K-shell Research Articles

Beyond The Sphere. Au20(PR3)8 as a Spherical Aromatic Cuboctahedron Cluster.

The icosahedral Au135+ core is a recurrent building block in ligand-protected gold clusters involving an 8-cluster electron 1S21P6 electronic shell. Such a prototypical structure enables a spherical aromatic behavior as given by long-range magnetic shielding. Recently, the Au20(tBu3P)8 cluster featuring a contrasting cuboctahedral core with formally neutral gold atoms appears as a novel core architecture with the potential to be considered as another potential building block towards functional nanostructures. Here, we explore the ligand-core interaction and spherical aromatic characteristics of Au20(tBu3P)8, in order to provide a direct connection to classical icosahedral spherical aromatic compounds, now involving a cuboctahedral core structure. Such characteristics suggest rationalization of their robustness in terms of certain electron counts, enabling a shielding cone property in ligand-protected metallic clusters, which favors bridging organic and inorganic planar/spherical aromatic species towards the unification of the aromaticity concept and designing guidelines for further achievements.

Coherent control of rare earth 4f shell wavefunctions in the quantum spin liquid Tb2Ti2O7

The resonant excitation of electronic transitions with coherent laser sources creates quantum coherent superpositions of the involved electronic states. Most time-resolved studies have focused on gases or isolated subsystems embedded in insulating solids, aiming for applications in quantum information. Here, we focus on the coherent control of orbital wavefunctions in the correlated quantum material Tb2Ti2O7, which forms an interacting spin liquid ground state. We show that resonant excitation with a strong THz pulse creates a coherent superposition of the lowest energy Tb 4f states. The coherence manifests itself as a macroscopic oscillating magnetic dipole, which is detected by ultrafast resonant x-ray diffraction. We envision the coherent control of orbital wavefunctions demonstrated here to become a new tool for the ultrafast manipulation and investigation of quantum materials.

Extension of the SpK atomic physics code to generate global equation of state data

Global microphysics models are required for the modelling of high-energy-density physics (HEDP) experiments, the improvement of which are critical to the path to inertial fusion energy. This work presents further developments to the atomic and microphysics code, SpK, part of the numerical modelling suite of Imperial College London and First Light Fusion. We extend the capabilities of SpK to allow the calculation of the equation of state (EoS). The detailed configuration accounting calculations are interpolated into finite-temperature Thomas–Fermi calculations at high coupling to form the electronic component of the model. The Cowan model provides the ionic contribution, modified to approximate the physics of diatomic molecular dissociation. By utilising bonding corrections and performing a Maxwell construction, SpK captures the EoS from states ranging from the zero-pressure solid, through the liquid–vapour coexistence region and into plasma states. This global approach offers the benefit of capturing electronic shell structure over large regions of parameter space, building highly-resolved tables in minutes on a simple desktop. We present shock Hugoniot and off-Hugoniot calculations for a number of materials, comparing SpK to other models and experimental data. We also apply EoS and opacity data generated by SpK in integrated simulations of indirectly-driven capsule implosions, highlighting physical sensitivities to the choice of EoS models.

Electronic shells regulate the geometric structures, stabilities and electronic properties of AlnCum (n = 10–15, m = 1–3) clusters

The properties of AlnCum (n = 10–15, m = 1–3) clusters are explored using genetic algorithm combined with density functional theory (GA-DFT). The geometric structures of Al-Cu binary clusters are spherical when the numbers of the valence electrons are around the magic number 40 predicted by Jellium model. The stabilities decrease as the number of doping Cu atom increases. The stabilities also relate to the electronic configurations. In Al13Cu-a cluster, the 3d orbitals of the central Cu atom interact strongly with the superatomic 1D orbitals and formed five bonding orbitals and five antibonding orbitals. The bonding orbitals are dominated by 3d orbitals of Cu, and antibonding orbitals are composed mainly of the superatomic 1D orbitals. The “50 valence electrons” (including 3d10 of Cu) form closed 1S21P63d101D102S21F142P6 shells, and this electronic configuration can be taken as the combination of 40-electron rule. The 3d electrons of Cu atom don’t affect the Jellium model of the cluster.

Electron diffraction of foam-like clusters between xenon and helium in superfluid helium droplets.

We report electron diffraction results of xenon clusters formed in superfluid helium droplets, with droplet sizes in the range of 105-106 atoms/droplet and xenon clusters from a few to a few hundred atoms. Under four different experimental conditions, the diffraction profiles can be fitted using four atom pairs of Xe. For the two experiments performed with higher helium contributions, the fittings with one pair of Xe-He and three pairs of Xe-Xe distances are statistically preferred compared with four pairs of Xe-Xe distances, while the other two experiments exhibit the opposite preference. In addition to the shortest pair distances corresponding to the van der Waals distances of Xe-He and Xe-Xe, the longer distances are in the range of the different arrangements of Xe-He-Xe and Xe-He-He-Xe. The number of independent atom pairs is too many for the small xenon clusters and too few for the large clusters. We consider these results evidence of xenon foam structures, with helium atoms stuck between Xe atoms. This possibility is confirmed by helium time-dependent density functional calculations. When the impact parameter of the second xenon atom is a few Angstroms or longer, the second xenon atom fails to penetrate the solvation shell of the first atom, resulting in a dimer with a few He atoms in between the two Xe atoms. In addition, our results for larger droplets point toward a multi-center growth process of dopant atoms or molecules, which is in agreement with previous proposals from theoretical calculations and experimental results.

Delay of Photoelectrons Emitted from the Ionized Ne 2s and 2p Shells Including Photoelectron Scattering

Delay of Photoelectrons Emitted from the Ionized Ne 2s and 2p Shells Including Photoelectron Scattering

Regulating Second-Shell Coordination in Cobalt Single-Atom Catalysts toward Highly Selective Hydrogenation.

Manipulating the local coordination environment of central metal atoms in single-atom catalysts (SACs) is a powerful strategy to exploit efficient SACs with optimal electronic structures for various applications. Herein, Co-SACs featured by Co single atoms with coordinating S atoms in the second shell dispersed in a nitrogen-doped carbon matrix have been developed toward the selective hydrogenation of halo-nitrobenzene. The location of the S atom in the model Co-SAC is verified through synchrotron-based X-ray absorption spectroscopy and theoretical calculations. The resultant Co-SACs containing second-coordination shell S atoms demonstrate excellent activity and outstanding durability for selective hydrogenation, superior to most precious metal-based catalysts. In situ characterizations and theoretical results verify that high activity and selectivity are attributed to the advantageous formation of the Co-O bond between p-chloronitrobenzene and Co atom at Co1N4-S moieties and the lower free energy and energy barriers of the reaction. Our findings unveil the correlation between the performance and second-shell coordination atom of SACs.

Geometric and Electronic Properties of P Atom-Doped Al Nanoclusters: Alkaline-like Superatom of P@Al12.

The geometric and electronic characteristics of phosphorus-atom doped aluminum nanoclusters, AlnPm (n = 7-17, m = 1 and 2), were investigated through a combination of experiments and theoretical calculations. The size dependences of the ionization energy (Ei) for AlnPm NCs exhibit a local minimum of 5.37 eV at Al12P1, attributed to an endohedral P@Al12 superatom (SA). This SA originates from an excess electron toward the 2P shell closing (40e), coexisting with an exohedral isomer featuring a vertex P atom. The stability of the endohedral P@Al12 is further enhanced in its cationic state compared to the exohedral isomer, when complexed with a fluorine (F) atom, forming an SA salt denoted as P@Al12+F- with an elevated Ei ranging from 6.42 to 7.90 eV. In contrast, for the anionic Al12P1-, the exohedral form is found to be more stable than the endohedral one using anion photoelectron spectroscopy and calculations. The geometric and electronic robustness of neutral P@Al12 SAs against electron donation and acceptance is discussed in comparison to rare-gas-like Si@Al12 SAs.

Ring Structures of Metal Atom Doped C9, C11, and C13 Clusters from Ab Initio Calculations-The Finding of a Gd@C13 Magnetic Superatom Ring.

Pure C9 clusters have a linear chain structure. However, here, we report using ab initio calculations the transformation of a chain into a cyclic ring structure with the capping of Ca, Sr, and Ba atoms. Further calculations on neutral and charged clusters doped with Sc, Y, and La atoms show stabilization of a cation C9 isoelectronic cyclic ring capped with the metal (M) atom, but anion clusters doped with these trivalent atoms form a C10 like MC9 ring, which deforms to a necklace structure. Both the ring structures correspond to electronic shell closing with 20 delocalized valence electrons in a disk jellium model and have a large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap. Calculations of IR and Raman spectra show no imaginary frequency, suggesting that the structures are stable. Addition of two C atoms to the ring structure of LaC9 leads to a capped ring structure of the cation LaC11 and an open ring structure of the LaC11 anion. Further addition of two C atoms leads to a La@C13 cation as well as Ca@C13 and Sr@C13 neutral wheel-shaped rings with endohedral doping of the M atom. These novel ring structures have a large HOMO-LUMO gap of more than 4 eV and are magic with electronic shell closing corresponding to 28 delocalized valence electrons in a disk jellium model. There is π aromaticity in this ring satisfying 4n+2 (n = 3) Hückel's rule with 14 valence electrons. Interestingly, when the dopant is a Gd atom, there is a formation of a magnetic superatom ring Gd@C13 + with 7 μB magnetic moments due to seven 4f up-spin states of Gd being fully occupied. Bonding in these novel ring structures is discussed.

Preparation of Neptunyl and Plutonyl Acetates To Access Nonaqueous Transuranium Coordination Chemistry.

Uranyl diacetate dihydrate is a useful reagent for the preparation of uranyl (UO22+) coordination complexes, as it is a well-defined stoichiometric compound featuring moderately basic acetates that can facilitate protonolysis reactivity, unlike other anions commonly used in synthetic actinide chemistry such as halides or nitrate. Despite these attractive features, analogous neptunium (Np) and plutonium (Pu) compounds are unknown to date. Here, a modular synthetic route is reported for accessing stoichiometric neptunyl(VI) and plutonyl(VI) diacetate compounds that can serve as starting materials for transuranic coordination chemistry. The new NpO22+ and PuO22+ complexes, as well as a corresponding molecular UO22+ complex, are isomorphous in the solid state, and in solution show similar solubility properties that facilitate their use in synthesis. In both solid and solution state, the +VI oxidation state (O.S.) is maintained, as demonstrated by vibrational and optical spectroscopy, confirming that acetate anions stabilize the oxidizing, high-valent +VI states of Np and Pu as they do for the more stable U(VI). All three acetate salts readily react with a model diprotic ligand, affording incorporation of U(VI), Np(VI), and Pu(VI) cores into molecular coordination compounds that occurs concomitantly with elimination of acetic acid; the new complexes are high-valent, yet overall charge neutral, facilitating entry into nonaqueous chemistry by rational synthesis. Computational studies reveal that the dianionic ligand framework assists in stabilizing the +VI O.S. via donation to the 5f shells of the actinides, highlighting the potential usefulness of protonolysis reactivity toward preparation of stabilized high-valent transuranic species.

Correlation between reactivity descriptors and electronic pressures: A different application of SBO orbitals

AbstractThis research has discovered relationships between atom reactivity parameters (such as electronegativity or hardness) and the pressure exerted on their electronic shells. These relationships are derived from the relationship between the radius of the atom confined within a spherical box and the pressure exerted on the box by its electrons. To determine the pressure corresponding to each radius, it was essential to formulate a set of new basis functions valid for calculating the energy of confined atoms. These new basis functions, Simplified Box Orbitals (SBOs), stem from the previously studied SBOs, but their coefficients are obtained variationally instead of being fitted to a Slater‐Type Orbital (STO), and the powers (R−r)k are selected differently. These differences make them especially suitable for treating confined atoms and distinguishing between “hard” and “soft” confinements. This methodology has proven useful for accurately calculating the properties of various atoms under different pressures, namely H, He, Li, Be, B, C, N, O, F, and Ne. Furthermore, we believe that the new basis functions are suitable for obtaining Gaussian expansions that will enable the treatment of confined molecules, which we intend to study in a subsequent work.

Effect of Ho3+ Substitution on Magnetic Properties of ZnCr2Se4.

A series of ZnCr2-xHoxSe4 microcrystalline spinels (where x = 0.05, 0.075, and 0.10) containing holmium ions in octahedral coordination were obtained by sintering of adequate reactants at high temperatures. The obtained doped materials were characterized by X-ray diffraction, Scanning Electron Microscopy, UV-Vis-NIR, molecular field approximation, and XPS spectroscopies. Their thermal properties were also investigated. The doping of the ZnCr2S4 matrix with paramagnetic Ho3+ ions with a content of not more than 0.1 and a screened 4f shell revealed a significant effect of orbital and Landau diamagnetism, a strong reduction in short-range ferromagnetic interactions, and a broadening and shift of the peak of the first critical field by simultaneous stabilization of the sharp peak in the second critical field. These results correlate well with FPLO calculations, which show that Cr sites have magnetic moments of 3.19 µB and Ho sites have significantly larger ones with a value of 3.95 µB. Zn has a negligible magnetic polarization of 0.02 µB, and Se induces a polarization of approximately -0.12 µB.

Interactions between Iron Minerals and Dissolved Organic Matter Derived from Microplastics Inhibited the Ferrihydrite Transformation as Revealed at the Molecular Scale.

Microplastic-derived dissolved organic matter (MP-DOM) is an emerging carbon source in the environment. Interactions between MP-DOM and iron minerals alter the transformation of ferrihydrite (Fh) as well as the distribution and fate of MP-DOM. However, these interactions and their effects on both two components are not fully elucidated. In this study, we selected three types of MP-DOM as model substances and utilized Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and extended X-ray absorption fine structure (EXAFS) spectroscopy to characterize the structural features of DOMs and DOM-mineral complexes at the molecular and atomic levels. Our results suggest that carboxyl and hydroxyl groups in MP-DOM increased the Fe-O bond length by 0.02-0.03 Å through interacting with Fe atoms in the first shell, thereby inhibiting the transformation of Fh to hematite (Hm). The most significant inhibition of Fh transformation was found in PS-DOM, followed by PBAT-DOM and PE-DOM. MP-DOM components, such as phenolic compounds and condensed polycyclic aromatics (MW > 360 Da) with high oxygen content and high unsaturation, exhibited stronger mineral adsorption affinity. These findings provide a profound theoretical basis for accurately predicting the behavior and fate of iron minerals as well as MP-DOM in complex natural environments.

Beyond Shell-Filling: Strong Enhancement of Electron Affinity of Metal Clusters through a Noninvasive Oriented External Electric Field.

Traditional electron counting rules, like the Jellium model, have long been successfully utilized in designing superhalogens by modifying clusters to have one electron less than a filled electronic shell. However, this shell-filling approach, which involves altering the intrinsic properties of the clusters, can be complex and challenging to control, especially in experiments. In this letter, we theoretically establish that the oriented external electric field (OEEF) can substantially enhance the electron affinity (EA) of diverse aluminum-based metal clusters with varying valence electron configurations, leading to the creation of superhalogen species without altering their shell arrangements. This OEEF approach offers a noninvasive alternative to traditional superatom design strategies, as it does not disrupt the clusters' geometrical structures and superatomic states. These findings contribute a vital piece to the puzzle of constructing superalkalis and superhalogens, extending beyond conventional shell-filling strategies and potentially expanding the range of applications for functional clusters.

Role of outer shell electron-nuclear distant of transition metal atoms (TMA) on band gap reduction and optical properties of TiO2 semiconductor

The reduction of the band gap of titanium dioxide (TiO2) via transition metal atoms (TMAs) is the focus of many research groups to increase its absorption to visible region of electromagnetic (EM) radiation. In this modeling it was shown that atoms having near outer electron shells affect strongly on the band gap of TiO2. The TiO2 has exceptional photoactivity, high stability, and cost-effectiveness. The results of the current study emphasized that adjustable TiO2 photocatalyst material can be produced through band gap reduction by Pt, Pd, and Ni dopants. The electronic configurations and optical properties of both undoped and (Ni, Pd and Pt)-doped TiO2 have been studied using first-principles calculations within the framework of Density Functional Theory (DFT) with the aid of the Vienna Ab initio Simulation Package (VASP). The role of electronegativity of TMAs on band gap drop was discussed for the first time. The band gap of clean TiO2 was found to be 3.2 eV using the band structure (BS) and Tauq model. The results indicate that Ni TMA with lowest electronegativity reduces the band gap of TiO2 to the lowest value (0.86 eV). The results of BS and density of state revealed the fact that small outer shell electron-nuclear distant TMA more affected the band gap of TiO2. The band gap determined from the imaginary part of the optical dielectric function closely aligns with those determined from the BS diagram. The increase in the refractive index (n) was observed with the increase of Ni, Pd, and Pt into the TiO2 structure, which indicates an increase in charge density within the TiO2 structure. The reflectance, absorbance, and transmittance results suggest that Pd and Pt-doped TiO2 are responsive to the visible region of electromagnetic radiation, whereas Ni-doped TiO2 exhibits responsiveness in the IR region. In its pure state; TiO2 was found almost transparent in the visible and IR regions of EM radiation.

Regulating Second Coordination Shell of Ce Atom Site and Reshaping of Carrier Enable Single-Atom Nanozyme to Efficiently Express Oxidase-like Activity.

Single-atom nanozymes (SANs) are considered to be ideal substitutes for natural enzymes due to their high atom utilization. This work reported a strategy to manipulate the second coordination shell of the Ce atom and reshape the carbon carrier to improve the oxidase-like activity of SANs. Internally, S atoms were symmetrically embedded into the second coordination layer to form a Ce-N4S2-C structure, which reduced the energy barrier for O2 reduction, promoted the electron transfer from the Ce atom to O atoms, and enhanced the interaction between the d orbital of the Ce atom and p orbital of O atoms. Externally, in situ polymerization of mussel-inspired polydopamine on the precursor helps capture metal sources and protects the 3D structure of the carrier during pyrolysis. On the other hand, polyethylene glycol (PEG) modulated the interface of the material to enhance water dispersion and mass transfer efficiency. As a proof of concept, the constructed PEG@P@Ce-N/S-C was applied to the multimodal assay of butyrylcholinesterase activity.

Examining the Electron Phase Transformation Theory in the Context of Atomic Structure

The shortcomings of the Rutherford atomic model can be eliminated by the suggested phase transformation of the electrons from point to non-revolving surface charge in the vicinity of the nucleus. The energy balance investigations of this atom model indicated that the stability of the surface charge valence electron shell is ensured by the one-dimensional Casimir effect. If this theoretical prediction is correct then the first ionization energies of the elements should correlate linearly to the inverse of atomic diameter. Classical physics approach, the electrostatic attraction of the nucleus and the repulsion of the surface charge electron shell result in an identical relationship. The problem with the classical physics approach is that it does not offer an adequate explanation for the photoelectric effect and the free electrons inside the metal. Therefore, classical electrostatics cannot be considered the right physical process responsible for the stability of the valence electron/s in neutral atoms. The derived theoretical relationship, between atomic diameter and ionization energy, was tested up to 86 elements of the periodic table. The correlation coefficient is 0.9187. The correlation is stronger for individual periods. The empirical relationship between the ionization energy and atomic radii is well known, resulting in the same correlation coefficients. However, the correlation to the atomic radii does not reproduce the theoretically derived constant multiplier, contrarily to the atomic diameter relationship. Thus, the first ionization energy is the function of the atomic diameter. The uncertainties in the reported atomic sizes are relatively high. Therefore, the correlation between theory and experiments should be considered as excellent. The theoretically derived relationship between the first ionization energy and atomic diameter is the consequence of the proposed phase transformation of the electron. Thus the detected strong correlation between theory and experiments adds further support to the proposed atomic structure.

Development of a cylindrical mirror analyser electron spectrometer and associated data acquisition system to study inner shell electron emission following ion-atom collision

In this paper we report on the development and performance of a cylindrical mirror analyser electron spectrometer for ion atom collision experiments. A low cost data acquisition system using Arduino microcontroller has also been developed and tested. We have measured the Auger emission spectra for various gaseous targets in collision with 1 MeV proton beams. Relative total Auger emission cross sections have also been measured for N2 molecular target as a function of incident proton energy. The projectile energy dependence agrees well with earlier reported measurements.

High‐Energy Nitrogen Rings Stabilized by Superatomic Properties

AbstractHow to stabilize nitrogen‐rich high‐energy‐density molecules under conventional conditions is particularly important for the energy storage and conversion of such systems and has attracted extensive attention. In this work, the theoretical study shows for the first time that the stabilization mechanism of the nitrogen ring conforms to the superatomic properties at the atomic level. This result occurs because the stabilized anionic nitrogen rings generally exhibit planar high symmetry and the injected electrons occupy the superatomic molecular orbitals (SAMOs) of the nitrogen rings. According to these results, the typical stabilized anionic nitrogen ring structures N64−, N5−, and N42− are identified, which possess delocalized molecular orbitals with the same symmetry as the atomic orbitals and electron arrangement consistent with the electron shell arrangement of the atom. On this basis, a pathway is further designed to stabilize nitrogen rings by introducing metal atoms as electron donors to form neutral ThN6, LiN5, and MgN4 structures, thereby replacing the anionization of systems. This study highlights the importance of developing nitrogen‐rich energetic materials from the perspective of superatoms.

Machine Learning Molecular Dynamics Shows Anomalous Entropic Effect on Catalysis through Surface Pre-melting of Nanoclusters.

Due to the superior catalytic activity and efficient utilization of noble metals, nanocatalysts are extensively used in the modern industrial production of chemicals. The surface structures of these materials are significantly influenced by reactive adsorbates, leading to dynamic behavior under experimental conditions. The dynamic nature poses significant challenges in studying the structure-activity relations of catalysts. Herein, we unveil an anomalous entropic effect on catalysis via surface pre-melting of nanoclusters through machine learning accelerated molecular dynamics and free energy calculation. We find that due to the pre-melting of shell atoms, there exists a non-linear variation in the catalytic activity of the nanoclusters with temperature. Consequently, two notable changes in catalyst activity occur at the respective temperatures of melting for the shell and core atoms. We further study the nanoclusters with surface point defects, i.e. vacancy and ad-atom, and observe significant decrease in the surface melting temperatures of the nanoclusters, enabling the reaction to take place under more favorable and milder conditions. These findings not only provide novel insights into dynamic catalysis of nanoclusters but also offer new understanding of the role of point defects in catalytic processes.