4S Atoms Research Articles

Structural evolution and hydrogen storage properties of ScHn− (n = 10–20) clusters

An unbiased Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) method combined with density functional theory (DFT) study was adopted to identify the ground state ScHn− (n = 10–20) isomers. We have performed a series of investigation of electronic properties, structural evolution, hydrogen storage properties of ScHn− (n = 10–20) clusters. Using time-dependent (TD-) DFT, the photoelectron spectra (PES) of the low-lying structures of ScHn− clusters have been simulated. The adsorption energies of ScHn− (n = 10–20) clusters are found in 2.52 eV-2.65 eV, and all hydrogen atoms are adsorbed on the Sc atom. Studied on the molecular orbitals and chemical bonds of the ScH12− cluster, it is found that the ScH12− cluster are mainly the interaction between the 3d atom orbital of Sc and the 1s atom orbital of H. The present study aims at novel nanomaterials for hydrogen storage.

Transport collision integrals for C(5S)-H(2S), C(1S)-H(2S), C(1D)-H(2S), and C(3P)-H(2S) interactions

Transport collision integrals of interacting atoms or ions are essential in modeling transport properties of high-temperature gases and plasmas. Here, we obtained the potential energy curves (PECs) of CH using the state-of-the-art ab initio methods. The PECs were also extrapolated to investigate the transport collision integrals for C(3P)-H(2S), C(5S)-H(2S), C(1S)-H(2S), and C(1D)-H(2S) interactions, in which the interactions between the excited C(5S), C(1S), and C(1D) atoms and the ground H(2S) atoms were calculated for the first time. The resulting transport collision integrals were fitted to simple functional forms for ease of use in plasma modeling. Our transport collision integrals can provide data references for computing transport properties of high-temperature plasmas involving C and H atoms.

Infrared Spectroscopic Studies of Oxygen Atom Quantum Diffusion in Solid Parahydrogen.

The thermally induced diffusion of atomic species in noble gas matrices was studied extensively in the 1990s to investigate low-temperature solid-state reactions and to synthesize reactive intermediates. In contrast, much less is known about the diffusion of atomic species in quantum solids such as solid parahydrogen (p-H2). While hydrogen atoms were shown to diffuse in normal-hydrogen solids at 4.2 K as early as 1989, the diffusion of other atomic species in solid p-H2 has not been reported in the literature. The in situ photogeneration of atomic oxygen, by ArF laser irradiation of an O2-doped p-H2 solid at 193 nm, is studied here to investigate the diffusion of O(3P) atoms in a quantum solid. The O(3P) atom mobility is detected by measuring the kinetics of the O(3P) + O2 → O3 reaction after photolysis via infrared spectroscopy of the O3 reaction product. This reaction is barrierless and is thus assumed to be diffusion-controlled under these conditions such that the reaction rate constant can be used to estimate the oxygen atom diffusion coefficient. The O3 growth curves are well fit by single exponential expressions allowing the pseudo-first-order rate constant for the O(3P) + O2 → O3 reaction to be extracted. The reaction rates are affected strongly by the p-H2 crystal morphology and display a non-Arrhenius-type temperature dependence consistent with quantum diffusion of the O(3P) atom. The experimental results are compared to H(2S) atom reaction studies in p-H2, analogous studies in noble gas matrices, and laboratory studies of atomic diffusion in astronomical ices and surfaces.

Electronic Structure Calculations with the Exact Pseudopotential and Interpolating Wavelet Basis

Electronic structure calculations are mostly carried out with Coulomb potential singularity adapted basis sets such as STO or contracted GTO. With another basis or for heavy elements, the pseudopotentials may appear as a practical alternative. Here, we introduce the exact pseudopotential (EPP) to remove the Coulomb singularity and test it for orbitals of small atoms with the interpolating wavelet basis set. We apply EPP to the Galerkin method with a basis set consisting of Deslauriers–Dubuc scaling functions on the half-infinite real interval. We demonstrate the EPP–Galerkin method by computing the hydrogen atom 1s, 2s, and 2p orbitals and helium atom configurations He1s2, He1s2s1S, and He1s2s3S. We compare the method to the ordinary interpolating wavelet Galerkin method (OIW–Galerkin), handling the singularity at the nucleus by excluding the scaling function located at the origin from the basis. We also compare the performance of our approach to that of finite-difference approach, which is another practical method for spherical atoms. We find the accuracy of the EPP–Galerkin method to be better than both of the above-mentioned methods.

Synthesis, electrochemical, and quantum chemical studies of some metal complexes: Mn(II), Co(II), and Zn(II) with 2-furaldehyde semicarbazone

In this study, we conducted experimental and computational investigations of a Schiff base and its metal complexes. The azote atom coordinates with the metal according to the FT-IR spectra. Powder X-ray Diffraction experiments show that the crystalline nature of metal complexes is responsible for their enhanced crystallinity. X-ray photoelectron spectroscopy shows that the binding energy of the nitrogen atom's 1s electrons is increased in metal complexes due to complexation. Cyclic voltammograms were used to determine the electroactivity of the ligand and its complexes in a solution of 10−1M DMSO/tetrabutylammonium hexafluorophosphate (NBu4PF6). The DPPH radical scavenging assay was used to determine the compounds' effectiveness as antioxidants. Transition metal complexes were much more effective than the free ligand FSC at scavenging DPPH radicals. The three investigated complexes have been analyzed as theoretical proof of the metal's bonding potential in the environment.

Kinetic Study of the Gas-Phase O(1D) + CH3OH and O(1D) + CH3CN Reactions: Low-Temperature Rate Constants and Atomic Hydrogen Product Yields.

Atomic oxygen in its first excited singlet state, O(1D), is an important species in the photochemistry of several planetary atmospheres and has been predicted to be a potentially important reactive species on interstellar ices. Here, we report the results of a kinetic study of the reactions of O(1D) with methanol, CH3OH, and acetonitrile, CH3CN, over the 50-296 K temperature range. A continuous supersonic flow reactor is used to attain these low temperatures coupled with pulsed laser photolysis and pulsed laser-induced fluorescence to generate and monitor O(1D) atoms, respectively. Secondary experiments examining the atomic hydrogen product channels of these reactions are also performed, through laser-induced fluorescence measurements of H(2S) atom formation. On the kinetic side, the rate constants for these reactions are seen to be large (>2 × 10-10 cm3 s-1) and consistent with barrierless reactions, although they display contrasting dependences as a function of temperature. On the product formation side, both reactions are seen to yield non-negligible quantities of atomic hydrogen. For the O(1D) + CH3OH reaction, the derived yields are in good agreement with the conclusions of previous experimental and theoretical works. For the O(1D) + CH3CN reaction, whose H-atom formation channels had not previously been investigated, electronic structure calculations of several new product formation channels are performed to explain the observed H-atom yields. These calculations demonstrate the barrierless and exothermic nature of the relevant exit channels, confirming that atomic hydrogen is also an important product of the O(1D) + CH3CN reaction.

Structural, Electronic and Magnetic Properties of Defected Water Adsorbed Single-Layer MoS2

Water adsorbed in MoS2 (wad-MoS2), 1S atom vacancy defect in wad-MoS2 (1S-wad-MoS2), 2S atoms vacancy defects in wad-MoS2 (2S-wad-MoS2), and 1Mo atom vacancy defect in wad-MoS2 (Mo-wad-MoS2) materials were constructed, and their structural, electronic, and magnetic properties were studied by spin-polarized density functional theory (DFT) based first-principles calculations. The wad-MoS2, 1S-wad-MoS2, 2S-wad-MoS2, and Mo-wad-MoS2 materials were found stable. From band structure calculations, wad-MoS2, 1S-wad-MoS2 and 2S-wad-MoS2 materials open energy bandgap of values 1.19 eV, 0.65 eV and 0.38 eV respectively. Also, it was found that the conductivity strength of the material increases with an increase in the concentration of S atom vacancy defects in the structure. On the other hand, the Mo-wad-MoS2 material has metallic properties because energy bands of electrons crossed the Fermi energy level in the band structure. For the investigation of magnetic properties, the density of states (DoS) and partial density of states (PDoS) calculations were used and found that wad-MoS2, 1S-wad-MoS2, and 2S-wad-MoS2 are non-magnetic materials, while Mo-wad-MoS2 is a magnetic material. The total magnetic moment of Mo-wad-MoS2 has a value of 2.66 µB/cell, due to the arrangement of unpaired up-spin and down-spin of electrons in 3s & 3p orbitals of S atoms; and 4p, 4d & 5s orbitals of Mo atoms in the material.

Structural, electronic and magnetic properties of S sites vacancy defects graphene/MoS2 van der Waals heterostructures: First-principles study

In this work, we investigated the geometrical structures, electronic and magnetic properties of S sites vacancy defects in heterostructure graphene/molybdenum disulphide ((HS)G/MoS[Formula: see text] material by performing first-principles calculations based on spin polarized Density Functional Theory (DFT) method within van der Waals (vdW) corrections (DFT-D2) approach. All the structures are optimized and relaxed by BFGS method using computational tool Quantum ESPRESSO (QE) package. We found that both (HS)G/MoS2 and S sites vacancy defects in (HS)G/MoS2 (D1S–(HS)G/MoS2, U1S–(HS)G/MoS2, 2S–(HS)G/MoS2 and 3S–(HS)G/MoS[Formula: see text] are stable materials, and atoms in defects structures are more compact than in pristine (HS)G/MoS2 structure. From band structure calculations, we found that (HS)G/MoS2, (D1S–(HS)G/MoS2, U1S–(HS)G/MoS2, 2S–(HS)G/MoS2 and 3S–(HS)G/MoS[Formula: see text] materials have [Formula: see text]-type Schottky contact. The Dirac cone is formed in conduction band of the materials mentioned above. The barrier height of Dirac cones from Fermi energy level of (HS)G/MoS2, (D1S–(HS)G/MoS2, U1S–(HS)G/MoS2, 2S–(HS)G/MoS2 and 3S–(HS)G/MoS[Formula: see text] materials have values 0.56[Formula: see text]eV, 0.62[Formula: see text]eV, 0.62[Formula: see text]eV, 0.64[Formula: see text]eV and 0.65[Formula: see text]eV, respectively, which means they have metallic properties. To study the magnetic properties of materials, we have carried out DoS and PDoS calculations. We found that (HS)G/MoS2, D1S–(HS)G/MoS2 and U1S–(HS)G/MoS2 materials have non-magnetic properties, and 2S–(HS)G/MoS2 and 3S–(HS)G/MoS2 materials have magnetic properties. Therefore, the non-magnetic (HS)G/MoS2 changes to magnetic 2S–(HS)G/MoS2 and 3S–(HS)G/MoS2 materials due to 2S and 3S atoms vacancy defects, respectively, in (HS)G/MoS2 material. Magnetic moment obtained in 2S–(HS)G/MoS2 and 3S–(HS)G/MoS2 materials due to the unequal distribution of up and down spin states of electrons in 2s and 2p orbitals of C atoms; 4p, 4d and 5s orbitals of Mo atoms; and 3s and 3p orbitals of S atoms in structures. Magnetic moment of 2S–(HS)G/MoS2 and 3S–(HS)G/MoS2 materials is −0.11[Formula: see text][Formula: see text]/cell and [Formula: see text]/cell, respectively, and spins of 2p orbital of C atoms, 3p orbital of S atoms and 4d orbital of Mo atoms have dominant role to create magnetism in 2S–(HS)G/MoS2 and 3S–(HS)G/MoS2 materials.

Kinetic Study of the Gas-Phase C(3P) + CH3CN Reaction at Low Temperatures: Rate Constants, H-Atom Product Yields, and Astrochemical Implications

Rate constants have been measured for the C(3P) + CH3CN reaction between 50 and 296 K using a continuous-flow supersonic reactor. C(3P) atoms were created by the in situ pulsed laser photolysis of CBr4 at 266 nm, while the kinetics of C(3P) atom loss were followed by direct vacuum ultraviolet laser-induced fluorescence at 115.8 nm. Secondary measurements of product H(2S) atom formation were also made, allowing absolute H-atom yields to be obtained by comparison with those obtained for the C(3P) + C2H4 reference reaction. In parallel, quantum chemical calculations were performed to obtain the various complexes, adducts, and transition states relevant to the title reaction over the 3A″ potential energy surface, allowing us to better understand the preferred reaction pathways. The reaction is seen to be very fast, with measured rate constants in the range of (3–4) × 10–10 cm3 s–1 with little or no observed temperature dependence. As the C + CH3CN reaction is not considered in the current astrochemical networks, we test its influence on interstellar methyl cyanide abundances using a gas-grain dense interstellar cloud model. Its inclusion leads to predicted CH3CN abundances that are significantly lower than the observed ones.

On the width of the K−D atom ground state

Experiments at DAΦNE-Frascati and at J-PARC are scheduled to produce K−D atoms and observe their X-ray cascade down to the 1S ground state (g.s.), thereby measuring its strong-interaction width and shift away from a purely Coulomb state. A width Γ1S≲1 keV will ensure good resolution of the X-ray transitions feeding the 1S g.s. Here we study the expected K−D 1S g.s. width from the perspective of global fits to level shifts and widths in heavier kaonic atoms across the periodic table, using K− nuclear optical potentials constructed from K¯N chiral interaction models. Special attention is paid to the subthreshold energy at which the K¯N subsystem interacts in the K−D atom g.s. Within this approach we predict strong-interaction upward level shift of close to 700 eV and width of about 1.2 to 1.3 keV for the K−D atom 1S g.s., in fair agreement with genuinely three-body K−D atom calculations. Comparison is made with π−D atom phenomenology.

Relativistic local hybrid functionals and their impact on 1s core orbital energies.

In this work, we report the first relativistic density functional theory calculations using relativistic local hybrid functionals. Besides outlining the construction of relativistic local hybrid exchange within a two-component-relativistic framework based on the picture-change transformation of the density matrix and a recently developed relativistic iso-orbital indicator, we investigate the influence of two-electron-relativistic effects, using relativistic functional ingredients in local hybrid functionals, and the choice of the exchange-correlation functional on atomic 1s core orbital energies of light and heavier elements. Finally, we discuss the applicability of relativistic 1s core orbital shifts for the relativistic correction of non-relativistic 1s core excitation energies.

Spin coherence and optical properties of alkali-metal atoms in solid parahydrogen

We present a joint experimental and theoretical study of spin coherence properties of 39K, 85Rb, 87Rb, and 133Cs atoms trapped in a solid parahydrogen matrix. We use optical pumping to prepare the spin states of the implanted atoms and circular dichroism to measure their spin states. Optical pumping signals show order-of-magnitude differences depending on both matrix growth conditions and atomic species. We measure the ensemble transverse relaxation times (T2*) of the spin states of the alkali-metal atoms. Different alkali species exhibit dramatically different T2* times, ranging from sub-microsecond coherence times for high mF states of 87Rb, to ~100 microseconds for 39K. These are the longest ensemble T2* times reported for an electron spin system at high densities (n > 10^16 cm^-3). To interpret these observations, we develop a theory of inhomogenous broadening of hyperfine transitions of ^2S atoms in weakly-interacting solid matrices. Our calculated ensemble transverse relaxation times agree well with experiment, and suggest ways to longer coherence times in future work.

Chemical interactions of heparin in porous polypyrrole, an example of drug–carrier destructive interaction

Plasma polypyrrole doped with iodine (PPy/I) and heparin (Hp) has been used as drugs in different ways in the human body. In this work, Hp was inserted into porous PPy/I to prepare biocompatible mixtures to potentially release Hp into the human body. The study was focused on the absorption and chemical interaction of Hp with PPy/I to find whether their structures suffer modifications in the carrying process. The absorption was performed by cryo-lyophilization immersing Hp in water with a 10:1 polymer/drug mass ratio. Once in contact, Hp went into the pores and partially coated the PPy/I surface. IR analyses of the mixtures on the surface showed that the functional Hp groups predominated over the polymer. The chemical interaction of both components was studied by XPS analyzing the energetic distribution of their atomic 1s or 2p orbitals. The results indicated that Hp interacted chemically with PPy/I losing the highest oxidized C1s, N1s and S2p chemical states in both components, forming new reduced ones with less multiple bonds or with more hydrogenated groups. S2p states were especially modified losing the most reactive states with 6 valences to form new more stable states with 2 valences. O1s orbitals behaved differently losing the most reduced chemical states forming new oxidized ones. These modifications suggest that the chemical reactivity of this PPy/I-Hp binary system was altered reducing the therapeutic capability of both components. This kind of studies should be done in all carrier–drug combinations to evaluate the possible chemical interaction between them.

Estimation of the Lyman-α signal of the EFILE diagnostic under static or radiofrequency electric field in vacuum

The electric field induced Lyman-α emission diagnostic aims to provide a non intrusive and precise measurement of the electric field in plasma, using a beam of hydrogen atoms prepared in the metastable 2s state. The metastable particles are obtained by means of a proton beam extracted from a hydrogen plasma source, and neutralised by interaction with vaporised caesium. When a 2s atom enters a region where an electric field is present, it undergoes a transition to the 2p state (Stark mixing). It then quickly decays to the ground level, emitting Lyman-α radiation, which is collected by a photomultiplier. The transition rate is proportional to the square of the magnitude of the electric field, and depends on the field oscillation frequency (with peaks around 1 GHz). By measuring the intensity of the Lyman-α radiation emitted by the beam it is possible to determine the magnitude of the field in a defined region. In this work, an analysis of the behaviour of the diagnostic under static or radiofrequency electric field is presented. Electric field simulations obtained with a finite element solver of Maxwell equations, combined with theoretical calculations of the Stark mixing transition rate, are used to develop a model for the interpretation of photomultiplier data. This method shows good agreement with experimental results for the static field case, and allows to measure the field magnitude for the oscillating case.

Rate Constants and H-Atom Product Yields for the Reactions of O(1D) Atoms with Ethane and Acetylene from 50 to 296 K

The gas phase reactions of atomic oxygen in its first excited state with ethane and acetylene have been investigated in a continuous supersonic flow reactor over the temperature range from 50 to 296 K. O(1D) atoms were produced by the pulsed laser photolysis of ozone at 266 nm. Two different types of experiments, kinetics measurements and H-atom product yield determinations, were performed by detecting O(1D) atoms and H(2S) atoms, respectively, by vacuum ultraviolet laser-induced fluorescence. The measured rate constants are in agreement with previous work at room temperature, and little or no temperature dependence was observed as the temperature was decreased to 50 K. H-atom yields were found to be independent of temperature for the reaction of O(1D) with ethane. These product yields are discussed in the context of earlier dynamics measurements at higher temperature. Due to the influence of secondary reactions, no H-atom yields could be obtained for the reaction of O(1D) with acetylene.

Interference between two resonant transitions with distinct initial and final states connected by radiative decay

The resonant line shape from driving a transition between two states, $|\rm{a}\rangle$ and $|\rm{b}\rangle$, can be distorted due to a quantum-mechanical interference effect involving a resonance between two different states, $|\rm{c}\rangle$ and $|\rm{d}\rangle$, if $|\rm{c}\rangle$ has a decay path to $|\rm{a}\rangle$ and $|\rm{d}\rangle$ has a decay path to $|\rm{b}\rangle$. This interference can cause a shift of the measured resonance, despite the fact that the two resonances do not have a common initial or final state. As an example, we demonstrate that such a shift affects measurements of the atomic hydrogen 2S$_{1/2}$-to-2P$_{1/2}$ Lamb-shift transition due to 3S-to-3P transitions if the 3S$_{1/2}$ state has some initial population.

A quasi-classical study of energy transfer in collisions of hyperthermal H atoms with SO2 molecules.

A deep understanding of energy transfer processes in molecular collisions is at central attention in physical chemistry. Particularly vibrational excitation of small molecules colliding with hot light atoms, via a metastable complex formation, has shown to be an efficient manner of enhancing reactivity. A quasi-classical trajectory study of translation-to-vibration energy transfer (T-V ET) in collisions of hyperthermal H(2S) atoms with SO2(X̃1A') molecules is presented here. For such a study, a double many-body expansion potential energy surface previously reported for HSO2(2A) is used. This work was motivated by recent experiments by Ma et al. studying collisions of H + SO2 at the translational energy of 59 kcal/mol [J. Ma et al., Phys. Rev. A 93, 040702 (2016)]. Calculations reproduce the experimental evidence that during majority of inelastic non-reactive collision processes, there is a metastable intermediate formation (HOSO or HSO2). Nevertheless, the analysis of the trajectories shows that there are two distinct mechanisms in the T-V ET process: direct and indirect. Direct T-V processes are responsible for the high population of SO2 with relatively low vibrational excitation energy, while indirect ones dominate the conversion from translational energy to high values of the vibrational counterpart.

Long-range interactions of hydrogen atoms in excited states. I.2S−1Sinteractions and Dirac-δperturbations

The theory of the long-range interaction of metastable excited atomic states with ground-state atoms is analyzed. We show that the long-range interaction is essentially modified when quasi-degenerate states are available for virtual transitions. A discrepancy in the literature regarding the van der Waals coefficient C_6(2S;1S) describing the interaction of metastable atomic hydrogen (2S state) with a ground-state hydrogen atom is resolved. In the the van der Waals range a_0 << R << a_0/alpha, where a_0= hbar/(alpha m c) is the Bohr radius and alpha is the fine structure constant, one finds the symmetry-dependent result E_{2S;1S}(R) ~ (-176.75 +/- 27.98) E_h (a_0/R)^6 (E_h denotes the Hartree energy). In the Casimir-Polder range a_0/alpha << R << hbar c/L, where L = E(2S_{1/2})-E(2P_{1/2}) is the Lamb shift energy, one finds E_{2S;1S}(R) ~ (-121.50 +/- 46.61) E_h (a_0/R)^6. In the the Lamb shift range R >> hbar c/L, we find an oscillatory tail with a negligible interaction energy below 10^(-36) Hz. Dirac-delta perturbations to the interaction are also evaluated and results are given for all asymptotic distance ranges; these effects describe the hyperfine modification of the interaction, or, expressed differently, the shift of the hydrogen 2S hyperfine frequency due to interactions with neighboring 1S atoms. The 2S hyperfine frequency has recently been measured very accurately in atomic beam experiments.

Proton and antiproton impact excitation of H(2s) atom

Direct excitations for atomic hydrogen 2s → 3s, 3p, and 3d transitions by proton and antiproton collisions have been investigated by using an impact parameter treatment. The calculations are performed within the impact parameter versions of the first- and second-order Born approximations, as well as the solution of the coupled differential equations arising from the one-center atomic-orbital close-coupling approach. We have considered calculations that allow couplings to the n = 1–5 states (up to g sub-levels) of the target atom as well as others that neglect the effect of all states other than the initial and final states of the target atom. The sensitivity of the cross sections to the sign of the projectile charge as well as the influence of the mechanism of possible electronic transitions allowed by the techniques under consideration have been studied. The calculated cross sections are compared with those obtained by previous calculations.

VUV irradiance measurement of a 2.45 GHz microwave-driven hydrogen discharge

Absolute values of VUV-emission of a 2.45 GHz microwave-driven hydrogen discharge are reported. The measurements were performed with a robust and straightforward method based on a photodiode and optical filters. It was found that the volumetric photon emission rate in the VUV-range (80–250 nm) is 1016–1017 cm−3 s−1, which corresponds to approximately 8% dissipation of injected microwave power by VUV photon emission. The volumetric emission of characteristic emission bands was utilized to diagnostics of molecular plasma processes including volumetric rates of ionization, dissociation and excitation to high vibrational levels and metastable states. The estimated reaction rates imply that each injected molecule experiences several inelastic electron impact collisions. The upper limit for the total density of metastable neutrals (2S atoms and molecules) was estimated to be approximately 0.5% of the neutral gas density.