Covalent Group IV Atomic Clusters

Atomic clusters containing from two to several hundred atoms offer the possibility of studying the transition from molecules to crystalline solids. The covalent group IV elements carbon, silicon, and germanium are now being examined with this long-range objective. These elements are particularly interesting because of the very different character of their crystalline solids and because they are intermediate between metals and insulators in the nature of their bonding. Small mass-selected atom cluster ions are formed by pulsed laser techniques and identified by time-of-flight methods. Laser photoexcitation is used to study the relative stability of these clusters and their modes of fragmentation. These modes for C(n)(+) clusters, which tend to fragment with a characteristic loss of a neutral C(3), are found to be different from the modes for Si(n)(+) and Ge(n)(+) clusters, which tend to fragment to "magic" clusters such as Si(4)(+), Si(6)(+) and Si(10)(+). These experimental results can be accounted for by recent theoretical calculations of the ground-state structure and stability of small silicon and carbon clusters. Several theoretical approaches give consistent results, showing that small silicon clusters are compact and different from small fragments of the bulk crystal. Calculations show that carbon clusters change from linear structures toward cyclic structures as the cluster size increases, but with significant odd-even differences.     

Gravitational lens optics

Several instances of multiple imaging of cosmologically distant sources by intervening galaxies and galaxy clusters have been discovered over the past decade. These "gravitational lenses" have distinctive optical properties. Pointlike sources such as quasars generally produce two or four images when lensed, whereas extended sources such as galaxies produce spectacular arcs and rings. The salient features of most of the observations can be reproduced with the use of simple elliptical lens models that approximate the lenses made by ellipsoidal mass distributions such as are common in the universe. In addition to illustrating simple optics in operation on a cosmological scale, multiple images and arcs provide useful probes of the lensing galaxies and clusters. Also, gravitational lenses can make magnified images of cosmologically distant sources and may eventually furnish important cosmographic data such as the Hubble constant.     

Crystal structure of a carbon monoxide dehydrogenase reveals a [Ni-4Fe-5S] cluster

The homodimeric nickel-containing CO dehydrogenase from the anaerobic bacterium Carboxydothermus hydrogenoformans catalyzes the oxidation of CO to CO2. A crystal structure of the reduced enzyme has been solved at 1.6 angstrom resolution. This structure represents the prototype for Ni-containing CO dehydrogenases from anaerobic bacteria and archaea. It contains five metal clusters of which clusters B, B', and a subunit-bridging, surface-exposed cluster D are cubane-type [4Fe-4S] clusters. The active-site clusters C and C' are novel, asymmetric [Ni-4Fe-5S] clusters. Their integral Ni ion, which is the likely site of CO oxidation, is coordinated by four sulfur ligands with square planar geometry.     

Spontaneous ordering of oxide nanostructures

We report the spontaneous formation of uniformly distributed arrays of "tips" (tall conical hillocks) upon oxidation of palladium (Pd) thin films. The formation of the palladium oxide tips depended on the thickness and granularity of the Pd film and on annealing and oxidation conditions. As the Pd film thickness increased from 40 to 200 nanometers, the average height of the tips increased from 0.5 to 1.2 micrometers, their height distribution became broader, and their density decreased from 55 x 10(6) to 12 x 10(6) per square centimeter. Enhanced photoelectron emission from locations corresponding to the tips suggests their possible use in field emission applications.     

Identification of active gold nanoclusters on iron oxide supports for CO oxidation

Gold nanocrystals absorbed on metal oxides have exceptional properties in oxidation catalysis, including the oxidation of carbon monoxide at ambient temperatures, but the identification of the active catalytic gold species among the many present on real catalysts is challenging. We have used aberration-corrected scanning transmission electron microscopy to analyze several iron oxide-supported catalyst samples, ranging from those with little or no activity to others with high activities. High catalytic activity for carbon monoxide oxidation is correlated with the presence of bilayer clusters that are approximately 0.5 nanometer in diameter and contain only approximately 10 gold atoms. The activity of these bilayer clusters is consistent with that demonstrated previously with the use of model catalyst systems.     

Infrared signature of structures associated with the H+(H2O)n (n = 6 to 27) clusters

We report the OH stretching vibrational spectra of size-selected H+(H2O)n clusters through the region of the pronounced "magic number" at n = 21 in the cluster distribution. Sharp features are observed in the spectra and assigned to excitation of the dangling OH groups throughout the size range 6 相似文献   

Stormy weather in galaxy clusters

Recent x-ray, optical, and radio observations coupled with particle and gas dynamics numerical simulations reveal an unexpectedly complex environment within clusters of galaxies, driven by ongoing accretion of matter from large-scale supercluster filaments. Mergers between clusters and continuous infall of dark matter and baryons from the cluster periphery produce long-lived "stormy weather" within the gaseous cluster atmosphere-shocks, turbulence, and winds of more than 1000 kilometers per second. This weather may be responsible for shaping a rich variety of extended radio sources, which in turn act as "barometers" and "anemometers" of cluster weather.     

巨峰葡萄开花前至落果期~(14)C—光合产物的运转分配及与落花落果的关系

在巨峰葡萄开花前,盛花期和落果期．将~(14)CO_2导入不摘心、摘心及花穗用GA_350mL／L,GA_350mL／L＋BA25mL／L处理结果枝花（果）穗的邻近叶中,然后测定新消各部位~(14)C—光合产物的放射性强度。结果表明：1．开花前和盛花期导入~(14)CO_2的两处理,表现出花穗邻近叶的~(14)C—光合产物的运输方向与落果期不同、开花前和盛花期~(14)C—光合产物主要供给花序生长发育,供给枝梢先端极少。而在落果高峰期则大量供应枝梢生长,仅少量供应果穗。2、初花期果枝摘心在落果期能显著促进~(14)C—光合产物运向果穗,同时表现营养生长受抑。3．盛花期用GA_350mL／L、GA_350mL／L＋BA25mL／L处理花穗,可促使~(14)C—光合产物大量运向果穗。     

The structure of catalytically active gold on titania

The high catalytic activity of gold clusters on oxides has been attributed to structural effects (including particle thickness and shape and metal oxidation state), as well as to support effects. We have created well-ordered gold mono-layers and bilayers that completely wet (cover) the oxide support, thus eliminating particle shape and direct support effects. High-resolution electron energy loss spectroscopy and carbon monoxide adsorption confirm that the gold atoms are bonded to titanium atoms. Kinetic measurements for the catalytic oxidation of carbon monoxide show that the gold bilayer structure is significantly more active (by more than an order of magnitude) than the monolayer.     

三峡库区产业集群发展前景分析与优化方向探讨

产业集群作为处于特定地理区域中的一种新经济现象,对地方的经济发展起着很大的促进作用。三峡库区产业集群在发挥优势的同时,应该注意一些发展中的负面影响。作为提升区域经济竞争力的重要途径,三峡库区产业集群的发展与优化方向应朝着产业集群的生态化、产业集群的品牌化、产业集群的升级、产业集群的融合化等四个方面发展。     

Halictine social evolution: the Australian enigma

Australian halictines belong to the primitive genus Lasioglossum or related subgenera. The underground nests have lined cells in series or clusters and sometimes at the end of laterals. Two full generations per year are produced in the communal nests. Overwintered and newly emerged females form unique "pseudosocieties" rather than matrifilial societies along Holarctic patterns. Several Chilalictus species produce a "male caste" of big-headed, flightless males, in addition to normal individuals. Oviposition of unfertilized eggs on large pollen balls causes such allometric bees.     

Charging effects on bonding and catalyzed oxidation of CO on Au8 clusters on MgO

Gold octamers (Au8) bound to oxygen-vacancy F-center defects on Mg(001) are the smallest clusters to catalyze the low-temperature oxidation of CO to CO2, whereas clusters deposited on close-to-perfect magnesia surfaces remain chemically inert. Charging of the supported clusters plays a key role in promoting their chemical activity. Infrared measurements of the stretch vibration of CO adsorbed on mass-selected gold octamers soft-landed on MgO(001) with coadsorbed O2 show a red shift on an F-center-rich surface with respect to the perfect surface. The experiments agree with quantum ab initio calculations that predict that a red shift of the C-O vibration should arise via electron back-donation to the CO antibonding orbital.     

The origin of galaxies and clusters of galaxies

Debate on how galaxies and clusters of galaxies formed has reached an interesting stage at which one can find arguments for quite different scenarios. The galaxy distribution has a complex "frothy" character that could be the fossil of a network of protoclusters or pancakes that produced galaxies. However, there are galaxies like our own that seem never to have been in a protocluster but are physically similar to the galaxies in dense clusters. Some clues to be assessed in resolving this dilemma are the possible existence of galaxy filaments, the relative ages of galaxies and clusters of galaxies, and the continuity between cluster and field galaxies and between galaxies and clusters of galaxies.     

Chemists cluster in chicago to confer on cagey compounds

Shrink a clump of matter to a few hundred atoms, and it enters a curious netherworld. Neither ordinary solids nor conventional small molecules, such atomic clusters have unique chemical, optical, and electronic properties. Since the mid-1970s, when advances in synthesizing and analyzing clusters first enabled researchers to explore this netherworld in earnest, clusters have nucleated a sizable field. Many of its participants gathered in Chicago on 15 to 22 September for the Sixth International Symposium on Small Particles and Inorganic Clusters, the first of the series to be held in the United States. At the top of the agenda for the 300 attendees were the field's old stalwarts, various kinds of metal clusters; its current darlings, the carbon clusters known as fullerenes; and a possible future superstar, silicon clusters.     

Quantum confinement and host/guest chemistry: probing a new dimension

Nanoparticulate metals and semiconductors that have atomic arrangements at the interface of molecular clusters and "infinite" solid-state arrays of atoms have distinctive properties determined by the extent of confinement of highly delocalized valence electrons. At this interface, the total number of atoms and the geometrical disposition of each atom can be used to significantly modify the electronic and photonic response of the medium. In addition to teh novel inherent physical properties of the quantum-confined moieties, their "packaging" into nanocomposite bulk materials can be used to define the confinement surface states and environment, intercluster interactions, the quantum-confinement geometry, and the effective charge-carrier density of the bulk. Current approaches for generating nanostructures of conducting materials are briefly reviewed, especially the use of three-dimensional crystalline superlattices as hosts for quantum-confined semiconductor atom arrays (such as quantum wires and dots) with controlled inter-quantum-structure tunneling.     

乌龙茶制造与品质形成的化学机理

乌龙茶制造与品质化学涉及制造原料理化性状、制造过程化学变化和成茶品质化学基础 3方面内容 .从原料、制造到成茶 ,贯穿其中的重要理论与技术原理是“控制”,即 :(1 )选择适当茶树品种和一定成熟度的新梢以限制原料中部分化学成分特别是多酚类物质的含量和比例 ;(2 )将多酚类物质制茶氧化控制在鲜叶局部和一定变化范围内进行 ;(3 )控制“做青”进程以促进内含物转移、转化 ;(4 )防止成茶色泽成分比例失调     

珠江口海岸带沉积物氨氧化细菌和古菌组成及定量研究

[目的]对珠江口海岸带沉积物中的氨氧化细菌和古菌的组成进行分析,并进行定量研究。[方法]用构建克隆文库和Q-PCR定量的方法对珠江口沉积物中氨氧化细菌和古菌amoA基因的含量和多样性特征进行研究。[结果]在2个沉积物表层,氨氧化古菌的含量是细菌的9和22倍,揭示氨氧化古菌在珠江口的氨氧化过程中起主导作用;系统发育分析表明大多数古菌和细菌的amoA基因序列与不可培养的源于河口区和污染区域的环境克隆子序列有较高的同源性;细菌amoA序列可分成5个类群(Cluster A、B、C、D和E),均属于Nitrosomonas类群,其中Cluster A是主要类群(72.1%);古菌amoA序列分析表明来自于表层的序列有52.2%属于"水/沉积物"簇,47.8%属于"土壤/沉积物"簇,而沉积物底层厌氧区,检测到的古菌amoA基因93.3%属于"土壤/沉积物"簇,6.7%属于"水/沉积物"簇,且amoA基因数量略高于表层。[结论]该研究有助于了解珠江口区域氮的循环过程,为氮的富营养化处理提供重要的理论依据。     

Enhanced bonding of gold nanoparticles on oxidized TiO2(110)

We studied the nucleation of gold clusters on TiO2(110) surfaces in three different oxidation states by high-resolution scanning tunneling microscopy. The three TiO2(110) supports chosen were (i) reduced (having bridging oxygen vacancies), (ii) hydrated (having bridging hydroxyl groups), and (iii) oxidized (having oxygen adatoms). At room temperature, gold nanoclusters nucleate homogeneously on the terraces of the reduced and oxidized supports, whereas on the hydrated TiO2(110) surface, clusters form preferentially at the step edges. From interplay with density functional theory calculations, we identified two different gold-TiO2(110) adhesion mechanisms for the reduced and oxidized supports. The adhesion of gold clusters is strongest on the oxidized support, and the implications of this finding for catalytic applications are discussed.     

Discrete sandwich compounds of monolayer palladium sheets

Despite the abundance of "sandwich" complexes, in which two cyclic aromatic hydrocarbon ligands flank a metal center, this motif has not been extended to sheets of multiple metal atoms. We prepared and isolated two such compounds. In the first, three palladium centers form a planar triangular array, capped by chlorides, between two cycloheptatrienyl ligands. In the second, a pentapalladium sheet adopts an edge-sharing triangle-trapezoid skeleton between two naphthacene rings. The compounds were characterized by x-ray crystallography and nuclear magnetic resonance spectroscopy. The nature of bonding in the clusters was analyzed by quantum calculations.