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   6037篇
  免费   56篇
  国内免费   14篇
化学   3565篇
晶体学   68篇
力学   171篇
数学   636篇
物理学   1667篇
  2018年   44篇
  2016年   51篇
  2015年   44篇
  2014年   63篇
  2013年   199篇
  2012年   147篇
  2011年   166篇
  2010年   112篇
  2009年   116篇
  2008年   178篇
  2007年   189篇
  2006年   184篇
  2005年   205篇
  2004年   187篇
  2003年   153篇
  2002年   140篇
  2001年   82篇
  2000年   72篇
  1999年   82篇
  1998年   54篇
  1997年   79篇
  1996年   106篇
  1995年   100篇
  1994年   61篇
  1993年   69篇
  1992年   64篇
  1991年   62篇
  1990年   70篇
  1989年   62篇
  1988年   72篇
  1987年   83篇
  1986年   57篇
  1985年   67篇
  1984年   83篇
  1983年   64篇
  1982年   80篇
  1981年   63篇
  1980年   89篇
  1979年   61篇
  1978年   84篇
  1977年   75篇
  1976年   64篇
  1975年   78篇
  1974年   50篇
  1973年   48篇
  1969年   42篇
  1968年   47篇
  1967年   47篇
  1955年   59篇
  1934年   44篇
排序方式: 共有6107条查询结果,搜索用时 15 毫秒
51.
Forward-backward multiplicity correlations in σ+,K + p andpp collisions at 250 GeV/c ( \(\sqrt s \) =22 GeV) are given for all charges and for the different charge combinations. The correlations are found to be caused predominantly by centrally produced particles. It is demonstrated that this result is an agreement with observations at the ISR and the CERNp \(\bar p\) -Collider. The results are compared to expectations from LUND, DPM and FRITIOF Monte Carlo models and a geometrical picture relating correlations in hadron-hadron collisions toe + e ? data in terms of impact parameters is tested.  相似文献   
52.
Results obtained in an experimental optically pumped cesium beam frequency standard in which a single semiconductor laser is used for the state selection and the atom detection are reported. The separation between the two interaction regions is equal to 21 cm. This gives a 500 Hz linewidth which is observed with a signal to noise ratio equal to 10,000 in a 1 Hz noise bandwidth. A quartz crystal oscillator is frequency controlled by the atomic transition. The measured short term frequency stability is given by y()=2×10–12–1/2 for 1 s<500 s. Prospects for improvement of this frequency stability are discussed.  相似文献   
53.
Time and spatially resolved absorption measurements of Xe* particle number densities performed at an X-ray preionized self-sustained XeCl*-laser discharge were used to investigate the reaction kinetics of such discharges and to test the validity of the commonly-made assumption that they are fit well by spatially homogeneous numerical models. It turned out that the Xe* density distribution over the discharge cross section remains homogeneous for less than 100 ns only. Comparing the experimental results of the homogeneous discharge phase with zero-dimensional model calculations, we conclude that induced emission modeled using two excited xenon levels must be taken into account for the quantitative interpretation of the absorption measurements. For the interpretation of the Xe 6s-6p absorption measurements even a multilevel model will be necessary. Further we conclude from the relative courses and the absolute values of the Xe* particle number densities that realistic HCl kinetics should contain three vibrationally excited levels and stepwise excitation processes as proposed by Dem'yanov et al. [8].  相似文献   
54.
We use tunable UV laser light in the region 200–320 nm, produced by frequency doubling the output of a dye laser, for the decomposition of organometallic compounds. This method has been applied to TMA, trimethylaluminum Al(CH3)3. Only the TMA monomer absorbs UV light for >220 nm. TMA decomposes by one-photon absorption mainly into two channels: aluminum atoms Al plus organic fragments, and aluminummonomethyl AlCH3 molecules plus organic fragments. The ratio [Al]/[AlCH3] is wavelength dependent. We present a mechanism to explain the photolysis of trimethyl compounds of group III elements (Al, Ga, In).  相似文献   
55.
Zusammenfassung Perimidon (1) reagiert mit monosubstit. Malonsäure-trichlorphenylestern (2) bei 250° zu 9-Hydroxy-5,7-dioxo-4,5-dihydro-7H-pyrido[1,2,3-cd]perimidinen (3), die durch saure Hydrolyse zu 10-Amino-4-hydroxy-benzo[h]carbostyrilen (4) gespalten werden.
Syntheses of heterocycles, CXXII: Reaction of perimidone with reactive malonates
Perimidone (1) reacts with monosubstituted 2.4.6-trichlorophenyl malonates at 250° to 9-hydroxy-5.7-dioxo-4.5-dihydro-7H-pyrido[1.2.3-cd]perimidines (3), which are cleaved by acid yielding 10-amino-4-hydroxy-benzo[h]quinolin-2-ones (4).
  相似文献   
56.
Positron lifetime and Doppler broadening of the annihilation line measurements were performed in highT c superconducting samples YBa2Cu3Cd x O y ,x=0, 0.05 and 6.9<y<7, as a function of temperature in the region of 14–300 K. It was found that the positron lifetime and theS parameter values are lower in the Cd doped sample than those in the undoped one. It was also observed that the positron annihilation parameters show similar temperature dependence for the undoped and Cd doped samples. We conclude that the Cd doping in highT c superconductor YBa2Cu3Cd x O y , 6.9<y<7 fills defects associated with oxygen vacancies probably in oxygen deficient regions which can trap positrons.  相似文献   
57.
58.
A novel heterocycloaddition merges 2-thiono-3-ketolactones with carbohydrate glycals to afford materials which resemble disaccharides with an O-glycosidic linkage at the anomeric center and a thioether linking both C-2 and C-2′, thus creating a third heterocyclic ring. Upon desulfurization, these novel cycloadducts afford materials which are models for 2-deoxydisaccharides. Studies with two keto lactones and seven glycals are described.  相似文献   
59.
60.
Based upon a thermodynamical approach, the generalized Onsager type of relaxation of van der Waals networks is presented. By linearly and identically coupling the set hidden variables to the network, the memory function of the system can be related to the equilibrium strain-energy function. The relaxation behavior of real networks on stretching can quantitatively be described by means of a distribution of relaxation times known from small strain experiments. Some new and interesting conclusions are discussed as to how the macroscopically non-linear visco-elastic response might be interpreted.  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