Asymmetric Electron Research Articles

Nonadiabatic dynamics of charge transfer in diatomic anion clusters

We have studied the photodissociation and recombination dynamics of the diatomic anions X(2)(-) and XY(-) designed to mimic I(2)(-) and ICl(-), respectively, by using a one-electron model in size-selected N(2)O clusters. The one-electron model is composed of two nuclei and an extra electron moving in a two-dimensional plane including the two nuclei. The main purpose of this study is to explain the salient features of various dynamical processes of molecular ions in clusters using a simple theoretical model. For heteronuclear diatomic anions, a mass disparity and asymmetric electron affinity between the X and Y atoms lead to different phenomena from the homonuclear case. The XY(-) anion shows efficient recombination for a smaller cluster size due to the effect of collision-mediated energy transfer and an inherent potential wall on excited state at asymptotic region, while the recombination for the X(2)(-) anion is due to rearrangement of solvent configuration and faster nonadiabatic transitions. The results of the present study illustrate the microscopic details of the electronically nonadiabatic processes which control the photodissociation dynamics of molecular ions in clusters.

Polymorphism in Bismuth Stannate: A First-Principles Study

From the wide range of SnIV-based pyrochlores, Bi2Sn2O7 stands out as a material that deviates from the standard cubic-lattice symmetry. At low temperatures, Bi2Sn2O7 adopts a distorted monoclinic √2 × √2 × 2 expansion of the pyrochlore structure (α-phase) and only favors the cubic lattice (γ-phase) above 900 K. In this study, we calculate and examine the electronic structure of both the α- and γ-phases of Bi2Sn2O7 and compare them to the results of two regular pyrochlore materials, La2Sn2O7 and Y2Sn2O7. Our analysis highlights the importance of covalent interactions between the electronic states of the metal with O 2p in Bi2Sn2O7, which are not present in the other oxides. The formation of an asymmetric electron density on Bi is observed as the driving force behind the distorted geometry favored by Bi2Sn2O7.

Coplanar symmetric and asymmetric electron impact ionization studies from the 1b1 state of H2O at low to intermediate impact energies

(e, 2e) ionization differential cross sections are presented for incident electron energies ranging from 15 eV to 95 eV above the ionization threshold of the 1b1 molecular state of H2O. Experimental results and theoretical analysis were derived for three energies in a coplanar symmetric geometry, and for three energies in an asymmetric geometry. The experimental data show a wide variation in the cross section over this range of energies, whereas the theoretical analysis carried out using a sophisticated molecular DWBA model, which includes the final state post collision interaction (PCI), shows best agreement at lower energies. The experimental techniques used to collect the data are described here as well as an improved theoretical approach using elastic scattering cross sections to evaluate the accuracy of the distorted waves utilized in the calculation of the ionization cross sections.

Catalytic, Asymmetric Inverse Electron Demand Hetero Diels-Alder Reactions of o-Benzoquinone Derivatives and Ketene Enolates

We present an array of [4+2] cycloaddition reactions between ketene enolates, catalytically derived from acid chlorides and cinchona alkaloid nucleophilic catalysts in the presence of stoichiometric base, and o-benzoquinone derivatives. These cycloaddition reactions proceed with excellent stereochemical control (up to >99% ee). In fact, for both the o-benzoquinone imide and the o-benzoquinone diimide manifolds, these Diels-Alder reactions occurred with uniformly >99% ee. The wide scope of this methodology provides access to a diverse range of biologically and synthetically useful chiral products, including ?-hydroxyesters, non-natural ?-amino acids, quinoxalinones, and many others, all with remarkable, catalytic control of regio- and stereochemistry.

Calculation of rate constants for asymmetric charge transfer, and their effect on relative sensitivity factors in glow discharge mass spectrometry

For this paper, we have calculated the rate coefficients for asymmetric charge transfer between Ar + ions and all elements of interest in analytical glow discharges, based on a semi-classical approach. These values were then used to make predictions on the relative sensitivity factors (RSFs) in glow discharge mass spectrometry (GDMS) (VG9000 discharge cell) for various elements. The RSFs were calculated based on a transport factor, and an ionization factor, which comprises asymmetric charge transfer, Penning ionization and electron impact ionization. The ionization rates of these three processes were calculated explicitly, based on our earlier computer simulations, in combination with the rate coefficients and cross sections of the ionization processes for different elements. In this way, we are able to offer a rationalization of the experimental RSFs. It is demonstrated that variations in RSFs are largely determined by the occurrence of asymmetric charge transfer in the glow discharge plasma.

Field-induced resistive switching based on space-charge-limited current

Polycrystalline (Ba,Sr)(Zr,Ti)O3 thin films sandwiched between two Pt electrodes have been revealed to exhibit hysteretic current-voltage (I-V) characteristics and resistive switching at room temperature. High- and low-resistance states, as well as a less abrupt state transition, occur during the voltage cycle. The maximum ratio between these two resistance states is about 230. Analyses of I-V behaviors have been executed, and it is proposed that space-charge-limited-current conduction in higher voltage region caused by asymmetric electron trapping centers is responsible for such transition of resistance states.

A new method of asymmetric Abel inversion for magnetic equilibrium configuration state in tokamaks

It is difficult to obtain the asymmetrical factor along the observation direction parallel to the plasma mid-plane when the detected radiation is also in the mid-plane. This paper considers the magnetic surfaces and Grad?Shafranov shift, and develops a new method for inverse asymmetric electron density information, during magnetic equilibrium configuration in a tokamak.

Frequency-Switching Inversion-Recovery for Severely Hyperfine-Shifted NMR: Evidence of Asymmetric Electron Relaxation in High-Spin Co(II)

A new method for reliably measuring longitudinal relaxation rates for severely hyperfine-shifted NMR signals in aqueous solutions is presented. The method is illustrated for a well-defined cobalt tetracysteinate, with relevance to cobalt-substituted metalloproteins. The relaxation measurements are indicative of asymmetric electronic relaxation of the high-spin Co(II) ion.

Plasmon enhanced electron drag and terahertz photoconductance in a grating-gated field-effect transistor with two-dimensional electron channel

The authors present a theory of dc photoresponse in two-dimensional (2D) electron channel in the grating-gated field-effect transistor irradiated by an electromagnetic wave of terahertz frequency. The authors determine photoinduced dc correction to the source-drain voltage and demonstrate that it has resonant peaks when the frequency of an external radiation coincides with 2D plasmon frequencies. The photoresponse is shown to depend on the asymmetric electron drag in the 2D channel with constant bias current. The amplitude of the resonant peaks has nonmonotonic temperature dependence with a maximum at elevated temperatures. The results explain qualitatively some important features of the photoresponse observed in recent experiments.

ENTANGLEMENT AND QUANTUM PHASE TRANSITION IN A ONE-DIMENSIONAL SYSTEM OF QUANTUM DOTS WITH DISORDER

We study the entanglement of formation and quantum phase transition in a one-dimensional quantum dot system with disorder modeled by the Hubbard Hamiltonian. The entanglement for three different cases is studied: the homogeneous case, the impurity case of symmetric electron hopping, and asymmetric electron hopping. We find that the local entanglement of the system can be tuned by introducing different impurities characterized by the physical parameters of the system. In particular, for certain parameters, the entanglement is negligible up to a critical point Uc, where a quantum phase transition occurs, and is different from zero above Uc.

RHIC’s future looks bright

Bertram Schwarzschild’s Issues and Events piece on the National Research Council’s report Revealing the Hidden Nature of Space and Time (Physics Today, June 2006, page 26) states, “Fermilab’s Tevatron is unlikely to outlive the decade. Neither is the PEP-II asymmetric electron–positron collider at SLAC nor the Relativistic Heavy Ion Collider at Brookhaven National Laboratory.”Placing RHIC in this context is odd since the NRC report nowhere mentions it. RHIC is funded by the Office of Nuclear Physics in the US Department of Energy’s Office of Science, not by the Office of High Energy Physics, for which the NRC committee was charged with recommending priorities for the next 15 years.More important, the notion that RHIC is “unlikely to outlive the decade” is misbegotten. The scientific impact of RHIC has been outstanding; its discovery of the “perfect liquid” of quarks and gluons was named the number-one physics story of 2005 by the American Institute of Physics publication Physics News Update and garnered media coverage around the world.Brookhaven National Laboratory is currently working with the Office of Nuclear Physics to implement for RHIC a strategy for the period 2006–11 aimed at a 10-fold luminosity upgrade and detector upgrades. This strategy will place RHIC at the forefront of research in high-temperature quantum chromodynamics (QCD) for at least another 10 years. Furthermore, RHIC is the first and only hadron collider with the ability to accelerate, store, and collide polarized protons at energies up to 500 GeV in the center-of-mass frame. It therefore provides unique opportunities to study the spin content of the nucleon—a program that also will extend into the next decade.Beyond that is the prospect of using RHIC as the basis for a polarized electron-ion collider, an option for an international next-generation facility for the study of QCD. That option will be discussed by the Nuclear Science Advisory Committee in 2007 as it develops its long-range plan for the field. If longevity is based on compelling science to be done, such a QCD facility—with ion–ion, proton–ion, polarized proton–proton, polarized electron–proton, and electron–ion collisions at high energy—would likely outlive the next decade.© 2006 American Institute of Physics.

Belle SVD2 vertex detector

A new silicon vertex detector for the Belle experiment has been in operation at the high-luminosity asymmetric energy electron–positron collider KEKB since October 2003. It provides a larger polar angle acceptance, a better vertex resolution and a higher radiation tolerance than the previous one. The obtained performances indicate that the SVD2 works reliably, in very good agreement with expectations.

Efficient synthesis of brominated tetrathiafulvalene (TTF) derivatives: solid-state structure and electrochemical behaviour

An efficient synthesis is reported for 4,5-dibromo-[1,3]dithiole-2-thione ( 1) and 4-bromo-1,3-dithiole-2-thione ( 7) by bromination of lithiated vinylene trithiocarbonate. Compound 1 acts as a convenient precursor to a number of asymmetric electron donors. This is exemplified by the formation of 4,5-dibromo-4′,5′-bis(2′-cyanoethylsulfanyl)TTF ( 3) by cross-coupling methodology and subsequent conversion into 4,5-dibromo-4′,5′-ethylenedithioTTF ( 4) by reaction with caesium hydroxide and 1,2-dibromoethane. The new donor 4,5-dibromo-4′,5′-ethylenedithiodiselenadithiafulvalene ( 5) was prepared by cross-coupling of 1 and 4,5-ethylenedithio-1,3-diselenol-2-one ( 6). The X-ray structures of 3 and 5 are reported.

Divinylphenylene-Bridged Diruthenium Complexes Bearing Ru(CO)Cl(PiPr3)2Entities

The divinylphenylene-bridged diruthenium complexes (E,E)-[{(PiPr3)2(CO)ClRu}2(μ-HCCHC6H4CHCH-1,3)] (m-2) and (E,E)-[{(PiPr3)2(CO)ClRu}2(μ-HCCHC6H4CHCH-1,4)] (p-2) have been prepared and compared to their PPh3-containing analogues m-1 and p-1. The higher electron density at the metal atoms increases the contribution of the metal end groups to the bridge-dominated occupied frontier orbitals and stabilizes the various oxidized forms with respect to those of m-1 and p-1. This has been confirmed and quantified electrochemically, because the two reversible oxidation waves were observed at considerably lower potentials than for the PPh3 complexes. Owing to their greater stability, the one- and two-electron-oxidized forms m-2n+ and p-2n+ of both complexes could be generated and spectroscopically characterized inside an optically transparent thin layer electrolysis cell. UV/vis/near-IR and ESR spectroelectrochemistry indicates that the oxidation processes are centered at the organic bridging ligand. σ-Bonded divinylphenylenes thus constitute an unusual class of “noninnocent” ligands for organometallic compounds. Electronic transitions observed for the mono- and dioxidized forms closely resemble those of donor-substituted phenylenevinylene compounds, including oligo(phenylenevinylenes) (OPVs) and poly(phenylenevinylene) (PPV) in the respective oxidation states. Strong ESR signals and nearly isotropic g tensors are observed for the monocations in fluid and frozen solutions. The metal contribution to the redox orbitals is illustrated by a shift of the CO stretching bands to notably higher energies upon stepwise oxidation. The shifts strongly exceed those observed for the PPh3 containing, six-coordinated species (E,E)-[{(PPh3)2(CO)Cl(L)Ru}2(μ-HCCHC6H4CHCH)]n+ (L = substituted pyridine). IR spectroelectrochemistry reveals the presence of two electronically different transition-metal moieties in m-2+, while they resemble each other more closely in p-2+. Differences in electronic coupling are illustrated by the charge distribution parameters calculated from the spectra. Bulk electrolysis experiments confirm the results from the in situ spectroelectrochemistry and the overall stoichiometry of the redox processes. Quantum-chemical calculations were performed in order to provide insight into the nature and composition of the frontier orbitals. The electronic transitions observed for the neutral forms were assigned by TD DFT. IR frequencies calculated for m-2 and p-2 in their various oxidation states retrace the experimental observations. They fail, however, in the case of m-2+, where a symmetrical structure is calculated, as opposed to the distinctly asymmetric electron distribution observed by IR spectroscopy. Geometry-optimized structures were calculated for all accessible oxidation states. The structural changes following stepwise oxidation agree well with the experimental findings: e.g., a successive low-energy shift of the CC stretching vibration of the bridge. The radical cation m-2+ displays a broad composite electronic absorption band at low energy that extends into the mid-IR region.

Tetrahedral structure or chains for liquid water.

It has been suggested, based on x-ray absorption spectroscopy (XAS) experiments on liquid water [Wernet, Ph., et al. (2004) Science 304, 995-999], that a condensed-phase water molecule's asymmetric electron density results in only two hydrogen bonds per water molecule on average. The larger implication of the XAS interpretation is that the conventional view of liquid water being a tetrahedrally coordinated random network is now replaced by a structural organization that instead strongly favors hydrogen-bonded water chains or large rings embedded in a weakly hydrogen-bonded disordered network. This work reports that the asymmetry of the hydrogen density exhibited in the XAS experiments agrees with reported x-ray scattering structure factors and intensities for Q > 6.5 A(-1). However, the assumption that the asymmetry in the hydrogen electron density does not fluctuate and is persistent in all local molecular liquid water environments is inconsistent with longer-ranged tetrahedral network signatures present in experimental x-ray scattering intensity and structure factor data for Q < 6.5 A(-1).

Experimental observation of the spin-Hall effect in a spin–orbit coupled two-dimensional hole gas

Electrically induced ordering and manipulation of electron spins in semiconductors has a number of practical advantages over the established techniques using circularly polarized light sources, external magnetic fields and spin injection from a ferromagnet. The spin-Hall effect utilizes spin–orbit coupling to induce edge spin accumulation in response to a longitudinal electric field which can be applied locally and lead to low energy consumption devices. We study spin accumulation near the edge of a weakly disordered two-dimensional hole gas (2DHG) in a GaAs/AlGaAs heterostructure where the magnitude of the transverse spin current approaches the intrinsic, disorder independent value, in contrast to the impurity dominated regime observed in 3D electron doped systems. In our experiment, the induced spin polarization is detected by the electroluminescence resulting from two p–n junctions bordering the 2DHG channel. When an electric field is applied across the 2DHG channel, a non-zero out-of-plane component of the spin is optically detected. The sign of the spin depends on the direction of the field and is opposite for the two edges, consistent with theory predictions. We also report and analyze an in-plane spin-polarization effect induced in the device by asymmetric electron–hole recombination.

Spontaneous Formation of Closed-Field Torus Equilibrium via Current Jump Observed in an Electron-Cyclotron-Heated Plasma

Spontaneous current jump resulting in the formation of closed field equilibrium has been observed in electron-cyclotron-heated toroidal plasmas under steady external fields composed of a toroidal field and a relatively weak vertical field in the low aspect ratio torus experiment device. This bridges the gap between the open field equilibrium maintained by a pressure-driven current in the external field and the closed field equilibrium at a larger current. Experimental results and theoretical analyses suggest a current jump model that is based on the asymmetric electron confinement along the field line appearing upon simultaneous transitions of field topology and equilibrium.

Field‐aligned electrons at the lobe/plasma sheet boundary in the mid‐to‐distant magnetotail and their association with reconnection

We have surveyed field‐aligned electrons at the lobe/plasma sheet boundary and their association with reconnection in the distant magnetotail where reconnection is quasi‐steady and large scale. Asymmetric (in energy) counterstreaming electrons are detected in ∼30% of the boundary crossings when high‐speed flows are present in the plasma sheet. In 98% of the electron beam cases the low‐energy electrons are directed toward and the higher‐energy electrons directed away from the X‐line. The well‐organized (by the signs of Bx and Vx) quadrupolar pattern of the electrons indicates that these electrons are associated with reconnection. The low‐energy electrons could be the outer part of the Hall current loop, similar to previous reports in the near‐Earth region. The mean electron energy in the distant tail is a factor of ten lower than in the near‐Earth tail. Our observations suggest that the Hall effect can be detected even at large distances from the diffusion region.

Spin-polarized electron transport through nanometer-scale Al grains

We investigate spin-polarized electron tunneling through ensembles of nanometer-scale Al grains embedded between two Co reservoirs at 4.2 K, and observe tunneling-magnetoresistance (TMR) and effects from spin precession in the perpendicular applied magnetic field (the Hanle effect). The spin-coherence time $({T}_{2}^{\ensuremath{\star}})$ measured using the Hanle effect is of order ns. The dephasing is attributed to electron spin precession in local magnetic fields. Dephasing process does not destroy TMR, which is strongly asymmetric with bias voltage. The asymmetric TMR is explained by spin relaxation in the Al grains and asymmetric electron dwell times.

The origin of the stereochemically active Pb(II) lone pair: DFT calculations on PbO and PbS

The concept of a chemically inert but stereochemically active 6 s 2 lone pair is commonly associated with Pb(II). We have performed density functional theory calculations on PbO and PbS in both the rocksalt and litharge structures which show anion dependence of the stereochemically active lone pair. PbO is more stable in litharge while PbS is not, and adopts the symmetric rocksalt structure showing no lone pair activity. Analysis of the electron density, density of states and crystal orbital overlap populations shows that the asymmetric electron density formed by Pb(II) is a direct result of anion–cation interactions. The formation has a strong dependence on the electronic states of the anion and while oxygen has the states required for interaction with Pb 6 s, sulphur does not. This explains for the first time why PbO forms distorted structures and possesses an asymmetric density and PbS forms symmetric structures with no lone pair activity. This analysis shows that distorted Pb(II) structures are not the result of chemically inert, sterically active lone pairs, but instead result from asymmetric electron densities that rely on direct electronic interaction with the coordinated anions.