共查询到20条相似文献，搜索用时 15 毫秒
1.
Edelstein AS Chow GM Altman EI Colton RJ Hwang DM 《Science (New York, N.Y.)》1991,251(5001):1590-1592
An unusual distribution of particle sizes has been observed following the formation of molybdenum particles by argon ion sputtering. Many of the molybdenum particles produced by sputtering at the threshold pressure for particle formation in the vapor appear to be single crystalline cubes. There are two prominent peaks in the edge length distribution of the cubes, one centered at 4.8 nanometers with a halfwidth of approximately 1.3 nanometers and the other at 17.5 nanometers. The peak for the larger cubes is approximately square and has a total width of 7.0 nanometers. Evidence is presented that the larger cubes are formed by a 3 by 3 by 3 self-arrangement of the smaller cubes, which contain approximately 7000 atoms. Self-arrangement in inorganic structures is normally only observed when the building blocks are atoms, molecules, or clusters of less than 100 atoms. 相似文献
2.
Entrainment of the body temperature rhythm in rats: effect of color and intensity of environmental light 总被引：2，自引：0，他引：2
The daily rhythm in body temperature in rats was continuously monitored during exposure to low-intensity environmental illumination of various colors in the visible and near-ultraviolet spectrum. The ability of phase shifts in the lighting schedule to induce concomitant changes in the rhythm was used to determine the spectral sensitivity of the retinal photoreceptor systems mediating rhythm entrainment. Green light (lambda = 530 +/- 45 nanometers) was most potent, and red (lambda = 660 +/- 19 nanometers) and ultraviolet (lambda = 360 +/- 34 nanometers) were least potent in entraining the temperature rhythm. 相似文献
3.
Polar projections of 50 images of Saturn at 889 nanometers and 25 images at 718 nanometers taken by the Hubble Space Telescope in November 1990, as well as 3 images at each wavelength taken in June 1991, have been examined. Among them, 31 show the north polar spot, which is associated with Saturn's polar hexagon, in locations suitable for measurement. In each image, planetocentric coordinates of the polar spot were determined, and the movement of the spot with respect to Saturn's system III rotation rate was studied. During the period of observation, the polar spot had first a short-term westward movement and then a long-term eastward drift. The rate of the long-term drift was -0.060 +/- 0.008 degrees per day with respect to system III, approximately 50 percent greater than previously determined from Voyager. The original 1980 and 1981 Voyager data were combined with the new Hubble images to form an 11-year base line. The eastward drift over the longer period was -0.0569 degrees per day. The long-term drift could be due to uncertainty in the standard value of the internal rotation period, which is 810.7939 +/- 0.148 degrees per 24-hour day. The short-term movement in November 1990 has a rate that is greater in magnitude but opposite in sign and probably represents a real, transient motion of the spot relative to the internal rotation system. 相似文献
4.
Clouds of impurity atoms near line defects are believed to affect the plastic deformation of alloys. Three-dimensional atom probe techniques were used to image these so-called Cottrell atmospheres directly. Ordered iron-aluminum alloys (40 atomic percent aluminum) doped with boron (400 atomic parts per million) were investigated on an atomic scale along the <001> direction. A boron enrichment was observed in the vicinity of an <001> edge dislocation. The enriched region appeared as a three-dimensional pipe 5 nanometers in diameter, tangent to the dislocation line. The dislocation was found to be boron-enriched by a factor of 50 (2 atomic percent) relative to the bulk. The local boron enrichment is accompanied by a strong aluminum depletion of 20 atomic percent. 相似文献
5.
The reaction of [Cp*Fe(eta5-P5)] with Cu(I)Cl in solvent mixtures of CH2Cl2/CH3CN leads to the formation of entirely inorganic fullerene-like molecules of the formula [[Cp*Fe(eta5:eta1:eta1:eta1:eta1:eta1-P5)]12[CuCl]10[Cu2Cl3]5[Cu(CH3CN)2]5] (1) possessing 90 inorganic core atoms. This compound represents a structural motif similar to that of C60: cyclo-P5 rings of [Cp*Fe(eta5-P5)] molecules are surrounded by six-membered P4Cu2 rings that result from the coordination of each of the phosphorus lone pairs to CuCl metal centers, which are further coordinated by P atoms of other cyclo-P5 rings. Thus, five- and six-membered rings alternate in a manner comparable to that observed in the fullerene molecules. The so-formed half shells are joined by [Cu2Cl3]- as well as by [Cu(CH3CN)2]+ units. The spherical body has an inside diameter of 1.25 nanometers and an outside diameter of 2.13 nanometers, which is about three times as large as that of C60. 相似文献
6.
We report on stimulated Raman scattering in an approximately 1-meter-long hollow-core photonic crystal fiber filled with hydrogen gas under pressure. Light was guided and confined in the 15-micrometer-diameter hollow core by a two-dimensional photonic bandgap. Using a pulsed laser source (pulse duration, 6 nanoseconds; wavelength, 532 nanometers), the threshold for Stokes (longer wavelength) generation was observed at pulse energies as low as 800 +/- 200 nanojoules, followed by a coherent anti-Stokes (shorter wavelength) generation threshold at 3.4 +/- 0.7 microjoules. The pump-to-Stokes conversion efficiency was 30 +/- 3% at a pulse energy of only 4.5 microjoules. These energies are almost two orders of magnitude lower than any other reported energy, moving gas-based nonlinear optics to previously inaccessible parameter regimes of high intensity and long interaction length. 相似文献
7.
Heath JR 《Science (New York, N.Y.)》1992,258(5085):1131-1133
A liquid-solution-phase technique for preparing submicrometer-sized silicon single crystals is presented. The synthesis is based on the reduction of SiCl(4) and RSiCl(3) (R = H, octyl) by sodium metal in a nonpolar organic solvent at high temperatures (385 degrees C) and high pressures (> 100 atmospheres). For R = H, the synthesis produces hexagonal-shaped silicon single crystals ranging from 5 to 3000 nanometers in size. For R = octyl, the synthesis also produces hexagonal-shaped silicon single crystals; however, the size range is controlled to 5.5 +/- 2.5 nanometers. 相似文献
8.
Nightglow in the upper atmosphere of Mars and implications for atmospheric transport 总被引：1，自引：0，他引：1
Bertaux JL Leblanc F Perrier S Quemerais E Korablev O Dimarellis E Reberac A Forget F Simon PC Stern SA Sandel B;SPICAM Team 《Science (New York, N.Y.)》2005,307(5709):566-569
We detected light emissions in the nightside martian atmosphere with the SPICAM (spectroscopy for the investigation of the characteristics of the atmosphere of Mars) ultraviolet (UV) spectrometer on board the Mars Express. The UV spectrum of this nightglow is composed of hydrogen Lyman alpha emission (121.6 nanometers) and the gamma and delta bands of nitric oxide (NO) (190 to 270 nanometers) produced when N and O atoms combine to produce the NO molecule. N and O atoms are produced by extreme UV photodissociation of O2, CO2, and N2 in the dayside upper atmosphere and transported to the night side. The NO emission is brightest in the winter south polar night because of continuous downward transport of air in this region at night during winter and because of freezing at ground level. 相似文献
9.
Spectra of the region just above the bright limb of the Moon show weak emission features that are attributed to resonant scattering of sunlight from sodium and potassium vapor in the lunar atmosphere. The maximum omnidirectional emission flux above the bright limb is 3.8 +/- 0.4 kilorayleighs for sodium and 1.8 +/- 0.4 kiloray-leighs for potassium. The zenith column densities above the subsolar point are estimated to be 8 +/- 3 x 10(8) atoms cm(-2) for sodium 1.4 +/- 0.3 x 10(8) atoms cm(-2) for potassium. Corresponding surface densities are 67 +/- 12 atoms cm(-3) and 15 +/- 3 atoms cm(-3), respectively. The scale height for the sodium atmosphere is 120 +/- 42 kilometers, and for potassium 90 +/- 20 kilometers, which implies that the effective temperature of the sodium and potassium is close to the lunar surface temperature. The sodium density at the south polar region was found to be similar to that at the subsolar point, indicating wide-spread distribution of sodium vapor over the lunar surface. The ratio of the density of sodium to the density of potassium is (6 +/- 3) to 1, which is close to the sodium to potassium ratio in the lunar surface, suggesting that the atmosphere originates from the vaporization of surface minerals. 相似文献
10.
Green line emission at 557.7 nanometers arising from the O(1S - 1D) transition of atomic oxygen has been observed on the nightside of Venus with HIRES, the echelle spectrograph on the W. M. Keck I 10-meter telescope. We also observe optical emissions of molecular oxygen, consistent with the spectra from the Venera orbiters, but our green line intensity is so high that we cannot explain how it could be inconspicuous in the Venera spectra. An upper limit for the intensity of the O(1D - 3P) oxygen red line at 630 nanometers has also been obtained. The large green/red ratio indicates that the source is not associated with the Venus ionosphere. An important conclusion is that observation of the green line in a planetary atmosphere is not an indicator of an atmosphere rich in molecular oxygen. 相似文献
11.
Ebert LB 《Science (New York, N.Y.)》1990,247(4949):1468-1471
Soot generated from diesel fuel in a combustion tube is characterized by microanalysis, x-ray diffraction, chemical reactivity, and nuclear magnetic resonance to address the recent proposal of the significance of carbon clusters in soot. The data support a traditional model of soot as polynuclear aromatic compounds rather than as clusters of carbon atoms with minimal edge site density. The amounts of noncarbon atoms in the soot (hydrogen, oxygen, nitrogen, and sulfur) are commensurate with the edge density of the crystallites (2 by 2 nanometers) inferred from diffraction. The chemistry of soot, in being reduced by potassium metal and alkylated by alkyl iodides, is that known for aromatic compounds and not that anticipated for materials such as graphite, with a small fraction of carbon atoms on edges. 相似文献
12.
Intracellular absorption difference spectrum of Limulus extra-ocular photolabile pigment 总被引：7，自引：0，他引：7
G C Murray 《Science (New York, N.Y.)》1966,154(753):1182-1183
Microspectrophotometric measurements on single cell bodies located on the surface of the"lateral olfactory nerve" of Limulus polyphemus show they contain a photolabile pigment with a absorption-difference spectrum similar in shape to that of most other visual pigments and with a maximum absorption at 529 +/- 5 nanometers. 相似文献
13.
Arpigny C Jehin E Manfroid J Hutsemékers D Schulz R Stüwe JA Zucconi JM Ilyin I 《Science (New York, N.Y.)》2003,301(5639):1522-1524
High-resolution spectra of the CN B2 summation operator +-X2 summation operator + (0,0) band at 390 nanometers yield isotopic ratios for comets C/1995 O1 (Hale-Bopp) and C/2000 WM1 (LINEAR) as follows: 165 +/- 40 and 115 +/- 20 for 12C/13C, 140 +/- 35 and 140 +/- 30 for 14N/15N. Our N isotopic measurements are lower than the terrestrial 14N/15N = 272 and the ratio for Hale-Bopp from measurements of HCN, the presumed parent species of CN. This isotopic anomaly suggests the existence of other parent(s) of CN, with an even lower N isotopic ratio. Organic compounds like those found in interplanetary dust particles are good candidates. 相似文献
14.
We used molecular dynamics simulations with system sizes up to 100 million atoms to simulate plastic deformation of nanocrystalline copper. By varying the grain size between 5 and 50 nanometers, we show that the flow stress and thus the strength exhibit a maximum at a grain size of 10 to 15 nanometers. This maximum is because of a shift in the microscopic deformation mechanism from dislocation-mediated plasticity in the coarse-grained material to grain boundary sliding in the nanocrystalline region. The simulations allow us to observe the mechanisms behind the grain-size dependence of the strength of polycrystalline metals. 相似文献
15.
Rosei F Schunack M Jiang P Gourdon A Laegsgaard E Stensgaard I Joachim C Besenbacher F 《Science (New York, N.Y.)》2002,296(5566):328-331
The electronic connection of single molecules to nanoelectrodes on a surface is a basic, unsolved problem in the emerging field of molecular nanoelectronics. By means of variable temperature scanning tunneling microscopy, we show that an organic molecule (C90H98), known as the Lander, can cause the rearrangement of atoms on a Cu(110) surface. These molecules act as templates accommodating metal atoms at the step edges of the copper substrate, forming metallic nanostructures (0.75 nanometers wide and 1.85 nanometers long) that are adapted to the dimensions of the molecule. 相似文献
16.
An unusual phycoerythrin from a marine cyanobacterium 总被引：1，自引：0，他引：1
Phycoerythrin conjugates are reagents for cell sorting and analyses in which the argon-ion laser line at 488 nanometers is used for excitation. Many marine Synechococcus strains contain phycoerythrins with absorption maxima at approximately 490 and 550 nanometers; these maxima indicate the presence of phycourobilin and phycoerythrobilin prosthetic groups in the protein. Phycoerythrins of red algae contain both groups, but those of freshwater and soil cyanobacteria contain only phycoerythrobilin. Phycoerythrin purified from Synechococcus WH8103 has molecular properties typical of red algal phycoerythrins, but its phycourobilin content is higher than that of other phycoerythrins. The protein has absorption maxima at 492 and 543 nanometers and corresponding molar extinction coefficients of 2.78 and 1.14 x 10(6); it fluoresces maximally at 565 nanometers with a quantum yield of 0.5. Conjugates of Synechococcus WH8103 phycoerythrin could increase the sensitivity of cell analysis techniques to almost twice that possible with other phycoerythrin conjugates. 相似文献
17.
Direct measurements of sliding between outer doublet microtubules in swimming sperm flagella 总被引：1，自引：0，他引：1
C J Brokaw 《Science (New York, N.Y.)》1989,243(4898):1593-1596
The relative motion of 40-nanometer gold beads bound to the exposed outer doublet microtubules of demembranated sea urchin sperm flagella has been observed and photographed during adenosine triphosphate (ATP)-reactivated swimming. This direct demonstration and measure of sliding displacements between outer doublet microtubules in actively bending flagella verifies the original sliding microtubule model for ciliary bending that was established by electron microscopy of fixed cilia and provides a new, functional measure for the diameter of the flagellar axoneme of 132 +/- 8 nanometers. 相似文献
18.
A Rundquist CG Durfee Z Chang C Herne S Backus MM Murnane HC Kapteyn 《Science (New York, N.Y.)》1998,280(5368):1412-1415
Phase-matched harmonic conversion of visible laser light into soft x-rays was demonstrated. The recently developed technique of guided-wave frequency conversion was used to upshift light from 800 nanometers to the range from 17 to 32 nanometers. This process increased the coherent x-ray output by factors of 10(2) to 10(3) compared to the non-phase-matched case. This source uses a small-scale (sub-millijoule) high repetition-rate laser and will enable a wide variety of new experimental investigations in linear and nonlinear x-ray science. 相似文献
19.
Roemmich D 《Science (New York, N.Y.)》1992,257(5068):373-375
Hydrographic time-series data recorded during the past 42 years in the upper 500 meters off the coast of southern California indicate that temperatures have increased by 0.8 degrees C uniformly in the upper 100 meters and that temperatures have risen significantly to depths of about 300 meters. The effect of warming the surface layer of the ocean and there by expanding the water column has been to raise sea level by 0.9 +/- 0.2 millimeter per year. Tide gauge records along the coast are coherent with steric height and show upward trends in sea level that vary from about 1 to 3 millimeters per year. 相似文献
20.
以高衣分染色体片段导入系IL-10-1以及背景亲本陆地棉遗传标准系TM-1和供体亲本海岛棉H7124为试验材料,用扫描电镜观察了纤维初始发育的特征,测定了单粒种子成熟纤维总重量、纤维平均长度、单位长度纤维重量和纤维总根数并分析了IL-10-1高衣分形成的原因。扫描电镜观察的结果表明,纤维突起启动的早晚与基因型关系密切,纤维伸长起始的早晚与衣分关系密切,较早的纤维伸长启动有利于高衣分的形成。高衣分导入系IL-10-1单粒种子成熟纤维总质量、纤维平均长度、单位长度纤维质量和纤维总根数均极显著高于TM-1。相关性分析和通径分析结果证明:纤维总质量、单位长度纤维质量和纤维平均长度与衣分关系密切。因此,材料较早的启动纤维突起和开始纤维伸长利于优良品质和高衣分的形成。 相似文献