Unclosed Shell Research Articles

Nanoparticulate Pt-catalyzed hydrogenation of Yb films: Towards hydrides with higher hydrogen compositions

• Moderate condition synthesis of β-YbH 3− δ was achieved by nanoparticulate Pt-catalyzed hydrogenation. • Hydrogenation using the Pt nanoparticle catalyst capping method demonstrated room-temperature synthesis of α-YbH 2± δ . • Thermodynamic kinetics in syntheses of β-YbH 3− δ and α-YbH 2± δ were analyzed. It is more difficult to synthesize β-phase ytterbium trihydride (β-YbH 3− δ ) under moderate conditions than other rare-earth metal trihydrides because it requires a valence change from a fully occupied 4 f shell ( 4 f 14 ) in Yb 2+ to an unclosed shell ( 4 f 13 ) in Yb 3+ . We synthesized β-YbH 3− δ at ambient pressure through hydrogenation of a Yb film capped with platinum (Pt) nanoparticle (NP) catalysts. In addition, Pt NP capping enabled us to synthesize α-phase Yb dihydride (α-YbH 2± δ ) at room temperature. The difficulty of trihydrogenating Yb to β-YbH 3− δ is explained by kinetic analyses based on temperature and time dependences of molar fractions of α-YbH 2± δ and β-YbH 3− δ . Pt NP capping can be used to synthesize other hydrogen-rich rare-earth metal hydrides under ambient pressure and lower temperatures.

Incoherent photoproduction of pseudoscalar mesons off nuclei at forward angles

Recent advances in the photon tagging facilities together with the novel, high-resolution fast calorimetry make it possible to perform photoproduction cross section measurements of pseudoscalar mesons on nuclei with a percent level accuracy. The extraction of the radiative decay widths, needed for testing the symmetry breaking effects in QCD, from these measurements at small angles is done by the Primakoff method. This method requires theoretical treatment of all processes participating in these reactions at the same percent level. The most updated description of general processes, including the nuclear coherent amplitude, is done in our previous paper. In this work, in the framework of the Glauber multiple scattering theory, we obtain analytical expressions for the incoherent cross section of the photoproduction of pseudoscalar mesons off nuclei accounting for the mesons absorption in nuclei and the Pauli suppression at forward production angles. As illustrations of the obtained formulas, we calculate the incoherent cross section for photoproduction from a closed shell nucleus, 16O, and from an unclosed shell nucleus, 12C. These calculations allow one to compare different approaches and estimate their impact on the incoherent cross section of the processes under consideration.

Zinc′s Exclusive Tetrahedral Coordination Governed by Its Electronic Structure

Zinc is a critical component of more than 300 proteins including farnesyltransferase, matrix metalloproteinases and endostatin that are involved in the front-line cancer research, and a host of proteins termed zinc fingers that mediate protein-protein and protein-nucleic acid interactions. Despite the growing appreciation of zinc in modern biology, the knowledge of zinc′s coordination nature in proteins remains controversial. It is typically assumed that Zn2+ coordinates to four to six ligands, which led to intensive debates about whether the catalysis of some zinc proteins is regulated by zinc′s four- or five-coordinate complex. Here we report the inherent uncertainty, due to the experimental resolution, in classifying zinc′s five- and six-coordinate complexes in protein crystal structures, and put forward a tetrahedral coordination concept that Zn2+ coordinates to only four ligands mainly because of its electronic structure that accommodates four pairs of electrons in its vacant 4s4p 3 orbitals. Experimental observations of five- and six-coordinate complexes were due to one or two pairs of ambidentate coordinates that exchanged over time and were averaged as bidentate coordinates. This concept advances understanding of zinc′s coordination nature in proteins and the means to study zinc proteins to unlock the secrets of Zn2+ in human biology. In particular, according to this concept, it is questionable to study zinc′s coordination in proteins with Co2+ as a surrogate of Zn2+ for spectroscopic measurements, since the former is a d7 unclosed shell divalent cation whereas the latter is a d10 closed shell divalent cation.

ANALYTICAL FORMULATION OF THE ION–ION INTERACTION POTENTIAL INCLUDING SPIN DENSITY TERM IN ENERGY DENSITY FORMALISM

The heavy-ion interaction potentials are calculated in sudden approximation, using the Fermi density distribution in the proximity theorem treatment of the Skyrme interaction energy density formalism. The contribution of the spin density term is included and the effects of varying the Skyrme force parameters, Fermi density parameters and the strength of the surface correction to Thomas–Fermi kinetic energy density are studied in detail. The universality of the "universal function" of proximity potential is tested and analytical formulas are obtained for both the proximity potential and spin density dependent potentials. The analytical formula for spin density potential works only for colliding nuclei belonging to the same shell. Adding the Coulomb potential, the interaction barriers and their positions can now be calculated analytically which are required for the calculations of fusion cross-sections. Our calculations are made for both the closed and unclosed shell nuclei consisting of light spherical nuclei from 0d-1s and 0f 7/2 shells. In all, over 100 cases are studied and comparisons with other model calculations are presented.

Fusion barriers using the energy-density formalism: Simple analytical formula and the calculation of fusion cross sections.

Fusion barriers are calculated within the sudden approximation, using the Skyrme interaction energy-density model. Both the closed and unclosed shell nuclei are considered and the role of spin-density term is studied in detail. For unclosed shell nuclei, the fusion cross sections are found to decrease by as much as {similar to}50 mb when spin-density effects are included. Very accurate, simple analytical expressions, in terms of only the masses and charges of the reacting nuclei, are obtained for the barrier heights and positions. This introduces a great simplification for the fusion barrier calculations.

Spin density contribution in heavy-ion interaction potentials using energy density formalism.

The result of adding the spin density term in heavy-ion interaction potentials is studied for the first time, especially for unclosed shell nuclei. For the energy density functional of Skyrme interaction, the spin density contribution is derived for nuclei with an even number of valence particles (or holes). The calculations are made for light nuclei, using both the shell-model and Fermi-type nucleon densities. The interaction potential is calculated in the proximity theorem, using corrected Thomas-Fermi approximation for the kinetic energy density. Only the repulsive spin density contribution is found to be significant, which cancels a part of attraction in the total interaction potential.

FEEDBACK STABILIZATION OF AXISYMMETRIC MODES IN NON-CIRCULAR TOKAMAKS

The feedback stability of axisymmetric MHD mode in tokamaks with unclosed shell has been studied analytically through solving directly the boundary problem of the perturbed magnetic surface in the vacuum region. For plasma torus with elliptical cross-section, a feedback system with conducting shell and active control coils has been suggested. The result shows that two unclosed shells arranged above and below the plasma have a very strong stabilizing effect. The re istive shell can suppress the modes with faster growth rate and the active coils can be used to stabilize the slower modes.

Deformation of unclosed shells of revolution with inhomogeneous boundary conditions

Deformation of unclosed shells of revolution with inhomogeneous boundary conditions

On the theory of the high temperature resistivity in the transition metals

The high-temperature deviation from the linear law for the electrical resistivity is explained by the temperature dependence of the one-site t-matrix. It is shown, that when the interatomic correlations between the states of the unclosed shells are absent, the high-temperature “Kondo-tail” remains in the full t-matrix of the translationally invariant system.

Analytic Hartree-Fock Wave Functions for the3p-Shell Atoms

Hartree-Fock wave functions have been obtained for the $3p$-row atoms, i.e., for neutral Al, Si, P, S, Cl, and Ar, and for ${\mathrm{Cl}}^{\ensuremath{-}}$. Solutions were determined in analytic form using a version of Nesbet's symmetry and equivalence restrictions to simplify the calculations for atoms with both closed and unclosed shells of the same $l$ value. These restrictions, the reason for their use, and their relation to other open-shell methods are discussed and the calculated one-electron wave functions and their eigenvalues are presented.

Centre-of-mass effects in the nuclear shell-model

An examination is made of the possible errors that may arise in nuclear shell-model calculations by failing to separate out centre-of-mass motions from the internal motions of the system; a central oscillator potential is used to make possible a simple analysis. The most important effect is the appearance of spurious states that do not refer to the internal motion at all; these must be considered whenever the state is described by two or more unclosed shells (in the sense of L-S coupling), as, for example, in the excited states of 16 O.

The calculation of the absolute strengths of spectral lines

It is shown that in calculating transition integrals it is permissible to neglect the departure of the potential of an atom or ion from its asymptotic Coulomb form. This enables a general analytical expression for the transition integral to be derived. Tables are compiled from which the absolute strengths of large numbers of spectral lines can at once be obtained if the term values of the upper and lower levels are known. s-p , p-d and d-f transitions are all treated. Comparison with experimental data shows that for the simpler systems (i. e. systems with a single electron outside closed shells) the method gives remarkably accurate results; indeed, it appears superior to the normal rather laborious procedure involving the computation of the necessary wave functions, in each individual case, by solution of the appropriate Hartree or Fock differential equation. The method (in its most elementary form) may not be so satisfactory for complex systems (i. e. systems with unclosed shells) owing to difficulties associated with the identification of certain energy parameters. However, the rather scanty comparison data available suggest that even for such systems it yields useful (and in some cases precise) information on the line strengths. Incidentally, in the course of the work the accuracy of a few wave functions based on the self consistent field approximation (including exchange) was tested by using them to evaluate line strengths from both the dipole moment and the dipole velocity formulae. Appreciable defects were revealed.