Imaging atomic structure and dynamics with ultrafast x-ray scattering

Measuring atomic-resolution images of materials with x-ray photons during chemical reactions or physical transformations resides at the technological forefront of x-ray science. New x-ray-based experimental capabilities have been closely linked with advances in x-ray sources, a trend that will continue with the impending arrival of x-ray-free electron lasers driven by electron accelerators. We discuss recent advances in ultrafast x-ray science and coherent imaging made possible by linear-accelerator-based light sources. These studies highlight the promise of ultrafast x-ray lasers, as well as the technical challenges and potential range of applications that will accompany these transformative x-ray light sources.     

Imaging and manipulating molecules on a zeolite surface with an atomic force microscope

The adsorption of neutral molecules and ions on the surfaces of zeolites was observed in real time with an atomic force microscope (AFM). Direct imaging of the surface of the zeolite clinoptilolite was possible by using a diluted tert-butyl ammonium chloride solution as a medium. Images of the crystal in different liquids revealed that molecules could be bound to the surface in different ways; neutral molecules of tert-butanol formed an ordered array, whereas tert-butyl ammonium ions formed clusters. These absorbed molecules were not rearranged by the AFM tip when used in an imaging mode. However, when a sufficiently large force was applied, the tip of the AFM could rearrange the tert-butyl ammonium ions on the zeolite surface. This demonstration of molecular manipulation suggests new applications, including biosensors and lithography.     

Imaging the Mott insulator shells by using atomic clock shifts

Microwave spectroscopy was used to probe the superfluid-Mott insulator transition of a Bose-Einstein condensate in a three-dimensional optical lattice. By using density-dependent transition frequency shifts, we were able to spectroscopically distinguish sites with different occupation numbers and to directly image sites with occupation numbers from one to five, revealing the shell structure of the Mott insulator phase. We used this spectroscopy to determine the onsite interaction and lifetime for individual shells.     

火焰原子吸收光谱法测定水样中铜含量的不确定度评定

测量不确定度定义为表征合理地赋予被测量之值的分散性,是对测量结果质量的定量表征。测量结果的可用性很大程度上取决于其不确定度的大小,通过采用火焰原子吸收光谱法测定水样中铜的不确定度各项来源和评定方法的分析,建立一种分析实验室不确定度的评定方法,使实验结果更具客观性和准确性。     

Imaging phonon excitation with atomic resolution

Inelastic electron tunneling spectroscopy at low temperatures was used to investigate vibrations of Au(111) and Cu(111). The low-energy peaks at 9 millielectron volts (meV) on Au(111) and 21 meV on Cu(111) are attributed to phonons at surfaces. On Au(111), the phonon energy is not influenced by the different stacking of the surface atoms, but it is considerably influenced by different atomic distances within the surface layer. The spatial variation of the phonon excitation is measured in inelastic electron tunneling maps on Au(111), which display atomic resolution. This atomic resolution is explained in terms of site-specific phonon excitation probabilities.     

Imaging the surface of Altair

Spatially resolving the surfaces of nearby stars promises to advance our knowledge of stellar physics. Using optical long-baseline interferometry, we constructed a near-infrared image of the rapidly rotating hot star Altair with a resolution of <1 milliarcsecond. The image clearly reveals the strong effect of gravity darkening on the highly distorted stellar photosphere. Standard models for a uniformly rotating star cannot explain our findings, which appear to result from differential rotation, alternative gravity-darkening laws, or both.     

Toward protein tertiary structure recognition by means of associative memory hamiltonians

The statistical mechanics of associative memories and spin glasses suggests ways to design Hamiltonians for protein folding. An associative memory Hamiltonian based on hydrophobicity patterns is shown to have a large capacity for recall and to be capable of recognizing tertiary structure for moderately variant sequences.     

Confinement of electrons to quantum corrals on a metal surface

A method for confining electrons to artificial structures at the nanometer lengthscale is presented. Surface state electrons on a copper(111) surface were confined to closed structures (corrals) defined by barriers built from iron adatoms. The barriers were assembled by individually positioning iron adatoms with the tip of a 4-kelvin scanning tunneling microscope (STM). A circular corral of radius 71.3 A was constructed in this way out of 48 iron adatoms. Tunneling spectroscopy performed inside of the corral revealed a series of discrete resonances, providing evidence for size quantization. STM images show that the corral's interior local density of states is dominated by the eigenstate density expected for an electron trapped in a round two-dimensional box.     

Imaging the pore structure of geomaterials

Laser scanning confocal microscopy can be used to image the pore structure of geologic materials in three dimensions at a resolution of 200 nanometers. The technique involves impregnation of the void space with an epoxy doped with a fluorochrome whose fluorescent wavelength matches the excitation wavelength. Optical sections with a thickness of less than 1 micrometer can be sliced from thick polished sections and combined to produce three-dimensional reconstructions. Application of the technique to rocks with porosities from 1 to 20 percent reveals the geometric complexity of the pore space. The technique can also be applied to other brittle solids such as ceramics.     

Quantum States and atomic structure of silicon surfaces

The electronic and geometric structures of surfaces are closely related to each other. Conventional surface science techniques can study one or the other, but not both at the same time. Recent developments in scanning tunneling microscopy have made it possible to study simultaneously the electronic and geometric structure of Si(111) and Si(001) surfaces. Surface states can be atomically resolved in space and energy; thus the electronic structure of single atoms on surfaces can be studied in detail. The various surface states observed on silicon surfaces are found to derive from different atomic-scale features in the surface geometric structure. Scanning tunneling microscopy has now bridged the gap between electronic and geometric structure, providing a unique opportunity to obtain a better understanding of many surface processes at the atomic level.     

山地果园便携式挖穴机的研制

为解决山地果园人工锄头挖穴劳动强度大、效率低,现有大型挖穴机及手持式汽油挖穴机在山地果园作业适应性不强等生产实际问题,加速我国山地果园挖穴作业机械化进程,在系统分析丘陵山区机械化挖穴作业特点的基础上,借鉴已有挖穴机的优点,设计了一种双立柱便携式挖穴机遥重点分析了该挖穴机的整机结构、工作原理以及关键部件的模块化设计原理。样机采用高强度轻质材料制作。样机测试试验结果表明,该挖穴机拆装方便、作业效率高、出土率达85%袁,挖穴效率为130穴/h袁各穴一致性变异系数为10%,达到了设计指标,能较好地解决现有挖穴机在坡地果园作业时搬移不便、对操作者产生损伤等问题。     

The problem with determining atomic structure at the nanoscale

Emerging complex functional materials often have atomic order limited to the nanoscale. Examples include nanoparticles, species encapsulated in mesoporous hosts, and bulk crystals with intrinsic nanoscale order. The powerful methods that we have for solving the atomic structure of bulk crystals fail for such materials. Currently, no broadly applicable, quantitative, and robust methods exist to replace crystallography at the nanoscale. We provide an overview of various classes of nanostructured materials and review the methods that are currently used to study their structure. We suggest that successful solutions to these nanostructure problems will involve interactions among researchers from materials science, physics, chemistry, computer science, and applied mathematics, working within a "complex modeling" paradigm that combines theory and experiment in a self-consistent computational framework.     

The atomic structure of Mengo virus at 3.0 A resolution

The structure of Mengo virus, a representative member of the cardio picornaviruses, is substantially different from the structures of rhino- and polioviruses. The structure of Mengo virus was solved with the use of human rhinovirus 14 as an 8 A resolution structural approximation. Phase information was then extended to 3 A resolution by use of the icosahedral symmetry. This procedure gives promise that many other virus structures also can be determined without the use of the isomorphous replacement technique. Although the organization of the major capsid proteins VP1, VP2, and VP3 of Mengo virus is essentially the same as in rhino- and polioviruses, large insertions and deletions, mostly in VP1, radically alter the surface features. In particular, the putative receptor binding "canyon" of human rhinovirus 14 becomes a deep "pit" in Mengo virus because of polypeptide insertions in VP1 that fill part of the canyon. The minor capsid peptide, VP4, is completely internal in Mengo virus, but its association with the other capsid proteins is substantially different from that in rhino- or poliovirus. However, its carboxyl terminus is located at a position similar to that in human rhinovirus 14 and poliovirus, suggesting the same autocatalytic cleavage of VP0 to VP4 and VP2 takes place during assembly in all these picornaviruses.     

Imaging sites of receptor dephosphorylation by PTP1B on the surface of the endoplasmic reticulum

When bound by extracellular ligands, receptor tyrosine kinases (RTKs) on the cell surface transmit critical signals to the cell interior. Although signal termination is less well understood, protein tyrosine phosphatase-1B (PTP1B) is implicated in the dephosphorylation and inactivation of several RTKs. However, PTP1B resides on the cytoplasmic surface of the endoplasmic reticulum (ER), so how and when it accesses RTKs has been unclear. Using fluorescence resonance energy transfer (FRET) methods, we monitored interactions between the epidermal- and platelet-derived growth factor receptors and PTP1B. PTP1B-catalyzed dephosphorylation required endocytosis of the receptors and occurred at specific sites on the surface of the ER. Most of the RTKs activated at the cell surface showed interaction with PTP1B after internalization, establishing that RTK activation and inactivation are spatially and temporally partitioned within cells.     

Imaging powders with the atomic force microscope: from biominerals to commercial materials

Atomically resolved images of pressed powder samples have been obtained with the atomic force microscope (AFM). The technique was successful in resolving the particle, domain, and atomic structure of pismo clam (Tivela stultorum) and sea urchin (Strongylocentrotus purpuratus) shells and of commercially available calcium carbonate (CaCO(3)) and strontium carbonate (SrCO(3)) powders. Grinding and subsequent pressing of the shells did not destroy the microstructure of these materials. The atomic-resolution imaging capabilities of AFM can be applied to polycrystalline samples by means of pressing powders with a grain size as small as 50 micrometers. These results illustrate that the AFM is a promising tool for material science and the study of biomineralization.     

山地果园便携式挖穴机的挖穴性能试验

为降低山地果园挖穴作业的劳动强度,提高挖穴效率,分析了山地果园便携式挖穴机整机结构和工作原理,并对挖穴机的挖穴性能进行试验,包括土壤坚实度试验、挖穴机振动试验及土壤回填率试验。结果表明,被测土壤的坚实度平均值为4.24 MPa;挖穴机携带直径为10、15、20和25cm钻头挖穴时土壤回填率分别为18.5%、19%、20.75%和22.75%,表明该机性能满足山地果园对挖穴作业孔径及深度的需求;便携式挖穴机工作过程中的平均振动加速度相对于手提式挖穴机减少了24%,且该振动直接传递到机架,不会存在手传振动引起操作者不舒适,表明该挖穴机可减轻作业时的劳动强度,提高安全性,适合长时间作业。试验结果为便携式挖穴机的进一步优化和推广应用提供了依据。     

An atomic seesaw switch formed by tilted asymmetric Sn-Ge dimers on a Ge (001) surface

When tin (Sn) atoms are deposited on a clean germanium (Ge) (001) surface at room temperature, buckled dimers originating from the Sn atoms are formed at the Ge-dimer position. We identified the dimer as a heterogeneous Sn-Ge dimer by reversing its buckling orientation with a scanning tunneling microscope (STM) at 80 kelvin. An atomic seesaw switch was formed for one-dimensional electronic conduction in the Ge dimer-row direction by using the STM to reversibly flip the buckling orientation of the Sn-Ge dimer and to set up standing-wave states.     

Interface structure and atomic bonding characteristics in silicon nitride ceramics

Direct atomic resolution images have been obtained that illustrate how a range of rare-earth atoms bond to the interface between the intergranular phase and the matrix grains in an advanced silicon nitride ceramic. It has been found that each rare-earth atom bonds to the interface at a different location, depending on atom size, electronic configuration, and the presence of oxygen at the interface. This is the key factor to understanding the origin of the mechanical properties in these ceramics and will enable precise tailoring in the future to critically improve the materials' performance in wide-ranging applications.     

Wet electrons at the H2O/TiO2(110) surface

At interfaces of metal oxide and water, partially hydrated or "wet-electron" states represent the lowest energy pathway for electron transfer. We studied the photoinduced electron transfer at the H2O/TiO2(110) interface by means of time-resolved two-photon photoemission spectroscopy and electronic structure theory. At approximately 1-monolayer coverage of water on partially hydroxylated TiO2 surfaces, we found an unoccupied electronic state 2.4 electron volts above the Fermi level. Density functional theory shows this to be a wet-electron state analogous to that reported in water clusters and which is distinct from hydrated electrons observed on water-covered metal surfaces. The decay of electrons from the wet-electron state to the conduction band of TiO2 occurs in 相似文献