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81.
Polarization‐sensitive CARS spectroscopy on free‐base porphyrins: coproporphyrin I tetramethyl ester
The application of polarization‐sensitive (PS) coherent anti‐Stokes Raman scattering (CARS) spectroscopy for the investigation of highly luminescent free‐base porphyrins under Qx band resonance is discussed. For coproporphyrin I tetramethyl ester (CP‐I‐TME), PS CARS spectra involving resonances with the electronic Qx absorption band as well as polarized spontaneous Raman spectra involving B band resonance are presented. A quantitative evaluation of the CP‐I‐TME spectra is performed and the results are compared to our previously presented data on free‐base octaethylporphine (OEP) and mesoporphyrin IX dimethyl ester (MP‐IX‐DME), which were obtained under identical excitation conditions. This comprehensive analysis reveals several spectral differences that can be attributed to the different β–substitution pattern of the porphyrin macrocycle. Additionally, the strong resonance enhancement of totally symmetric modes under Qx band excitation is identified as a common property for OEP, CP‐I‐TME, and MP‐IX‐DME; this enhancement selectivity distinguishes the investigated substances from what is generally observed for metallo porphyrins. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
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84.
Peter van de Weijer Dr. Douwe van der Meer Johan L. Koster 《Theoretical chemistry accounts》1975,38(3):223-229
The protonation of azanaphthalenes and azabenzenes has been studied theoretically using CNDO/2 wavefunctions and perturbation theory in order to examine the correlation between pKa values and quantum-mechanical quantities.[/p] 相似文献
85.
J. N. Koster 《Heat and Mass Transfer》1997,33(3):193-199
The stability of Rayleigh-Bénard convection in a transparent model alloy was investigated with holographic interferometry.
The dispersed minority phase is the heavier component. The majority component is lighter. A vertical temperature difference
was applied to the liquid layer keeping the hot temperature always above phase separation. The upper cold side temperature
was kept above or below the phase separation temperature. Fluid separation occurs when the cold temperature drops below consolute
temperature. When the liquid is kept at higher than consolute temperature, it is noticed that the stability of the layer is
increased when the average temperature approaches the consolute temperature.
Received on 13 January 1997 相似文献
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87.
Enantioseparation of chiral products has become increasingly important in a large diversity of academic and industrial applications. The separation of chiral compounds is inherently challenging and thus requires a suitable analytical technique that can achieve high resolution and sensitivity. In this context, CE has shown remarkable results so far. Chiral CE offers an orthogonal enantioselectivity and is typically considered less costly than chromatographic techniques, since only minute amounts of chiral selectors are needed. Several CE approaches have been developed for chiral analysis, including chiral EKC and chiral CEC. Enantioseparations by EKC benefit from the wide variety of possible pseudostationary phases that can be employed. Chiral CEC, on the other hand, combines chromatographic separation principles with the bulk fluid movement of CE, benefitting from reduced band broadening as compared to pressure-driven systems. Although UV detection is conventionally used for these approaches, MS can also be considered. CE-MS represents a promising alternative due to the increased sensitivity and selectivity, enabling the chiral analysis of complex samples. The potential contamination of the MS ion source in EKC-MS can be overcome using partial-filling and counter-migration techniques. However, chiral analysis using monolithic and open-tubular CEC-MS awaits additional method validation and a dedicated commercial interface. Further efforts in chiral CE are expected toward the improvement of existing techniques, the development of novel pseudostationary phases, and establishing the use of chiral ionic liquids, molecular imprinted polymers, and metal-organic frameworks. These developments will certainly foster the adoption of CE(-MS) as a well-established technique in routine chiral analysis. 相似文献
88.
Origin of charge density at LaAlO3 on SrTiO3 heterointerfaces: possibility of intrinsic doping 总被引:1,自引:0,他引:1
Siemons W Koster G Yamamoto H Harrison WA Lucovsky G Geballe TH Blank DH Beasley MR 《Physical review letters》2007,98(19):196802
As discovered by Ohtomo and Hwang, a large sheet charge density with high mobility exists at the interface between SrTiO3 and LaAlO3. Based on transport, spectroscopic, and oxygen-annealing experiments, we conclude that extrinsic defects in the form of oxygen vacancies introduced by the pulsed laser deposition process used by all researchers to date to make these samples is the source of the large carrier densities. Annealing experiments show a limiting carrier density. We also present a model that explains the high mobility based on carrier redistribution due to an increased dielectric constant. 相似文献
89.
Holger Braunschweig Margot Koster Karl W. Klinkhammer 《Angewandte Chemie (International ed. in English)》1999,38(15):2229-2231
Nucleophilic attack of a carbonyl oxygen atom of the complexes K[(η5-C5H5)M(CO)3] (M=Mo, W) on each boron center gave the oxycarbyne complexes 1 . These novel products undergo a unique type of rearrangement with quantitative formation of the diborane(4)yl complexes 2 . Complex 2 (M=Mo) is the first structurally characterized boryl complex with a Mo–B bond (see structure). 相似文献
90.
Local interface velocities are tracked radioscopically in the III-V semiconductor compound indium-antimony grown in a vertical Bridgman-Stockbarger furnace. Comparisons are made of interface velocities from five different compositions (40, 49, 50, 55, and 60 at.% Sb). Under specific growth conditions, the growth velocity for stoichiometric melts was comparatively constant and very close to the translation velocity. Measured chemical homogeneity was excellent, though polycrystallinity could occur when concentration boundary layers formed ahead of the interface. Off-stoichiometric melts exhibited initial supercooling, resulting in transient interface velocities and polycrystallinity. The observed supercooling is governed by chemical segregation in the melt. Thus, local growth velocity fluctuations are unambiguously attributed to a coupling of ompositional effects in the melt and crystal facetting kinetics. 相似文献