Core Electron Density Research Articles

Accurate charge densities from powder X-ray diffraction - a new version of the Aarhus vacuum imaging-plate diffractometer.

In recent years powder X-ray diffraction has proven to be a valuable alternative to single-crystal X-ray diffraction for determining electron-density distributions in high-symmetry inorganic materials, including subtle deformation in the core electron density. This was made possible by performing diffraction measurements in vacuum using high-energy X-rays at a synchrotron-radiation facility. Here we present a new version of our custom-built in-vacuum powder diffractometer with the sample-to-detector distance increased by a factor of four. In practice this is found to give a reduction in instrumental peak broadening by approximately a factor of three and a large improvement in signal-to-background ratio compared to the previous instrument. Structure factors of silicon at room temperature are extracted using a combined multipole-Rietveld procedure and compared with ab initio calculations and the results from the previous diffractometer. Despite some remaining issues regarding peak asymmetry, the new diffractometer yields structure factors of comparable accuracy to the previous diffractometer at low angles and improved accuracy at high angles. The high quality of the structure factors is further assessed by modelling of core electron deformation with results in good agreement with previous investigations.

Abstract 159: Eicosapentaenoic Acid and Docosahexaenoic Acid Have Distinct Membrane Locations and Lipid Interactions as Determined by X-ray Diffraction

Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) differentially influence lipid oxidation, signal transduction, fluidity, and cholesterol domain formation, potentially due to distinct membrane interactions. We used small angle x-ray diffraction to test EPA and DHA effects on model membrane structure. Vesicles were prepared to model human peripheral cell membranes using 1-palmitoyl-2-oleoyl- sn -glycero-3-phosphocholine (POPC) and cholesterol (C) (0.3 C:POPC mole ratio), and treated with vehicle, EPA, or DHA (1:10 mole ratio to POPC). Membrane width ( d- space) was 59.5 Å at 10°C, decreasing to 54.7 Å at 30°C due to increased acyl chain dynamics. EPA or DHA had no effect on membrane d -space (<1 Å change). Electron density profiles from diffraction data were superimposed on corresponding vehicle profiles (Fig. 1). EPA increased membrane hydrocarbon core electron density over a broad area, up to ± 20 Å from the membrane center, indicating an energetically favorable extended membrane orientation for EPA, stabilized by van der Waals interactions. By contrast, DHA increased electron density in the phospholipid head group region starting at ± 12 Å from the membrane center, due to DHA-surface interactions, resulting in a pronounced electron density reduction and increased disorder in the membrane hydrocarbon core centered ± 7-9 Å from the membrane center. DHA disordering effects were less apparent at higher temperatures, likely due to greater rotational dynamics, while EPA effects were stable. The contrasting EPA and DHA effects on membrane structure signify distinct molecular locations/orientations and may contribute to observed differences in biological activity.

Core-valence stockholder AIM analysis and its connection to nonadiabatic effects in small molecules.

A previous theory of separation of motions of core and valence fractions of electrons in a molecule [J. R. Mohallem et al., Chem. Phys. Lett. 501, 575 (2011)] is invoked as basis for the useful concept of Atoms-in-Molecules (AIM) in the stockholder scheme. The output is a new tool for the analysis of the chemical bond that identifies core and valence electron density fractions (core-valence stockholder AIM (CVSAIM)). One-electron effective potentials for each atom are developed, which allow the identification of the parts of the AIM which move along with the nuclei (cores). This procedure results in a general method for obtaining effective masses that yields accurate non-adiabatic corrections to vibrational energies, necessary to attain cm-1 accuracy in molecular spectroscopy. The clear-cut determination of the core masses is exemplified for either homonuclear (H2+, H2) or heteronuclear (HeH+, LiH) molecules. The connection of CVSAIM with independent physically meaningful quantities can resume the question of whether they are observable or not.

Contrasting Behavior of the Z Bonds in X-Z···Y Weak Interactions: Z = Main Group Elements Versus the Transition Metals.

In contrast to the increasing family of weak intermolecular interactions in main-group compounds (X-Z···Y, Z = main-group elements), an analysis of the Cambridge Structural Database indicates that electron-saturated (18-electron) transition-metal complexes show reluctance toward weak M bond formation (X-M···Y, M = transition metal). In particular, weak M bonds involving electron-saturated (18-electron) complexes of transition metals with partially filled d-orbitals are not found. We propose that the nature of valence electron density distribution in transition-metal complexes is the primary reason for this reluctance. A survey of the interaction of selected electron-saturated transition-metal complexes with electron-rich molecules (Y) demonstrates the following: shielding the possible σ-hole on the metal center by the core electron density in 3d series, and enhanced electronegativity and relativistic effects in 4d and 5d series, hinders the formation of the M bond. A balance in all the destabilizing effects has been found in the 4d series due to its moderate polarizability and primogenic repulsion from inner core d-electrons. A changeover in the donor-acceptor nature of the metal center toward different types of incoming molecules is also unveiled here. The present study confirms the possibility of M bond as a new supramolecular force in designing the crystal structures of electron-saturated transition-metal complexes by invoking extreme ligand conditions.

Atmospheric neutrinos for investigation of Earth interior

The neutrino experiment IceCube at South pole can distinguish between Bulk Silicate Earth model and Hydridic Earth model predictions about inner Earth density by using neutrino absorption tomography. The result can be obtained during less than 10 years of operation. The experiment PINGU at South pole can check the Hydridic Earth model predictions about Earth core electron density by using neutrino oscillation tomography. The result will be obtained for the time of operation much shorter than 10 years. The combination of IceCube and PINGU results will give the information about the Earth core chemical element composition.

Positron probing of disordered regions in neutron‐irradiated silicon

The vacancy‐rich disordered regions (DR) playing a key role in improving the thermoelectric figure‐of‐merit of silicon thermoelectric generators by reducing (by ∼90%) the thermal conductivity, have been probed with positrons. The DR were created by irradiating n‐Cz‐Si(P) material with the fast reactor neutrons. The parameter of the electron density rs′ ≈ 2.18 a.u. contacting positrons in DR has been reconstructed using the data of the angular correlation of the annihilation radiation (ACAR); the amendments to the rs′ value associated with the ion core electrons were taken into account. It is argued that the ion cores of atoms of silicon as well as the ones of the as‐grown impurities (O, C) are involved in the open vacancy volume to be probed with positrons: a relaxation of the ion cores directed inward toward the vacancy volume seems to take place. These positron traps are formed beyond the vacancy‐rich area of the disordered region. In the course of isochronal annealing, the traps are stable up to Tanneal. ≈ 520 °C when a recovery of ACAR parameters begins and then it continues up to ∼1050 °C.A vacancy center containing positrons in the open vacancy volume beyond the vacancy‐rich area of the disordered region is shown schematically. The emission of electron–positron annihilation gamma‐quanta dominates from within these positron traps. The values of both the electron–positron ion core radius (rm) and electron density (parameter rs′) suggest a relaxation directed toward inwards the open vacancy volume: the impurity atoms of oxygen and carbon are, perhaps, involved in the center.

Surface Functionalization of Metal Nanoparticles by Conjugated Metal-Ligand Interfacial Bonds: Impacts on Intraparticle Charge Transfer.

Noble metal nanoparticles represent a unique class of functional nanomaterials with physical and chemical properties that deviate markedly from those of their atomic and bulk forms. In order to stabilize the nanoparticles and further manipulate the materials properties, surface functionalization with organic molecules has been utilized as a powerful tool. Among those, mercapto derivatives have been used extensively as the ligands of choice for nanoparticle surface functionalization by taking advantage of the strong affinity of thiol moieties to transition metal surfaces forming (polar) metal-thiolate linkages. Yet, the nanoparticle material properties are generally discussed within the context of the two structural components, the metal cores and the organic capping layers, whereas the impacts of the metal-sulfur interfacial bonds are largely ignored because of the lack of interesting chemistry. In recent years, it has been found that metal nanoparticles may also be functionalized by stable metal-carbon (or even -nitrogen) covalent bonds. Because of the formation of dπ-pπ interactions between the transition-metal nanoparticles and terminal carbon moieties, the interfacial resistance at the metal-ligand interface is markedly reduced, leading to the emergence of unprecedented optical and electronic properties. In this Account, we summarize recent progress in the studies of metal nanoparticles functionalized by conjugated metal-ligand interfacial bonds that include metal-carbene (M═C) and metal-acetylide (M-C≡)/metal-vinylidene (M═C═C) bonds. Such interfacial bonds are readily formed by ligand self-assembly onto nanoparticle metal cores. The resulting nanoparticles exhibit apparent intraparticle charge delocalization between the particle-bound functional moieties, leading to the emergence of optical and electronic properties that are analogous to those of their dimeric counterparts, as manifested in spectroscopic and electrochemical measurements. This is first highlighted by ferrocene-functionalized nanoparticles that exhibit nanoparticle-mediated intervalence charge transfer (IVCT) among the particle-bound ferrocenyl moieties, as manifested in electrochemical and spectroscopic measurements. Such intraparticle charge delocalization has also been observed with other functional moieties such as pyrene and anthracene, where the photoluminescence emissions are consistent with those of their dimeric derivatives. Importantly, as such electronic communication occurs via a through-bond pathway, it may be readily manipulated by the valence states of the nanoparticle cores as well as specific binding of selective molecules/ions to the organic capping shells. These fundamental insights may be exploited for diverse applications, ranging from chemical sensing to (nano)electronics and fuel cell electrochemistry. Several examples are included, such as sensitive detection of nitroaromatic derivatives, metal cations, and fluoride anions by fluorophore-functionalized metal nanoparticles, fabrication of nanoparticle-bridged molecular dyads by, for instance, using nanoparticles cofunctionalized with 4-ethynyl-N,N-diphenyl-aniline (electron donor) and 9-vinylanthracene (electron acceptor), and enhanced electrocatalytic activity of acetylene derivatives-functionalized metal/alloy nanoparticles for oxygen reduction reaction by manipulation of the metal core electron density and hence interactions with reaction intermediates. We conclude this Account with a perspective where inspiration from conventional organometallic chemistry may be exploited for more complicated nanoparticle surface functionalization through the formation of diverse metal-nonmetal bonds. This is a unique platform for ready manipulation of nanoparticle properties and applications.

Far-infrared tangential interferometer/polarimeter design and installation for NSTX-U.

The Far-infrared Tangential Interferometer/Polarimeter (FIReTIP) system has been refurbished and is being reinstalled on the National Spherical Torus Experiment-Upgrade (NSTX-U) to supply real-time line-integrated core electron density measurements for use in the NSTX-U plasma control system (PCS) to facilitate real-time density feedback control of the NSTX-U plasma. Inclusion of a visible light heterodyne interferometer in the FIReTIP system allows for real-time vibration compensation due to movement of an internally mounted retroreflector and the FIReTIP front-end optics. Real-time signal correction is achieved through use of a National Instruments CompactRIO field-programmable gate array.

Comparison of two treatment couch model in Monaco treatment planning system

Objective To construct the uniform electron density couch model (model A ED=0.25) and two components non uniform electron density couch model (model B FD=0.5and foam core=0.1) in the Monaco treatment planning system for the iBEAM® evo Extension 415, and to compare which model can better quantify the treatment couch influence on radiation dose. Methods Phantom was positioned in the center of the couch, the attenuation of the couch was evaluated with 6 MV for a field size of 10 cm×10 cm. Dose measurements of couch attenuation were performed at gantry angles from 180.0° to 122.8°, using a 0.125cc semiflex ionization chamber (PTW), isocentrically placed in the center of a homogeneous cylindrical phantom. Each experimental setup was first measured on the linear accelerator and then reproduced in the TPS.By adjusting the relative-to water electron density (ED) values of the couch, the measured attenuation was replicated. The model accuracies of the model A and model B were evaluated by comparing the measured and calculated results at the minimum computational grid (2 mm) and maximum computing grid (5 mm), respectively. Results The maximum measured and calculated percentage deviation for the central phantom position was 4.01%.The couch model was included in the TPS with a uniform ED of 0.25 or a 2 component model with a fiber ED=0.5 and foam core ED=0.1.For model A and B under 2 and 5 mm voxel grid size, the mean absorbed dose with couch was reduced to 0.61%, 0.84%, 0.71% and 0.92% from 2.8% without couch. Conclusions Model A has a good agreement between measured and calculated dose distributions for all different voxel grid sizes and gantry angles. It can accurately describes the dose perturbations due to the presence of the couch and should therefore be used during treatment planning. Key words: Carbon fiber treatment couch; Treatment couch model; Monaco treatment planning system; Dose attenuation

Tailoring the core electron density in modulation-doped core−multi-shell nanowires

We show how a proper radial modulation of the composition of core-multi-shell nanowires (NWs) critically enhances the control of the free-carrier density in the high-mobility core with respect to core−single-shell structures, thus overcoming the technological difficulty of fine tuning the remote doping density. We calculate the electron population of the different NW layers as a function of the doping density and of several geometrical parameters by means of a self-consistent Schrödinger–Poisson approach: free carriers tend to localize in the outer shell and screen the core from the electric field of the dopants.

SU‐E‐T‐650: Quantification and Modeling of the Dosimetric Impact of the IBEAM Evo Treatment Couchtop EP (Elekta) in VMAT

Purpose:quantification and modelling of the dosimetric impact of the treatment couch in Monaco Treatment Planning System.Methods:The attenuation characteristics of couchtop EP was evaluated for two different photon acceleration potentials (6MV and 10MV) for a field size of (10×10) cm2. Phantom positions in A‐B direction: on the left half, in the center and on the right half of the couch. Dose measurements of couch attenuation were performed at gantry angles from 180° to 122°, using a 0.125cc semiflex ionization chamber isocentrically placed in the center of a homogeneous cylindric sliced RW3 phantom. Each experimental setup was first measured on the LINAC and then reproduced in the TPS. By adjusting the relative‐to‐water electron density (ED) values of the couch, the measured attenuation was replicated. The simulated results were evaluated by comparing the measurements and simulations.Results:Without the couch model included the maximum difference between measured and calculated dose was 5.5% (5.1%) and 6.6% (6.1%) for 2 mm and 5 mm voxel size, when the phantom was positioned on the left (center). The couch model was included in the TPS with a uniform ED of 0.18 or a 2 component model with a fiber ED= 0.6 and foam core ED= 0.1. After including the treatment couch, the mean dose attenuation was reduced from 2.8% without couch included to (0.0, 0.8, −0.2, 0.6)%. The 4 different values represent the 1 and 2 components model and 2 and 5 mm voxel grid size.Conclusion:For a uniform relative‐to‐water couch electron density of 0.18 a good agreement between measured and calculated dose distributions was obtained for all different energies, voxel grid spacings and gantry angles. Therefore, we conclude that the Monaco couch model accurately describes the dose perturbations due to the presence of the patient couch and should therefore be used during treatment planning.This project is supported by Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Hebei Personnel of China

Plasma Core Electron Density and Temperature Measurements Using CVI Line Emissions in TCABR Tokamak

In this work, we present results of electron temperature (Te) and density (ne) measurements obtained in Tokamak Chauffage Alfven Bresilien (TCABR) tokamak using visible spectroscopy from CVI line emissions which occurs mainly near the center of the plasma column. The presented method is based on a well-known relationship between the particle flux (Γion) and the photon flux (oion) emitted by an ion species combined with ionizations per photon atomic data provided by the atomic data and analysis structure (ADAS) database. In the experiment, we measured the photon fluxes of three different CVI spectral line emissions, 4685.2, 5290.5, and 6200.6 A (one line per shot). Using this method it was possible to find out the temporal evolution of Te and ne in the plasma. The results achieved are in good agreement with Te and ne measurements made using other diagnostic tools.

Go protein subunit Goα and the secretory process of the natriuretic peptide hormones ANF and BNP.

Expression of the G protein subunit Goα has been shown to be prominent in the atria of the rat heart and to be significantly associated with atrial natriuretic factor (ANF)-containing atrial-specific secretory granules by immunocytochemistry. In addition, differential expression profile analysis using oligonucleotide arrays has shown that the Goα isoform 1 (Goα1) is 2.3-fold more abundant in the atria than it is in the ventricles. In the present report, we show protein-protein interaction between Goα and ANF by yeast two-hybrid and by immunoprecipitation. A cardiac conditional Goα knockout model developed for the present study showed a 90% decrease in Goα expression and decreased atrial expression and ANF and brain natriuretic peptides (BNP) content. Expression of chromogranin A, a specific atrial granule core constituent, was not affected. Morphometric assessment of atrial tissue showed a very significant decrease in atrial-specific granule density as well as granule core electron density. Atrial electrical activity was not affected. The results obtained are compatible with the suggestion that Goα plays a role in ANF sorting during intracellular vectorial transport and with the presence of a mechanism that preserves the molar relationship between cellular ANF and BNP stores in the face of the decreased production of these hormones.

Powder X-ray Diffraction Electron Density of Cubic Boron Nitride

Conventionally, the core electron density (ED) of atoms in molecules is considered to be virtually unperturbed by chemical bonding effects. Here we report a combined experimental and theoretical investigation of the ED of cubic boron nitride including a detailed modeling of the core ED. By modeling structure factors obtained from very-high-resolution synchrotron powder X-ray diffraction data, it is possible to model not only the valence ED but also the response of the core ED to the effects of chemical bonding. The biggest challenge when studying the core ED is the deconvolution of the thermal motion from the experimental structure factors, since the thermal motion is strongly correlated to core ED deformation. However, atomic displacement parameters could be estimated from a full pattern Rietveld-multipolar refinement, and they are shown to be in good correspondence with ab initio lattice dynamics calculations. The corresponding extended multipole model including both core and valence ED refinement sugge...

Core electron temperature and density in the innermost Saturn's magnetosphere from HF power spectra analysis on Cassini

We analyze the large‐scale structures of electrons in Saturn's inner magnetosphere equatorial plane, from 2.8 to about 10 Saturnian radii (RS). The electron total density and core temperature are obtained using the quasi‐thermal noise spectroscopy method, based on the HF power spectra measurements acquired with the Cassini/Radio and Plasma Wave Science instrument around the local plasma frequency from July 2004 to May 2012. The results reveal the existence of two regions. An inner region around Enceladus orbit (3–5 RS) is characterized by a high variability of the electron density, an increasing core temperature profile (∝ R2.7), and a strong correlation of the density and temperature. An outer region, beyond 5 RS, is characterized by a decrease of the density (∝ R−4.19) and a slight decrease of the core temperature with distance from the planet (∝ R−0.3). The electron temperature profile is consistent with heating by thermalization of the electrons with the ions in the inner region and with thermalization balanced by cooling due to outward radial transport in the outer region. In the outer region, we identify a local time asymmetry of both the density and the temperature: higher core temperatures are observed in the nightside while higher densities are observed in the dayside. We finally estimate the plasma scale height from a few orbits at quasi‐constant altitude. We find that it varies as R1.53 inside Dione's orbit and displays a bell‐shaped profile between 7 and 9 RS, consistent with the maximum in the corotation lag previously observed.

Strain engineering enhancement of surface plasmon polariton propagation lengths for gold nanowires

We present in this work the notion of using elastic strain engineering to reduce the intrinsic losses in a metal for subwavelength optical signal processing. By using a simple, analytical waveguide model, we demonstrate that application of uniaxial tensile strains below the yield strain of gold nanowires results in substantial increases of more than 70% in the surface plasmon polariton propagation lengths at optical frequencies. The enhancement is primarily due to a reduction in the core electron density, and is found to be size-independent for a wide range of nanowire diameters, while exhibiting a linear dependence on the applied tensile strain.

Edge and core Thomson scattering systems and their calibration on the ASDEX Upgrade tokamak

A new 10 channel Thomson scattering (TS) system was installed on the ASDEX Upgrade tokamak to measure radial profiles of electron density and temperature at the plasma edge with high radial resolution. Together with the already existing TS system, which is now used for the core plasma, electron density and temperature profiles extending from the edge to the core are now obtained in a single discharge. The TS systems are relatively calibrated by an optical parametric oscillator.

Subshell Fitting of Relativistic Atomic Core Electron Densities for Use in QTAIM Analyses of ECP-Based Wave Functions

Scalar-relativistic, all-electron density functional theory (DFT) calculations were done for free, neutral atoms of all elements of the periodic table using the universal Gaussian basis set. Each core, closed-subshell contribution to a total atomic electron density distribution was separately fitted to a spherical electron density function: a linear combination of s-type Gaussian functions. The resulting core subshell electron densities are useful for systematically and compactly approximating total core electron densities of atoms in molecules, for any atomic core defined in terms of closed subshells. When used to augment the electron density from a wave function based on a calculation using effective core potentials (ECPs) in the Hamiltonian, the atomic core electron densities are sufficient to restore the otherwise-absent electron density maxima at the nuclear positions and eliminate spurious critical points in the neighborhood of the atom, thus enabling quantum theory of atoms in molecules (QTAIM) analyses to be done in the neighborhoods of atoms for which ECPs were used. Comparison of results from QTAIM analyses with all-electron, relativistic and nonrelativistic molecular wave functions validates the use of the atomic core electron densities for augmenting electron densities from ECP-based wave functions. For an atom in a molecule for which a small-core or medium-core ECPs is used, simply representing the core using a simplistic, tightly localized electron density function is actually sufficient to obtain a correct electron density topology and perform QTAIM analyses to obtain at least semiquantitatively meaningful results, but this is often not true when a large-core ECP is used. Comparison of QTAIM results from augmenting ECP-based molecular wave functions with the realistic atomic core electron densities presented here versus augmenting with the limiting case of tight core densities may be useful for diagnosing the reliability of large-core ECP models in particular cases. For molecules containing atoms of any elements of the periodic table, the production of extended wave function files that include the appropriate atomic core densities for ECP-based calculations, and the use of these wave functions for QTAIM analyses, has been automated.

Core-electron tunneling in diatomics interacting with intense ultrashort-pulsed XUV and X-ray radiation: Theoretical studies

New X-ray free-electron and HHG lasers producing ultra-short pulses of intense XUV/X-ray radiation present a unique opportunity for developing novel techniques which would allow to trace the time evolution of the electronic density in molecular systems and identify signatures of core-electron transitions during the probe pulse. The intensity of XFEL emission is sufficient to influence the field-induced bound-state tunneling of core hole states generated by one-photon ionization. Since molecular imaging experiments at atomic resolution are sensitive to the core-electron density in the target, any density modification has potential implications for the single-shot imaging experiments utilizing femtosecond X-ray pulses. In this work, we discuss the effects of field-induced core-hole transport on X-ray scattering properties in molecular systems. As an example, we consider inter-well tunneling of a core electronic density through the Coulomb barrier between nuclei in a single-electron dicarbon ion under influence of an intense XUV laser field. We employ a simple numerical two-state model which is further corroborated by a numerical solution of the time-dependent three-dimensional Schrödinger equation. Our calculations show pronounced coherent suppression of core-hole delocalization dynamics by very intense XUV laser fields. The laser field parameters determining core-hole tunneling times are scalable for the higher intensity/shorter wavelength regimes. Finally, we discuss the implications of this study for the reconstruction of molecular structures by analysis of scattering data in single-shot XFEL experiments.

Analysis of Ar line spectra from indirectly-driven implosion experiments on SGII facility

This paper reports on the indirectly-driven implosion experiments on SGII laser facility in which Ar emission spectrum from Ar-doped D-filled plastic capsule is recorded with the crystal spectrometer. Spectral features of Ar Heβ line and its associated satellites are analysed to extract the electron temperature and density of the implosion core. Non local thermal equilibrium (NLTE) collisional-radiative atomic kinetics and Strark broadening line shape are included in the present calculation. By comparing the calculated spectrum with the measured one, the core electron temperature and density are inferred to be 700 eV and 2.5 × 1023 cm−3 respectively. With these inferred values of electron temperature and density, neutron yield can be estimated to agree with the measured value in magnitude despite of the very simple model used for the estimation.