首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   249篇
  免费   10篇
  国内免费   1篇
化学   186篇
晶体学   1篇
力学   4篇
数学   43篇
物理学   26篇
  2022年   10篇
  2021年   9篇
  2019年   6篇
  2018年   4篇
  2017年   3篇
  2016年   9篇
  2015年   10篇
  2014年   7篇
  2013年   16篇
  2012年   17篇
  2011年   15篇
  2010年   14篇
  2009年   11篇
  2008年   15篇
  2007年   20篇
  2006年   13篇
  2005年   13篇
  2004年   12篇
  2003年   9篇
  2002年   7篇
  2001年   2篇
  2000年   5篇
  1999年   1篇
  1998年   5篇
  1997年   2篇
  1996年   5篇
  1995年   1篇
  1994年   1篇
  1992年   1篇
  1988年   1篇
  1985年   1篇
  1983年   2篇
  1982年   1篇
  1980年   5篇
  1979年   1篇
  1978年   3篇
  1975年   1篇
  1973年   2篇
排序方式: 共有260条查询结果,搜索用时 31 毫秒
81.
Energy transfer from the excited aminophthalate dianion species to fluorescein at pH 8.32 (Tris-HCl buffer) was studied. The excited aminophthalate dianion species was obtained either by excitation with UV radiation (330 nm), with fluorescence emission, or by the well-known chemical reaction luminol-hydrogen peroxide in an alkaline medium, with chemiluminescent emission, both with max at 425 nm. The influence of Co2+ and Mg2+ on fluorescence and chemiluminescence (CL) was studied. It was found that at low concentrations (10–7–10–9 M), these ions do not modify the fluorescein fluorescence, however, the CL is strong affected. The effect of the concentration of these elements, which exert an influence on CL even at a high dilution (nanomolar concentration), was determined. In the case of Co2+ the prooxidant character is stronger than in the case of Mg2+, and therefore the CL enhancer effect is higher. Compared to the system without catalyst, their presence ensures stronger, prolonged, and stable light emission. The emission spectra, in the presence of fluorescein, show two bands with maxima at 425 and 520 nm, the second one being specific to fluorescein emission. The intensity of aminophthalate dianion luminescence is lower and the duration shorter in the presence of fluorescein. The influence of Co2+ and Mg2+ catalyst and fluorescein concentration on the energy transfer process was studied. The efficiency of the energy transfer process for these two situations (fluorescence and CL) was compared. An attempt was made to replace hydrogen peroxide with superoxide anion (solubilized by means of crown ether) and its effect upon the energy transfer process was observed.  相似文献   
82.
83.
It has been shown (see [3]) that every finite incidence structure can be represented in a suitable way by a polynomial over a convenient finite field. In this paper we present a FORTRAN interactive program which examines the incidence structure associated to a given polynomial over GF(9). Further, a second program is exhibited, which determines a complete system of mutually orthogonal latin squares related to a polynomial associated to a projective plane of order nine.  相似文献   
84.
While isobutyl vinyl ketone reacts with α-acetyl-α'-methyl succinate to give a normal Michael adduct, the Mannich base or its quaternary derivative as a source of vinylketonc gives the butenolide derivative 8a. Isobutyl cyclohexenoneacetic acid (1b) previously stated to be the reaction product was present only as a by-product. When α-acetyl-α'-methyl succinate is treated with acrylic acid derivatives only butenolides 8c–8f were obtained.  相似文献   
85.
Goldstein  S.  Ciupitoiu  A.  Vasilescu  V.  Duca  Al. 《Mikrochimica acta》1975,63(1):117-123
Microchimica Acta - Vitamin B12 has been determined by measurement of the infrared absorption of the cyano-group band at 2137 cm?1, the samples being pelleted with KBr. The error is within...  相似文献   
86.
We report on a new random laser phenomenon that gives rise to narrow emission modes without requiring optical cavities. Sharp emission peaks are observed experimentally over a broad range of scattering strengths and analyzed in numerical calculations. We find that the introduction of exponential gain in a multiple light scattering process strongly increases the importance of very long light paths. Such long paths are rare and often neglected in passive disordered materials but we show that they can dominate the emission spectrum from an amplifying disordered system.  相似文献   
87.
Luminol solution in DMSO in the presence of [18C6…K]+ O2 supramolecular complex (achieved from KO2 and 18-Crown-6 (18C6)-ether) is chemiluminescent, and its intensity depends on the complex concentration. Using fluorescein (Fl) as an energy acceptor in this system, the luminescence energy transfer process from chemically excited species, aminophtalate dianion, to Fl could be evidenced. On the basis of Förster theory, the critical energy transfer distance, for both fluorescence and chemiluminescence, has been determined. The values are comparable: 22.41 Å (for chemiluminescence) and 20.88 Å (for fluorescence).  相似文献   
88.

Photodegradation of two sulfonylurea herbicides, triasulfuron and thifensulfuron-methyl adsorbed on Preveza or Nea Malgara soils (Greece) was studied in outdoor and laboratory experiments. Herbicides on adsorbed phase and kept in the dark were characterised by a high reactivity, giving depletion curves that can be all described by a first order equation. In the irradiation experiments the kinetic behaviour of photodegradation was complex and characterised by a double step photoreaction. After a first period varying from 8 to 24 h the rate of reaction was reduced to 7-31% of the initial rate. The kinetic constant related to the degradation of triasulfuron practically showed the same values of those obtained for thifensulfuron-methyl. The half-lives obtained on Nea Malgara soil were generally higher than those obtained using Preveza soil. The observed behaviour is explained considering the photochemical properties of the herbicides, and the organic matter content of the soils.  相似文献   
89.
Models of inflation in which non-Gaussianity is generated outside the horizon, such as curvaton models, generate distinctive higher-order correlation functions in the cosmic microwave background and other cosmological observables. Testing for violation of the Suyama-Yamaguchi inequality τ(NL) ≥ (6/5f (NL))(2), where f(NL) and f(NL) denote the amplitude of the three-point and four-point functions in certain limits, has been proposed as a way to distinguish qualitative classes of models. This inequality has been proved for a wide range of models, but only weaker versions have been proved in general. In this Letter, we give a proof that the Suyama-Yamaguchi inequality is always satisfied. We discuss scenarios in which the inequality may appear to be violated in an experiment such as Planck and how this apparent violation should be interpreted.  相似文献   
90.
We report quantum chemical calculations providing the exchange coupling constants of the V[TCNE]2 model system, describing the amorphous room temperature molecular magnet V[TCNE] x (TCNE = tetracyanoethylene, x ~ 2). The geometry is optimized for the ideal lattice using density functional theory (DFT) calculations with periodic boundary conditions. Broken-symmetry DFT calculations indicate antiparallel spin alignment resulting in ferrimagnetic ordering, but heavily overestimate the value of the exchange coupling. Better estimates of the exchange coupling parameters between the V(II) ion and the [TCNE]? anionic radical are obtained by means of multiconfigurational calculations performed on smaller molecular models cut from the optimized crystal lattice. Complete active space self-consistent field and multireference second-order perturbation theory calculations provide the sign and the strength of the nearest-neighbor as well as next-nearest-neighbor interactions along all three crystallographic directions. We are able to explain also intuitively the mechanism for antiferromagnetic spin coupling in terms of the superexchange pathways, discussing the role of the main four types of contributions to superexchange. Moreover, we clarify the influence of the transition metal ion on the strength of the exchange interaction and on the critical temperature for long-range ferrimagnetic ordering.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号