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排序方式: 共有140条查询结果,搜索用时 390 毫秒
31.
Andrew S. Davis Achilles N. Leontakianakos Farida Benmakroha Ping Wang Rabia Haider John F. Alder Gunnar Thirup 《Analytica chimica acta》1987
A Fabry-Perot semiconfocal cavity resonator containing air at reduced pressure forms part of a Stark-modulated microwave spectrometer operated near the water 22.235-GHz (616–523) and ammonia 23.870 GHz (J = 3, K = 3) absorption lines. A glow discharge is formed in the cavity by an 8.333-kHz sinusoidal electric field of strength up to 140 kV m?1. Under these conditions in the pressure region near 0.01–1 mbar, microwave emission from water and ammonia is observed when microwave energy between 4 to 6 dBm (source output power) at frequencies near that of the line is coupled into the cavity. Spectral features indicate that the emitting molecules arise from recombination of H + OH and H + NH2. Predischarge features indicate that H2O is excited directly by electron impact with simultaneous dissociation to OH and its excitation. NH3 appears to be excited indirectly by energy transfer possibly from a metastable state resulting in dissociation to radicals in the discharge. Significant signal/noise treatment of the signals is found, compared with signals arising from coventional rotational (H2O) or inversion (NH3) absorption at these lines. 相似文献
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Farida Benabdellah Alexandre Seyer Loïc Quinton David Touboul Alain Brunelle Olivier Laprévote 《Analytical and bioanalytical chemistry》2010,396(1):151-162
Mass spectrometry imaging is becoming a more and more widely used method for chemical mapping of organic and inorganic compounds
from various surfaces, especially tissue sections. Two main different techniques are now available: matrix-assisted laser
desorption/ionizaton, where the sample, preliminary coated by an organic matrix, is analyzed by a UV laser beam; and secondary
ion mass spectrometry, for which the target is directly submitted to a focused ion beam. Both techniques revealed excellent
performances for lipid mapping of tissue surfaces. This article will discuss similarities, differences, and specificities
of ion images generated by these two techniques in terms of sample preparation, sensitivity, ultimate spatial resolution,
and structural analysis. 相似文献
34.
Niccolai S Mirazita M Rossi P Baltzell NA Carman DS Hicks K McKinnon B Mibe T Stepanyan S Tedeschi DJ Adams G Ambrozewicz P Anefalos Pereira S Anghinolfi M Asryan G Avakian H Bagdasaryan H Baillie N Ball JP Batourine V Battaglieri M Bedlinskiy I Bektasoglu M Bellis M Benmouna N Berman BL Biselli AS Boiarinov S Bouchigny S Bradford R Branford D Briscoe WJ Brooks WK Bültmann S Burkert VD Butuceanu C Calarco JR Careccia SL Carnahan B Chen S Cole PL Collins P Coltharp P Crabb D Crannell H Crede V 《Physical review letters》2006,97(3):032001
For the first time, the reaction gammad-->DeltanK+ has been analyzed in order to search for the exotic pentaquark baryon Theta+(1540). The data were taken at Jefferson Laboratory, using the Hall-B tagged-photon beam of energy between 0.8 and 3.6 GeV and the CEBAF Large Acceptance Spectrometer (CLAS). No statistically significant structures were observed in the nK+ invariant-mass distribution. The upper limit on the gammad-->DeltaTheta+ integrated cross section has been calculated and found to be between 5 and 25 nb, depending on the production model assumed. The upper limit on the differential cross section is also reported. 相似文献
35.
Farida Benmouna Abdelylah Daoudi Frdrick Roussel Jean‐Marc Buisine Xavier Coqueret Ulrich Maschke 《Journal of Polymer Science.Polymer Physics》1999,37(15):1841-1848
The experimental equilibrium phase diagram of a mixture of linear polystyrene of molecular weight Mw = 44,000 g/mol and 4‐cyano‐4′‐n‐octyl‐biphenyl (8CB) is established. The three transitions smectic A‐nematic, nematic‐isotropic, and isotropic‐isotropic are observed. The first two are observed both by optical microscopy and differential scanning calorimetry (DSC) while the isotropic‐isotropic transition could be seen only via optical microscopy. Two series of samples with the same compositions were independently prepared and yielded consistent results both by microscopy and DSC. Measurements of sample compositions with less than 50 weight % of 8CB were influenced by the vicinity of the glass transition temperature (Tg) of the polymer in the mixture. This quantity is also determined by DSC as a function of composition. A single Tg is observed, which decreases with composition of the LC. Other thermodynamic quantities such as the enthalpy variations of LC in the nematic‐isotropic transition and the fraction of LC contained in the droplets are also considered. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1841–1848, 1999 相似文献
36.
We present new results on the behaviour of single chains in homo-and copolymer blends which are beyond the rendom phase approximation (RPA). The radius of gyration of a chain in the blend was found to depend strongly on the interactions as well as the effective diffusion constant of a single chain in the blend or copolymer melt. Note that these results cannot be obtained in the original formulation of the RPA. They are based on the effective Hamiltonian of the chain in the medium. In the equation found for the self diffusion constant of a single chain, Ds, the second term is directly the value of the effective potential at zero wave vector. Thus the measurement of Ds would detect the screened potential at q = 0. Experimental verification of these results is in progress. 相似文献
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38.
Mustapha Benmouna Thomas A. Vilgis Henri Benoit 《Macromolecular theory and simulations》1992,1(6):333-358
Theoretical models describing the dynamics of multicomponent polymer mixtures are reviewed. Some detailed derivations are shown to make the basic assumptions clear and to ease the comparison between these models. The effects of random noise and memory functions on the time evolution of dynamical correlation functions are examined. The long-range hydrodynamic interactions or mode-coupling effects are also included. A possible extension of these models to the kinetics of spinodal decomposition in solutions of polymer mixtures is briefly discussed. 相似文献
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