Search results for " clusters"
showing 10 items of 1091 documents
Heterometallic CoII-CoIII-MII alkoxido-bridged heptanuclear motifs (M = Cu, Zn). Syntheses, crystal structures and magnetic properties
2018
Two new alkoxido-bridged heterometallic complexes of formula [CoIICoIII 3CuII 3(dea)6(CH3COO)3](ClO4)0.75(CH3COO)1.25 (1) and [CoII 2CoIII 2ZnII 3(tea)2(piv)6(CH3O)2(OH)2(CH3OH)2]·H2O 2 (H2dea=diethanolamine, H3tea=triethanolamine and Hpiv=pivalic acid) have been assembled using aminoalcohol ligands. The cationic core in 1 possesses a threefold crystallographic axis, and it exhibits a set of three copper(II), one cobalt(II) and three cobalt(III) ions arranged as a hexagon of alternating copper(II) and cobalt(III) ions around the central cobalt(II) ion. Each edge of the hexagon is defined by a double alkoxido bridge, the outer one being bis-monodentate with copper(II)-cobalt(III) pair wherea…
Fast, low-level detection of strontium-90 and strontium-89 in environmental samples by collinear resonance ionization spectroscopy
1993
Environmental assessment in the wake of a nuclear accident requires the rapid determination of the radiotoxic isotopes 89Sr and 90Sr. Useful measurements must be able to detect 108 atoms in the presence of about 1018 atoms of the stable, naturally occurring isotopes. This paper describes a new approach to this problem using resonance ionization spectroscopy in collinear geometry, combined with classical mass separation. After collection and chemical separation, the strontium from a sample is surface-ionized and the ions are accelerated to an energy of about 30 keV. Initially, a magnetic mass separator provides an isotopic selectivity of about 106. The ions are then neutralized by charge exc…
Resonant Photoelectron Diffraction
2013
A layout of a resonant photoelectron diffraction, RESPED, experiment is described from the theoretical basis to the data acquisition and analysis procedures. The theory of the resonance between the directly emitted photoelectron of a selected valence band and the electron emitted by autoionization (Auger) of the same valence band is presented within a formal frame. The critical issue of the angular symmetry and distribution of the resonating electron is discussed in connection with the current computational protocols for photoelectron diffraction, PED, analysis. A few representative applications are presented, where RESPED is shown to overcome some limitations of conventional PED thanks to …
Analysis of Dissolved-Gas Atomization: Supercritical CO2 Dissolved in Water
2010
Supercritical dissolved-gas atomization is an atomization process in which carbon dioxide at temperature and pressure above its critical point is used as the atomizing gas. The spray characteristics in terms of droplets size and distribution have been experimentally studied using a laser diffraction method based on a Malvern apparatus. The main parameter that influences the droplets size is the gas-to-liquid mass ratio (GLR); the injection pressure in the range of 7.4-13 MPa has a minor effect. Upon variation of the GLR from 0.5 to 3, the droplet mean diameter changes from about 8.0 to 2.0 μm; very narrow droplet size distributions are also produced. From the point of view of the atomizatio…
Enhanced Surface Ligands Reactivity of Metal Clusters by Bulky Ligands for Controlling Optical and Chiral Properties.
2021
Surface ligands play critical roles in determining the surface properties of metal clusters. However, modulating the properties and controlling the surface structure of clusters through surface‐capping agent displacement remain a challenge. In this work, a silver cluster, [Ag 14 (SPh(CF 3 ) 2 ) 12 (PPh 3 ) 4 (DMF) 4 ] ( Ag 14 ‐DMF , where HSPh(CF 3 ) 2 is 3,5‐bis(trifluoromethyl)benzenethiol, PPh 3 is triphenylphosphine and DMF is N,N‐Dimethylformamide), with weakly coordinated DMF ligands on the surface silver sites, was synthesized by using a mixed ligands strategy (bulky thiolates, phosphines and small solvents). The as‐prepared Ag 14 ‐DMF is a racemic mixture of chiral molecules. Owing …
Magic numbers, excitation levels, and other properties of small neutral math clusters (N < 50)
2006
The ground-state energies and the radial and pair distribution functions of neutral math clusters are systematically calculated by the diffusion Monte Carlo method in steps of one math atom from 3 to 50 atoms. In addition the chemical potential and the low-lying excitation levels of each cluster are determined with high precision. These calculations reveal that the “magic numbers” observed in experimental math cluster size distributions, measured for free jet gas expansions by nondestructive matter-wave diffraction, are not caused by enhanced stabilities. Instead they are explained in terms of an enhanced growth due to sharp peaks in the equilibrium concentrations in the early part of the e…
Vibrations of weakly coupled nanoparticles
2009
The vibrations of a coupled pair of isotropic silver spheres are investigated and compared with the vibrations of the single isolated spheres. Situations of both strong coupling and also weak coupling are investigated using continuum elasticity and perturbation theory. The numerical calculation of the eigenmodes of such dimers is augmented with a symmetry analysis. This checks the convergence and applicability of the numerical method and shows how the eigenmodes of the dimer are constructed from those of the isolated spheres. The frequencies of the lowest frequency vibrations of such dimers are shown to be very sensitive to the strength of the coupling between the spheres. Some of these mod…
‘I don’t know the answer to that question’: a corpus-assisted discourse analysis of White House Press Briefings
2013
White House Press Briefings, daily meetings with the press held by the White House Press Secretary, are the main information conduit for the White House (Kumar 2007). They are considered a “political chess game” where the Press Secretary and the press face a “wrestling match” (Partington 2006: 16). Our analysis is carried out on a corpus comprising all the Press Briefings across three presidencies from Clinton to Obama. The additional mark-up includes information about individual speakers and their role, allowing us to compare different discourse strategies adopted by the participants in the briefings at different points in time. This leads us to determine the extent of the differences in t…
Evidence of superatom electronic shells in ligand-stabilized aluminum clusters
2011
Ligand-stabilized aluminum clusters are investigated by density functional theory calculations. Analysis of Kohn-Sham molecular orbitals and projected density of states uncovers an electronic shell structure that adheres to the superatom complex model for ligand-stabilized aluminum clusters. In this current study, we explain how the superatom complex electron-counting rule is influenced by the electron-withdrawing ligand and a dopant atom in the metallic core. The results may guide the prediction of new stable ligand-stabilized (superatom) complexes, regardless of core and electron-withdrawing ligand composition.
Laser resonant ionization of plutonium
1985
Resonance ionization mass spectrometry (RIMS) has been tested for the isotope-selective determination of trace amounts of plutonium. An atomic beam is formed by evaporating plutonium atoms from a rhenium-filament heated to 1800 °C. The radiation of a pulsed dye laser excites the atoms in a two-photon process (λ=595.2 nm) followed by photoionization of the excited atoms. Mass selectivity is obtained by use of a time-of-flight spectrometer. A resonant signal of239Pu was measured with 1013 atoms deposited on the filament.