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排序方式: 共有192条查询结果,搜索用时 109 毫秒
91.
92.
Back Cover: Synthesis and Properties of Semiconducting Bispyrrolothiophenes for Organic Field‐Effect Transistors (Chem. Eur. J. 20/2014)
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93.
James G. Hockridge Franca Jones Mitch Loan William R. Richmond 《Journal of Crystal Growth》2009,311(15):3876-3882
The formation of the iron oxyhydroxide schwertmannite has been monitored by time-resolved transmission electron microscopy (TEM) studies. Schwertmannite aggregates are found to form initially as spherical agglomerates of ferrihydrite crystallites, which then begin to grow characteristic needles on their surfaces. High-resolution images of the needles show that they are initially comprised of aligned goethite nanocrystals, which subsequently coarsen to form crystallographically coherent needles of goethite. Thus, needle formation on schwertmannite aggregates can be considered as the first stage in the phase transformation from schwertmannite to goethite. The results of this study suggest that schwertmannite is not a distinct mineral phase, but may be a mixture of ferrihydrite and poorly crystalline goethite with a distinctive morphology directed by the presence of surface-adsorbed sulfate anions. 相似文献
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96.
Laboratory, ground-based, and airborne tunable diode laser systems: performance characteristics and applications in atmospheric studies 总被引:4,自引:0,他引:4
A. Fried B. Henry B. Wert S. Sewell J.R. Drummond 《Applied physics. B, Lasers and optics》1998,67(3):317-330
2 O) used as the target gas. One method utilizes the Allan variance. When our present aircraft system is operated in the laboratory,
the Allan variance indicates a CH2O detection limit of 0.031 ppbv for integration times of 25 s. This sensitivity corresponds to a minimum detectable absorbance
of 1.0×10-6, and this is within a factor of two of that reported by Werle et al. who used high-frequency modulation spectroscopy. The
present instrument utilizes conventional low-frequency (2f=100 kHz) wavelength modulation. Instrument performance, obtained
from replicate measurements of CH2O standards acquired over time periods as long as 1.5 hours, on average resulted in a factor of two poorer precision than
indicated by the Allan variance. Since replicate measurements precisely simulate the acquisition procedures employed, including
the acquisition of sample and background spectra, they present a more meaningful measure of instrument performance. Preliminary
evidence suggests that slow drifts in the laser wavelength control during acquisition of replicate measurements may play an
important role in the above disparity. The resultant laser wavelength correction voltage, which is applied to counter such
drifts, may also be a factor in this disparity. A limited number of replicate measurements with minimal drift in the laser
wavelength yield much closer agreement between replicate and Allan variance precisions.
Received: 19 March 1998/Revised version: 2 June 1998 相似文献
97.
Normal vibrational modes for a three-dimensional defect-free crystal of the high explosive pentaerythritol tetranitrate were obtained in the framework of classical mechanics using a previously published unreactive potential-energy surface [J. Phys. Chem. B 112, 734 (2008)]. Using these results the vibrational density of states was obtained for the entire vibrational frequency range. Relaxation of selectively excited terahertz-active modes was studied using isochoric-isoergic (NVE) molecular dynamics simulations for energy and density conditions corresponding to room temperature and atmospheric pressure. Dependence of the relaxation time on the initial modal excitation was considered for five excitation energies between 10 and 500 kT and shown to be relatively weak. The terahertz absorption spectrum was constructed directly using linewidths obtained from the relaxation times of the excited modes for the case of 10 kT excitation. The spectrum shows reasonably good agreement with experimental results. Dynamics of redistribution of the excited mode energy among the other normal modes was also studied. The results indicate that, for the four terahertz-active initially excited modes considered, there is a small subset of zero wave vector (k = 0) modes that preferentially absorb the energy on a few-picosecond time scale. The majority of the excitation energy, however, is transferred nonspecifically to the bath modes of the system. 相似文献
98.
Johnson HC Robertson AP Chaplin AB Sewell LJ Thompson AL Haddow MF Manners I Weller AS 《Journal of the American Chemical Society》2011,133(29):11076-11079
We report the first insertion step at a metal center for the catalytic dehydropolymerization of H(3)B·NMeH(2) to form the simplest oligomeric species, H(3)B·NMeHBH(2)·NMeH(2), by the addition of 1 equiv of H(3)B·NMeH(2) to [Ir(PCy(3))(2)(H)(2)(η(2)-H(3)B·NMeH(2))][BAr(F)(4)] to give [Ir(PCy(3))(2)(H)(2)(η(2)-H(3)B·NMeHBH(2)·NMeH(2))][BAr(F)(4)]. This reaction is also catalytic for the formation of the free linear diborazane, but this is best obtained by an alternative stoichiometric synthesis. 相似文献
99.
Heinz H. Bauschke Mason S. Macklem Jason B. Sewell Xianfu Wang 《Journal of Approximation Theory》2010,162(6):1225-1244
We systematically investigate the farthest distance function, farthest points, Klee sets, and Chebyshev centers, with respect to Bregman distances induced by Legendre functions. These objects are of considerable interest in Information Geometry and Machine Learning; when the Legendre function is specialized to the energy, one obtains classical notions from Approximation Theory and Convex Analysis.The contribution of this paper is twofold. First, we provide an affirmative answer to a recently-posed question on whether or not every Klee set with respect to the right Bregman distance is a singleton. Second, we prove uniqueness of the Chebyshev center and we present a characterization that relates to previous works by Garkavi, by Klee, and by Nielsen and Nock. 相似文献
100.
Nataliya K. Sakhnenko Alexander G. Nerukh Trevor M. Benson Phillip Sewell 《Optical and Quantum Electronics》2008,40(11-12):813-820
This article presents an analytical approach to the 2D problem in which a circular whispering-gallery mode (WGM) resonator is subject to an abrupt temporal change in refractive index in a circular coaxial region or in a ring region near the rim. 相似文献