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81.
Potassium oxalate acts as a specific reagent in enhancing the fluorescence intensity of terbium in aqueous solutions. Maximum fluorescence intensity is obtained by irradiating (at 255 mμ) terbium(III) dissolved in 0.01 M potassium oxalate solution at pH 7.8. The enhancement and quenching phenomena caused by other lanthanides, errors in the determination, and various examples of spectrofluorimetric analysis of traces of terbium in mixtures with other lanthanides are described. The sensitivity of the method is 5·10-2μg/ml of terbium.  相似文献   
82.
Continuous 295 nm excitation of whey protein bovine apo-α-lactalbumin (apo-bLA) results in an increase of tryptophan fluorescence emission intensity, in a progressive red-shift of tryptophan fluorescence emission, and breakage of disulphide bridges (SS), yielding free thiol groups. The increase in fluorescence emission intensity upon continuous UV-excitation is correlated with the increase in concentration of free thiol groups in apo-bLA. UV-excitation and consequent SS breakage induce conformational changes on apo-bLA molecules, which after prolonged illumination display molten globule spectral features. The rate of tryptophan fluorescence emission intensity increase at 340 nm with excitation time increases with temperature in the interval 9.3–29.9°C. The temperature-dependent 340 nm emission kinetic traces were fitted by a 1st order reaction model. Native apo-bLA molecules with intact SS bonds and low tryptophan emission intensity are gradually converted upon excitation into apo-bLA molecules with disrupted SS, molten-globule-like conformation, high tryptophan emission intensity and red-shifted tryptophan emission. Experimental Ahrrenius activation energy was 21.8 ± 2.3 kJ.mol−1. Data suggests that tryptophan photoionization from the S1 state is the likely pathway leading to photolysis of SS in apo-bLA. Photoionization mechanism(s) of tryptophan in proteins and in solution and the activation energy of tryptophan photoionization from S1 leading to SS disruption in proteins are discussed. The observations present in this paper raise concern regarding UV-light pasteurization of milk products. Though UV-light pasteurization is a faster and cheaper method than traditional thermal denaturation, it may also lead to loss of structure and functionality of milk proteins.  相似文献   
83.
We present a new method to measure the triplet exciton diffusion length in organic semiconductors. N,N'-di-[(1-naphthyl)-N,N'-diphenyl]-1,1'-biphenyl)-4,4'-diamine (NPD) has been used as a model system. Triplet excitons are injected into a thin film of NPD by a phosphorescent thin film, which is optically excited and forms a sharp interface with the NPD layer. The penetration profile of the triplet excitons density is recorded by measuring the emission intensity of another phosphorescent material (detector), which is doped into the NPD film at variable distances from the injecting interface. From the obtained triplet penetration profile we extracted a triplet exciton diffusion length of 87±2.7 nm. For excitation power densities >1 mW/mm(2) triplet-triplet annihilation processes can significantly limit the triplet penetration depth into organic semiconductor. The proposed sample structure can be further used to study excitonic spin degree of freedom.  相似文献   
84.
We develop an efficient computational algorithm that produces efficient Markov chain Monte Carlo (MCMC) transition matrices. The first level of efficiency is measured in terms of the number of operations needed to produce the resulting matrix. The second level of efficiency is evaluated in terms of the asymptotic variance of the resulting MCMC estimators. Results are first given for transition matrices in finite state spaces and then extended to transition kernels in more general settings.  相似文献   
85.
We developed a sensitive spectroscopic method to probe triplet concentration in thin films of polyfluorene (PF) at room temperature. The energy of photoexcited triplet excitons is transferred to the guest metal-organic complex, meso-tetratolylporphyrin-Pd (PdTPP), and detected as phosphorescent emission. The phosphorescence intensity of PdTPP-PF blends is proportional to the independently measured triplet concentration using photoinduced absorption experiments. The high sensitivity of this method allows room temperature detection of triplet excitons in spin-coated polymer films as thin as 10 nm. We found that the triplet lifetime is independent of PdTPP concentration and therefore this method is nearly non-perturbing for the triplet population.  相似文献   
86.
Nanocrystalline TiO(2) samples, prepared for smart textiles applications by the sol-gel technique in acidic or basic media, have been characterized by synchrotron X-ray powder diffraction and total scattering methods based on a fast implementation of the Debye function and original algorithms for sampling interatomic distances. Compared to the popular and widely used Rietveld-based approaches, our method is able to simultaneously model both Bragg and diffuse contributions and to quantitatively extract either sizes and size distribution information from the experimental data. The photocatalytic activity of the investigated samples is here systematically correlated to the average sizes and size distributions of anisotropically shaped coherent domains, modeled according to bivariate populations of nanocrystals grown along two normal directions.  相似文献   
87.
The direct, non ex post synthesis of a novel phthalocyanine decorated with eight thiohexyl-nido-carborane functions, nido-[H(2)MCHESPc]K(8), where the anionic polyhedra are in the form of K(+) salt, is reported and discussed. The solution properties of this compound, including the unprecedented exchange between the pyrrolic protons and the peripheral alkali-metal ions, are also analyzed.  相似文献   
88.
In the recent years the so-called designer drugs have been diffused dramatically worldwide. Amphetamine psychostimulants are one of the major classes of illicit drugs consumed for recreational purposes in the world. The detection of the illegal substances is important both in the management of their misuse and in the forensic field. Urine matrix is a reliable biological matrix for verifying amphetamines intake in the short and medium term because the excretion of these drugs mainly occurs in urine, where substantial amounts of unchanged drug are present. In response to a growing demand for reliable evidence of amphetamine use, a method for identification and quantitation of amphetamine, methamphetamine, 3,4-methylenedioxy-N-methylamphetamine, 3,4-methylenedioxyamphetamine, 3,4-methylenedioxy-N-ethylamphetamine in urine has been developed by nuclear magnetic resonance spectroscopy, using electronic reference to access in vivo concentration method. The method showed good linearity. The limits of detection and quantitation of amphetamines analyzed ranging from 1.9–7.9 to 5.8–23.8 μg mL?1, respectively. These data demonstrate that the present method is suitable for detection of these drugs in urine samples, allowing to quantitate the target analytes without any purification step of the matrix.  相似文献   
89.
Correlation between microstructure noise and latent financial logarithmic returns is an empirically relevant phenomenon with sound theoretical justification. With few notable exceptions, all integrated variance estimators proposed in the financial literature are not designed to explicitly handle such a dependence, or handle it only in special settings. We provide an integrated variance estimator that is robust to correlated noise and returns. For this purpose, a generalization of the forward filtering backward sampling algorithm is proposed, to provide a sampling technique for a latent conditionally Gaussian random sequence. We apply our methodology to intraday Microsoft prices and compare it in a simulation study with established alternatives, showing an advantage in terms of root‐mean‐square error and dispersion.  相似文献   
90.
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