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排序方式: 共有855条查询结果,搜索用时 15 毫秒
61.
Shelton RK Ma LS Kapteyn HC Murnane MM Hall JL Ye J 《Science (New York, N.Y.)》2001,293(5533):1286-1289
We generated a coherently synthesized optical pulse from two independent mode-locked femtosecond lasers, providing a route to extend the coherent bandwidth available for ultrafast science. The two separate lasers (one centered at 760 nanometers wavelength, the other at 810 nanometers) are tightly synchronized and phase-locked. Coherence between the two lasers is demonstrated via spectral interferometry and second-order field cross-correlation. Measurements reveal a coherently synthesized pulse that has a temporally narrower second-order autocorrelation width and that exhibits a larger amplitude than the individual laser outputs. This work represents a new and flexible approach to the synthesis of coherent light. 相似文献
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64.
Secular variation of iron isotopes in north atlantic deep water 总被引:3,自引:0,他引:3
A high-precision iron isotope time series for a ferromanganese crust demonstrates that the iron isotope composition in North Atlantic Deep Water has changed substantially over the past 6 million years and that iron isotope variations in the crust are closely correlated to those of lead isotopes. The close correlation between the two isotope series indicates that the observed iron isotope variations predominantly reflect those of iron input from terrigenous sources and provides no evidence for biologically induced mass fractionation within North Atlantic Deep Water. 相似文献
65.
The photochemistry of diacetylene (C4H2), the largest hydrocarbon to be unambiguously identified in planetary atmospheres, is of considerable importance to understanding the mechanisms by which complex molecules are formed in the solar system. In this work, the primary products of C4H2's ultraviolet photochemistry were determined in a two-laser pump-probe scheme in which the products of C4H2 photoexcitation are detected by vacuum ultraviolet photoionization in a time-of-flight mass spectrometer. Three larger hydrocarbon primary products were observed with good yield in the C4H2* + C4H2 reaction: C6H2, C812, and C8H3. Neither C6H2 nor C8H3 is anticipated by current photochemical models of the atmospheres of Titan, Uranus, Neptune, Pluto, and Triton. The free hydrogen atoms that are released during the formation of the C8H3 and C8H2 products also may partially offset the role of C4H2 in catalysing the recombination of free hydrogen atoms in the planetary atmospheres. 相似文献
66.
Flasar FM Achterberg RK Conrath BJ Pearl JC Bjoraker GL Jennings DE Romani PN Simon-Miller AA Kunde VG Nixon CA Bézard B Orton GS Spilker LJ Spencer JR Irwin PG Teanby NA Owen TC Brasunas J Segura ME Carlson RC Mamoutkine A Gierasch PJ Schinder PJ Showalter MR Ferrari C Barucci A Courtin R Coustenis A Fouchet T Gautier D Lellouch E Marten A Prangé R Strobel DF Calcutt SB Read PL Taylor FW Bowles N Samuelson RE Abbas MM Raulin F Ade P Edgington S Pilorz S Wallis B Wishnow EH 《Science (New York, N.Y.)》2005,307(5713):1247-1251
Stratospheric temperatures on Saturn imply a strong decay of the equatorial winds with altitude. If the decrease in winds reported from recent Hubble Space Telescope images is not a temporal change, then the features tracked must have been at least 130 kilometers higher than in earlier studies. Saturn's south polar stratosphere is warmer than predicted from simple radiative models. The C/H ratio on Saturn is seven times solar, twice Jupiter's. Saturn's ring temperatures have radial variations down to the smallest scale resolved (100 kilometers). Diurnal surface temperature variations on Phoebe suggest a more porous regolith than on the jovian satellites. 相似文献
67.
Multilineage potential of adult human mesenchymal stem cells 总被引:7,自引:0,他引:7
Pittenger MF Mackay AM Beck SC Jaiswal RK Douglas R Mosca JD Moorman MA Simonetti DW Craig S Marshak DR 《Science (New York, N.Y.)》1999,284(5411):143-147
Human mesenchymal stem cells are thought to be multipotent cells, which are present in adult marrow, that can replicate as undifferentiated cells and that have the potential to differentiate to lineages of mesenchymal tissues, including bone, cartilage, fat, tendon, muscle, and marrow stroma. Cells that have the characteristics of human mesenchymal stem cells were isolated from marrow aspirates of volunteer donors. These cells displayed a stable phenotype and remained as a monolayer in vitro. These adult stem cells could be induced to differentiate exclusively into the adipocytic, chondrocytic, or osteocytic lineages. Individual stem cells were identified that, when expanded to colonies, retained their multilineage potential. 相似文献
68.
Ernst RK Yi EC Guo L Lim KB Burns JL Hackett M Miller SI 《Science (New York, N.Y.)》1999,286(5444):1561-1565
Cystic fibrosis (CF) patients develop chronic airway infections with Pseudomonas aeruginosa (PA). Pseudomonas aeruginosa synthesized lipopolysaccharide (LPS) with a variety of penta- and hexa-acylated lipid A structures under different environmental conditions. CF patient PA synthesized LPS with specific lipid A structures indicating unique recognition of the CF airway environment. CF-specific lipid A forms containing palmitate and aminoarabinose were associated with resistance to cationic antimicrobial peptides and increased inflammatory responses, indicating that they are likely to be involved in airway disease. 相似文献
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70.
Sutter NB Bustamante CD Chase K Gray MM Zhao K Zhu L Padhukasahasram B Karlins E Davis S Jones PG Quignon P Johnson GS Parker HG Fretwell N Mosher DS Lawler DF Satyaraj E Nordborg M Lark KG Wayne RK Ostrander EA 《Science (New York, N.Y.)》2007,316(5821):112-115
The domestic dog exhibits greater diversity in body size than any other terrestrial vertebrate. We used a strategy that exploits the breed structure of dogs to investigate the genetic basis of size. First, through a genome-wide scan, we identified a major quantitative trait locus (QTL) on chromosome 15 influencing size variation within a single breed. Second, we examined genetic variation in the 15-megabase interval surrounding the QTL in small and giant breeds and found marked evidence for a selective sweep spanning a single gene (IGF1), encoding insulin-like growth factor 1. A single IGF1 single-nucleotide polymorphism haplotype is common to all small breeds and nearly absent from giant breeds, suggesting that the same causal sequence variant is a major contributor to body size in all small dogs. 相似文献