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排序方式: 共有126条查询结果,搜索用时 15 毫秒
31.
Bradley J. Rowe Jeffrey Scholten Christopher D. Spilling 《Phosphorus, sulfur, and silicon and the related elements》2013,188(6-7):1881-1884
Reaction of nonracemic allylic hydroxy phosphonates, prepared by the asymmetric phosphonylation of unsaturated aldehydes, with methyl chloroformate in pyridine yields the corresponding carbonates. The carbonates are excellent substrates for the palladium-catalyzed addition of nucleophiles. Addition of the nucleophile is highly regioselective, resulting in n -substituted vinyl phosphonates. The reaction of the allylic carbonates with aryl stannanes and malonates has been investigated. Progress in the application of these reactions to the synthesis of turmerone and enterolactone is reported. 相似文献
32.
R. Van Langh E. Lehmann S. Hartmann A. Kaestner F. Scholten 《Analytical and bioanalytical chemistry》2009,395(7):1949-1959
Until recently fabrication techniques of Renaissance bronzes have been studied only with the naked eye, microscopically, videoscopically and with X-radiography. These techniques provide information on production techniques, yet much important detail remains unclear. As part of an interdisciplinary study of Renaissance bronzes undertaken by the Rijksmuseum Amsterdam, neutron-imaging techniques have been applied with the aim of obtaining a better understanding of bronze workmanship during the Renaissance period. Therefore, an explanation of the fabrication techniques is given to better understand the data collected by these neutron-imaging techniques. The data was used for tomography studies, which reveal hidden aspects that could not at all or scarcely be seen using X-radiography. For this specific study, the representative bronze ‘Hercules Pomarius’ of Willem van Tetrode (ca 1520–1588) has been examined, along with 20 other Renaissance bronzes from the Rijksmuseum collection. 相似文献
33.
B.?Smeets Email author R.C.M.?Bosch P.?van der Straten E.?te Sligte R.E.?Scholten H.C.W.?Beijerinck K.A.H.?van Leeuwen 《Applied physics. B, Lasers and optics》2003,76(8):815-819
Polarization spectroscopy of an Fe-Ar hollow cathode discharge cell was used to lock a frequency-doubled Ti:sapphire laser to the 372-nm5D45F5 transition of 56Fe. The discharge cell produced a density of 1018 m-3 ground-state 56Fe atoms at a temperature of 650 K, this density being comparable to a conventional oven at 1500 K. Saturated absorption spectroscopy and two schemes of polarization spectroscopy were compared with respect to signal-to-background ratio and the effect of velocity-changing collisions. The laser was locked within 0.2 MHz for hours by feedback of the dispersive polarization spectroscopy signal. PACS 33.55.Ad; 42.62.Fi; 52.25.Ya 相似文献
34.
Several basic parameters of a photochemical reactor coupled to a high-performance liquid Chromatographie system are discussed. The non-fluorescent clobazam and desmethylclobazam and three phenothiazines, which exhibit native fluorescence, are used as model compounds. On irradiation with ultra-violet light, the reaction products formed display fluorescence (clobazam, desmethylclobazam) or unproved fluorescence characteristics (phenothiazines). The effects of carrier stream (mobile-phase) composition, time of irradiation and band broadening in the reactor on the fluorescence signal are described. The polarity of the organic solvents used (methanol, ethanol, acetonitrile) appears to have an important effect on the fluorescence intensity. For clobazam and desmethylclobazam, detection limits of 70 and 120 pg, respectively, were calculated after an irradiation time of 28 s with methanol—0.01 M (pH 5) acetate buffer (1:1) as mobile phase. The method is applied to the determination of both compounds in serum and urine samples. 相似文献
35.
Ohne Zusammenfassung 相似文献
36.
Sub-Doppler bandwidth atomic optical filter 总被引:1,自引:0,他引:1
We demonstrate what is to our knowledge the first atomic optical filter that uses velocity selection to achieve a passband width that is less than the Doppler width of the filtering transition. A narrow-linewidth pump laser is used to induce circular birefringence in a narrow velocity class of atoms in a dense potassium vapor for 694-nm light resonant with the 4P(3/2)-6S(1/2) transition. The filter displays a single 170-MHz passband at a peak transmittance of 9.5%. The bandwidth is an order of magnitude lower than that of previously demonstrated atomic optical filters. 相似文献
37.
Subwavelength-resolution phase images of phase dislocations at the focal region of a 20x , 0.4-N.A. lens have been obtained by use of an optical fiber interferometer with a tapered probe in one arm. A phase-stepping algorithm is used to determine a quantitative value of the phase at each point in the scan, clearly showing the presence of edge dislocations between the Airy rings of the diffraction pattern near the lens focus, as well as four isolated screw-type singularties caused by astigmatism in the lens. 相似文献
38.
Very energetic cosmic rays entering the atmosphere of Earth will create a plasma cloud moving with almost the speed of light. The magnetic field of Earth induces an electric current in this cloud which is responsible for the emission of coherent electromagnetic radiation. We propose to search for a new effect: Because of the index of refraction of air, this radiation is collimated in a Cherenkov cone. To express the difference from usual Cherenkov radiation, i.e., the emission from a fast-moving electric charge, we call this magnetically induced Cherenkov radiation. We indicate its signature and possible experimental verification. 相似文献
39.
Clark JN Putkunz CT Curwood EK Vine DJ Scholten R McNulty I Nugent KA Peele AG 《Optics letters》2011,36(11):1954-1956
As the resolution in coherent diffractive imaging improves, interexposure and intraexposure sample dynamics, such as motion, degrade the quality of the reconstructed image. Selecting data sets that include only exposures where tolerably little motion has occurred is an inefficient use of time and flux, especially when detector readout time is significant. We provide an experimental demonstration of an approach in which all images of a data set exhibiting sample motion are combined to improve the quality of a reconstruction. This approach is applicable to more general sample dynamics (including sample damage) that occur during measurement. 相似文献
40.
A. F. Barfield P. von Brentano A. Dewald K. O. Zell N. V. Zamfir D. Bucurescu M. Ivaşcu O. Scholten 《Zeitschrift für Physik A Hadrons and Nuclei》1989,332(1):29-32
Two new absolute transition rates are reported for the nucleus144Sm following an (α, α′) Coulomb excitation study. They are B(E3; 3?→ 0+)=(38±3) W.u. and B(E1;3? → 2+)=(2.8±0.4)×10?3 W.u. This large E1 matrix element, along with the previously known B(E1; 1? →+) value support the interpretation of the 1? state in this nucleus as 2-phonon 2+ × 3? excitation. In the frame of the IBM-1 +f-boson model we show the need for a two-body term in the E1 transition operator. Estimates for the strengths of the one and two-body parts of the E1 transition operator are obtained from these experimental data. 相似文献