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排序方式: 共有271条查询结果,搜索用时 15 毫秒
61.
Christopher S. Withers Saralees Nadarajah 《Statistical Inference for Stochastic Processes》2012,15(2):127-132
Suppose ${\widehat{\theta}_1}$ and ${\widehat{\theta}_2}$ are asymptotically independent non-lattice with a joint second order Edgeworth expansion in n ?1/2. Then the ?? dependency coefficient is $$\alpha \left(\widehat{\theta}_1, \widehat{\theta}_2 \right) = n^{-1/2} C + O \left(n^{-1} \right),$$ where ${C = (4 \pi)^{-1}\exp (-1/2) (\tau^2_1 + \tau^2_2) ^{1/2}}$ for ${\tau_1, \tau_2}$ their joint skewness coefficients. 相似文献
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本文报导了采用氩离子激光器来泵浦Pr3+:YLF晶体,应用声光调制器实现了主动锁模;同时应用振动─高反射平面镜也实现了被动锁模,两种锁模均得到了ps光脉冲.据作者了解这是这种晶体材料的第一次锁模运转. 相似文献
64.
M. L. Mena C. W. McLeod P. Jones A. Withers V. Minganti R. Capelli P. Quevauviller 《Fresenius' Journal of Analytical Chemistry》1995,351(4-5):456-460
A novel method for the direct determination of mercury species at the ng l–1 level in natural waters is described. Methyl-, ethyl- and inorganic mercury are preconcentrated on a sulphhydryl cotton microcolumn incorporated in a flow injection system. Retained mercury species are then eluted with hydrochloric acid solution (3 mol/l) and subjected to phenylation before determination by gas chromatography-microwave induced plasma-atomic emission spectrometry. Limits of detection for mercury species are 10 ng l–1 for methyl- and ethyl-mercury and 16 ng l–1 for inorganic mercury based on processing 200 ml of sample. Application of the methodology to waters of the Manchester Ship Canal revealed elevated levels of methylmercury and inorganic mercury. 相似文献
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68.
Ray L. Withers Thomas Höche Saeid Esmaeilzadeh Brian Sales 《Journal of solid state chemistry》2004,177(10):3316-3323
High-purity Rb2V3O8 has been grown and temperature-dependent electron and single-crystal X-ray diffraction used to carefully investigate its fresnoite-type reciprocal lattice. In contrast to other recently investigated representatives of the fresnoite family of compounds, Rb2V3O8 is not incommensurately modulated with an incommensurate basal plane primary modulation wave vector given by q∼0.3 〈110〉*. A careful low-temperature electron diffraction study has, however, revealed the existence of weak incommensurate satellite reflections characterized by the primitive primary modulation wave vector q1∼0.16c*. The reciprocal space positioning of these incommensurate satellite reflections, the overall (3+1)-d superspace group symmetry, as well as the shapes of the refined displacement ellipsoids determined from single-crystal XRD refinement, are all consistent with their arising from a distinct type of condensed rigid unit modes (RUMs) of distortion of the Rb2V3O8 parent structure. 相似文献
69.
THE AVAILABILITY OF SOLAR RADIATION BELOW 290 nm AND ITS IMPORTANCE IN PHOTOMODIFICATION OF POLYMERS
R. E. BARKER JR. 《Photochemistry and photobiology》1968,7(3):275-295
Abstract— On a percentage basis, ozone is a very minor component of the atmosphere; at STP it would make a layer only about 2 mm thick. On almost every other basis (biological, meteorological, paleontological, photochemical, etc.) it is a major component, due mainly to the tremendous reduction in solar ultraviolet flux which it causes in the 220–290 nm region. Since no data are available for Λ < 285 nm, a rational basis for estimating the flux reaching the earth's surface in this region is discussed. Variations in ozone concentration are of great importance, and it is possible to have more radiation with Λ < 270 nm fall on a surface in one extreme day than in several years of typical days. Often, persons involved in studies of polymer degradation by sunlight mention that a negligible fraction (1 ppm) of the radiant flux reaching the earth's surface is associated with wavelengths below 290 nm and infer that studies at shorter wavelengths will not be of much practical value. Such inferences are questionable for at least two reasons. (1) The quantum flux density below 290 nm is about 1016 photons cm-2 month-1 , so that considerable long-term damage is possible since most of the flux will be absorbed in a layer only a few microns thick. (2) Even if solar radiation below 290 nm were completely absent, the existence of correlations between absorption peaks in the near ultraviolet and visible, and in the infrared with ionization potentials typically 6–12 eV or 200-100 nm) is evidence that we may expect studies in the ultraviolet and extreme ultraviolet (EUV) to provide important clues to the problem of improving the resistance of polymers to sunlight. 相似文献
70.
EFFECT OF ULTRAVIOLET RADIATION ON THE ENERGY METABOLISM OF THE CORNEAL EPITHELIUM OF THE RABBIT 总被引:2,自引:0,他引:2
The present research was directed at quantifying possible alterations in corneal epithelial metabolic activity secondary to in vivo exposure to ultraviolet radiation (UVR). Microfluorometric energy metabolite assays on microgram (microgram) sized, freeze-dried tissue samples were used as an in vitro means of assessing overall metabolic activity in the epithelium of control rabbit corneas and in the epithelium of UVR-exposed rabbit corneas 2 min after discontinuation of exposure. The specific assays were for glucose, glycogen, adenosine triphosphate (ATP), and phosphocreatine (PCr). The radiant exposures were kept constant at 0.05 J cm-2 for all UVR wavelengths utilized (290, 300, 310 and 360 nm). Experimental UVR exposure conditions served to increase epithelial glucose and glycogen concentrations. Although the epithelial ATP concentrations were unchanged, the epithelial PCr concentrations (a high energy phosphate bond reservoir) decreased as a result of UVR exposure. Overall, the data demonstrate a decrease in corneal epithelial metabolic activity, which may be wavelength-dependent, as a result of UVR exposure. It is suggested that immediate metabolic stress can be responsible for the pattern of epithelial cell loss seen in photokeratitis. 相似文献