Surface charge--induced ordering of the au(111) surface

Synchrotron surface x-ray scattering (SXS) studies have been carried out at the Au(lll)/electrolyte interface to determine the influence of surface charge on the microscopic arrangement of gold surface atoms. At the electrochemical interface, the surface charge density can be continuously varied by controlling the applied potential. The top layer of gold atoms undergoes a reversible phase transition between the (1 x 1) bulk termination and a (23 x radical3) reconstructed phase on changing the electrode potential. In order to differentiate the respective roles of surface charge and adsorbates, studies were carried out in 0.1 M NaF, NaCl, and NaBr solutions. The phase transition occurs at an induced surface charge density of 0.07 +/- 0.02 electron per atom in all three solutions.     

Xenon as a complex ligand: the tetra xenono Gold(II) cation in AuXe(4)2+(Sb(2)F(11)-)(2)

The first metal-xenon compound with direct gold-xenon bonds is achieved by reduction of AuF(3) with elemental xenon. The square planar AuXe(4)2+ cation is established by a single-crystal structure determination, with a gold-xenon bond length of approximately 274 picometers. The bonding between gold and xenon is of the final sigma donor type, resulting in a charge of approximately 0.4 per xenon atom.     

The response of electrons to structural changes

The properties of a molecule are determined by the distribution of its electrons. This distribution can be described by the charge density, which is readily obtained from the wave functions derived by ab initio molecular orbital calculations. The charge density may be analyzed in a number of different fashions to give information about the effects of substituents, structural changes, and electronic excitation on the properties of molecules; one common procedure makes use of projection density or charge difference plots. Charge density also may be partitioned among atoms, and by numerical integration over appropriate volume elements one may obtain atomic charges, dipoles, kinetic energies, and other properties of the atoms in a molecule. Many chemical phenomena have been analyzed in terms of charge densities.     

Controlling the charge state of individual gold adatoms

The nature and control of individual metal atoms on insulators are of great importance in emerging atomic-scale technologies. Individual gold atoms on an ultrathin insulating sodium chloride film supported by a copper surface exhibit two different charge states, which are stabilized by the large ionic polarizability of the film. The charge state and associated physical and chemical properties such as diffusion can be controlled by adding or removing a single electron to or from the adatom with a scanning tunneling microscope tip. The simple physical mechanism behind the charge bistability in this case suggests that this is a common phenomenon for adsorbates on polar insulating films.     

Controlled atomic doping of a single C60 molecule

We report a method for controllably attaching an arbitrary number of charge dopant atoms directly to a single, isolated molecule. Charge-donating K atoms adsorbed on a silver surface were reversibly attached to a C60 molecule by moving it over K atoms with a scanning tunneling microscope tip. Spectroscopic measurements reveal that each attached K atom donates a constant amount of charge (approximately 0.6 electron charge) to the C60 host, thereby enabling its molecular electronic structure to be precisely and reversibly tuned.     

Charge distribution in the light-atom mineral kernite

A charge density analysis of accurate x-ray data for the mineral kernite Na(2)B(4)O(6)(OH)(2). 3H(2)O indicates that the sodium and boron atoms have partial positive charges of 0.4 to 0.5 unit and 0.4 to 0.7 unit, respectively, whereas the oxygen atoms have negative charges of about 0.4 to 0.5 unit. The best agreement with the intensities and with the experimental scale factor is obtained with contracted molecule-optimized atomic orbitals. Difference density maps based on high-order parameters show more density in B-O than in Na-O bonds, thus supporting the covalent nature of the bonds between boron and oxygen atoms.     

植物原生质体培养中的粘连现象与其质膜电性

植物原生质体在培养中的粘连程度与其质膜所带负电荷的多寡密切关联，较易发生原生质体粘连的作物如胡萝卜、谷子，其原生质体表面负电性一般较弱；相反，不易发生原生质粘连的作物如烟草等，其原生质体表面负电性一般较强。在培养基中外源添加一些可以增强或削弱植物原生质体负电性的物质，能够对原生质体的粘连情况产生明显的影响。     

Formation of Al13I-: evidence for the superhalogen character of Al13

Al13- is a cluster known for the pronounced stability that arises from coincident closures of its geometric and electronic shells. We present experimental evidence for a very stable cluster corresponding to Al13I-. Ab initio calculations show that the cluster features a structurally unperturbed Al13- core and a region of high charge density on the aluminum vertex opposite from the iodine atom. This ionically bound magic cluster can be understood by considering that Al13 has an electronic structure reminiscent of a halogen atom. Comparisons to polyhalides provide a sound explanation for our chemical observations.     

Nanoscale imaging of molecular adsorption

In situ atomic force microscope observations were made of the adsorption of anions (1- or 2-) of the organic diacid 5-benzoyl-4-hydroxy-2-methoxybenzenesulfonic acid from aqueous solution onto the (0001) surface of hydrotalcite (HT), a layered clay. This adsorption process is believed to mimic the ion-exchange reactions that occur within the layers of HT and other layered clays. Atomic force microscope images of the (0001) surfaces of HT, acquired in aqueous solutions, reveal an ordered structure with respect to magnesium and aluminum atoms. In the presence of the anions, atomic force microscopy indicates pH-dependent adsorption onto the formally cationic HT surface. The anion coverage is governed by electroneutrality and steric interactions between the bulky anions within the adsorbed layer, whereas the orientation of the anions with respect to the HT surface is dictated by coulombic interactions and hydrogen bonding between the anion's sulfonate moiety and clay hydroxyl triads. These observations reveal that the reversible adsorption of molecular species can be examined directly by in situ atomic force microscopy, providing details of surface stoichiometry and adlayer symmetry on the local, molecular level.     

Atomistic mechanisms and dynamics of adhesion, nanoindentation, and fracture

Molecular dynamics simulations and atomic force microscopy are used to investigate the atomistic mechanisms of adhesion, contact formation, nanoindentation, separation, and fracture that occur when a nickel tip interacts with a gold surface. The theoretically predicted and experimentally measured hysteresis in the force versus tip-to-sample distance relationship, found upon approach and subsequent separation of the tip from the sample, is related to inelastic deformation of the sample surface characterized by adhesion of gold atoms to the nickel tip and formation of a connective neck of atoms. At small tipsample distances, mechanical instability causes the tip and surface to jump-to-contact, which in turn leads to adhesion-induced wetting of the nickel tip by gold atoms. Subsequent indentation of the substrate results in the onset of plastic deformation of the gold surface. The atomic-scale mechanisms underlying the formation and elongation of a connective neck, which forms upon separation, consist of structural transformations involving elastic and yielding stages.     

Microporous and photoluminescent chalcogenide zeolite analogs

Crystalline semiconducting sulfide and selenide zeolite analogs were synthesized that possess four-connected, three-dimensional tetrahedral networks built from tetravalent (M4+ = Ge4+ or Sn4+, where M = meta) and trivalent (M3+ = Ga3+ or In3+) cations. Microporous materials were obtained in all four combinations of M4+ and M3+, and some of them were thermally stable up to at least 380 degrees C. These materials exhibit framework topologies with pore size ranging from 12 to 24 tetrahedral atoms, high surface area, high framework charge density and ion exchange capacity, and tunable electronic and optical properties.     

Multiphoton ionization of atoms

Studies of multiphoton ionization of atoms have revealed several unexpected characteristics. The confluence of the experimental evidence leads to the hypothesis that the basic character of the atomic response involves highly organized, coherent motions of entire atomic shells. The important regime, for which the radiative field strength is greater than an atomic unit (e/a(2)(0)), can be viewed in approximate correspondence with the physics of fast (approximately 10 MeV per atomic mass unit) atom-atom scattering. This physical picture provides a basis for the expectation that stimulated emission in the x-ray range can be produced by direct, highly nonlinear coupling of ultraviolet radiation to atoms.     

喂入量(初始密度)对压捆机压缩过程的影响

为了研究喂入量（初始密度）对压捆机压缩过程的影响，对苜蓿、羊草，碾过的麦秸在不同喂入量下送行了压缩试验，试验表明喂入量对压缩过程确有影晌。通述分析其压缩压力与压缩量及压缩密度的关系，建立了模型方程，提出了喂入量对压缩过程的影响规律。     

Direct sub-angstrom imaging of a crystal lattice

Despite the use of electrons with wavelengths of just a few picometers, spatial resolution in a transmission electron microscope (TEM) has been limited by spherical aberration to typically around 0.15 nanometer. Individual atomic columns in a crystalline lattice can therefore only be imaged for a few low-order orientations, limiting the range of defects that can be imaged at atomic resolution. The recent development of spherical aberration correctors for transmission electron microscopy allows this limit to be overcome. We present direct images from an aberration-corrected scanning TEM that resolve a lattice in which the atomic columns are separated by less than 0.1 nanometer.     

Complex patterning by vertical interchange atom manipulation using atomic force microscopy

The ability to incorporate individual atoms in a surface following predetermined arrangements may bring future atom-based technological enterprises closer to reality. Here, we report the assembling of complex atomic patterns at room temperature by the vertical interchange of atoms between the tip apex of an atomic force microscope and a semiconductor surface. At variance with previous methods, these manipulations were produced by exploring the repulsive part of the short-range chemical interaction between the closest tip-surface atoms. By using first-principles calculations, we clarified the basic mechanisms behind the vertical interchange of atoms, characterizing the key atomistic processes involved and estimating the magnitude of the energy barriers between the relevant atomic configurations that leads to these manipulations.     

Diffuse-double layer at a membrane-aqueous interface measured with x-ray standing waves

The ion distribution in an electrolyte solution in contact with a charged polymerized phospholipid membrane was directly measured with long-period x-ray standing waves. The 27-angstrom-thick lipid monolayer was supported on a tungsten/silicon mirror. X-ray standing waves were generated above the mirror surface by total external reflection of a 9.8-kiloelectron volt x-ray beam from a synchrotron undulator. The membrane surface, which contained negatively charged phosphate headgroups, was bathed in a dilute ZnCl2 solution. The concentration of Zn2+ in the condensed layer at the membrane surface and the Zn2+ distribution in the diffuse layer were measured as a function of headgroup charge. The Debye length of the diffuse layer varied between 3 and 58 angstroms. The results qualitatively agree with the Gouy-Chapman-Stern model.     

Real-space imaging of two-dimensional antiferromagnetism on the atomic scale

A two-dimensional antiferromagnetic structure within a pseudomorphic monolayer film of chemically identical manganese atoms on tungsten(110) was observed with atomic resolution by spin-polarized scanning tunneling microscopy at 16 kelvin. A magnetic superstructure changes the translational symmetry of the surface lattice with respect to the chemical unit cell. It is shown, with the aid of first-principles calculations, that as a result of this, spin-polarized tunneling electrons give rise to an image corresponding to the magnetic superstructure and not to the chemical unit cell. These investigations demonstrate a powerful technique for the understanding of complicated magnetic configurations of nanomagnets and thin films engineered from ferromagnetic and antiferromagnetic materials used for magnetoelectronics.     

Discovery of sodium in the atmosphere of mercury

The spectrum of Mercury at the Fraunhofer sodium D lines shows strong emission features that are attributed to resonant scattering of sunlight from sodium vapor in the atmosphere of the planet. The total column abundance of sodium was estimated to be 8.1 x 10(11) atoms per square centimeter, which corresponds to a surface density at the subsolar point of about 1.5 x 10(5) atoms per cubic centimeter. The most abundant atmospheric species found by the Mariner 10 mission to Mercury was helium, with a surface density of 4.5 x 10(3) atoms per cubic centimeter. It now appears that sodium vapor is a major constituent of Mercury's atmosphere.     

SO2在Ni(111)表面分解吸附的理论研究

采用密度泛函理论(DFT)的广义梯度近似(GGA)方法对SO2及其分解片段的吸附结构和性质进行了理论研究.计算结果表明:在Ni(111)表面上SO2以S,O原子顶位平铺吸附最稳定;SO3以C3v轴倾斜表面吸附;SO自由基存在两种可能的吸附结构,一种是平铺吸附于Ni(111)面,另一种是以S端垂直吸附穴位,S原子在穴位为最优吸附位.对各吸附体系的态密度及电子转移分析表明,SOx吸附中均为衬底镍原子向硫的氧化物转移电子,改变了S—O键之间的电子分布,导致其键长变长,这有利于硫氧化物在镍表面的解离反应.S原子在Ni(111)表面的吸附中,Ni原子的s轨道向S原子转移电子,这与O的吸附机理有所不同.     

Contact electrification and adhesion between dissimilar materials

Simultaneous measurements of surface force and surface charge demonstrate strong attraction due to the spontaneous transfer of electrical charge from one smooth insulator (mica) to another (silica) as a result of simple, nonsliding contact in dry nitrogen. The measured surface charge densities are 5 to 20 millicoulombs per square meter after contact. The work required to separate the charged surfaces is typically 6 to 9 joules per square meter, comparable to the fracture energies of ionic-covalent materials. Observation of partial gas discharges when the surfaces are approximately 1 micrometer apart gives valuable insight into the charge separation processes underlying static electrical phenomena in general.