control theory to the proposed scheme. Our numerical simulations show that the average fidelity could be 99.98% for single ensemble qubit gate and 99.94% for two-qubit gate even when the Rabi frequency of the gate laser acquires 10% fluctuations. We also find that the optimized scheme can also reduce errors caused by higher-order perturbation terms in deriving the Hamiltonian of the ensemble atoms. To address the experimental issue of decoherence error between the ground state and Rydberg levels in Rydberg ensemble, we introduce a dispersive coupling regime between Rydberg and ground levels, based on which the Rydberg state is adiabatically discarded. The numerical simulation demonstrate that the quantum gate is enhanced. By combining strong Rydberg atom interactions, nonadiabatic geometric quantum computation, dynamical invariant and optimal control theory together, our scheme shows a new route to construct fast and robust quantum gates with mesoscopic atomic ensembles. Our study contributes to the ongoing effort in developing quantum information processing with Rydberg atoms trapped in optical lattices or tweezer arrays.