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   1229篇
  免费   46篇
  国内免费   3篇
化学   919篇
晶体学   15篇
力学   14篇
数学   122篇
物理学   208篇
  2023年   5篇
  2022年   13篇
  2021年   19篇
  2020年   9篇
  2019年   21篇
  2018年   17篇
  2017年   14篇
  2016年   28篇
  2015年   21篇
  2014年   17篇
  2013年   57篇
  2012年   70篇
  2011年   68篇
  2010年   46篇
  2009年   61篇
  2008年   79篇
  2007年   82篇
  2006年   83篇
  2005年   72篇
  2004年   57篇
  2003年   60篇
  2002年   44篇
  2001年   19篇
  2000年   18篇
  1999年   16篇
  1998年   18篇
  1997年   8篇
  1996年   18篇
  1995年   14篇
  1994年   18篇
  1993年   15篇
  1992年   15篇
  1991年   7篇
  1990年   8篇
  1989年   8篇
  1988年   12篇
  1987年   8篇
  1986年   10篇
  1985年   15篇
  1984年   19篇
  1982年   8篇
  1981年   12篇
  1980年   11篇
  1979年   11篇
  1978年   7篇
  1977年   6篇
  1976年   7篇
  1975年   4篇
  1974年   5篇
  1973年   4篇
排序方式: 共有1278条查询结果,搜索用时 46 毫秒
131.
The two-photon absorption (TPA) properties of four TPEB [tetrakis(phenylethynyl)benzene] derivatives (TD, para, ortho, and meta) with different donor/acceptor substitution patterns have been investigated experimentally by the femtosecond open-aperture Z-scan method and theoretically by the time-dependent density-functional theory (TDDFT) method. The four compounds show relatively large TPA cross sections, and the all-donor substituted species (TD) displays the largest TPA cross-section σ(2) = 520 ± 30 GM. On the basis of the calculated electronic structure, TD shows no TPA band in the lower energy region of the spectrum because the transition density is concentrated on particular transitions due to the high symmetry of the molecular structure. The centrosymmetric donor-acceptor TPEB para shows excitations resulting from transitions centered on D-π-D and A-π-A moieties, as well as transition between the D-π-D and A-π-A moieties; this accounts for the broad nature of the TPA bands for this compound. Calculations for two noncentrosymmetric TPEBs (ortho and meta) reveal that the diminished TPA intensities of higher-energy bands result from destructive interference between the dipolar and three-state terms. The molecular orbitals (MOs) of the TPEBs are derivable with linear combinations of the MOs of the two crossing BPEB [bis(phenylethynyl)benzene] derivatives. Overall, the characteristics of the experimental spectra are well-described based on the theoretical analysis.  相似文献   
132.
Understanding the effect of codoping on the properties of photonic glasses is important for improving their properties. The effect of codoping on the ligand field around Cu(2+) ions in a sodium borate glass is examined using optical absorption spectroscopy, continuous-wave electron paramagnetic resonance, and three-pulse electron-spin-echo envelope-modulation. Glass with a composition of 0.1CuO·5Na(2)O·95B(2)O(3) was codoped with 2 mol % of Al(3+), Si(4+), P(5+), Zr(4+), or La(3+) oxide. Three codoping effects are found: strengthening the ligand field, as observed for Zr-codoping, which induces a large blue shift of the optical absorption peak of Cu(2+); weakening the ligand field, as observed for P-codoping, which causes a red shift of the Cu(2+) absorption peak; and almost no effect on the ligand field, which is observed for Al-, Si-, and La-codoping. Coordination structure models based on local charge neutrality are proposed for the codoped glasses. The mechanism of the codoping effect is revealed by elucidating the local structure around Cu(2+).  相似文献   
133.
A new all-fiber optical add-drop multiplexer (ADM) using a pair of fiber grating couplers (FGCs) was proposed and first demonstrated. It was theoretically and experimentally shown that both drop and add efficiencies could be maximized in the proposed ADM. It was also clarified that the crosstalk between dropped and added signals could be improved in the proposed ADM compared with that in the single FGC. Drop and add operations in the ADM for 3-channel WDM signals with 1.6 nm wavelength spacing were confirmed. The ADM with signal amplification ability, which consists of a pair of FGCs, an EDF, and a pump-LD, was proposed and demonstrated. It was confirmed that the dropped and transmitted optical signals could be amplified in the ADM, and would be able to expand fiber routing spans.  相似文献   
134.
135.
136.
2,3-Diphenylpropionic acid library for VLA-4 antagonist was synthesized on solid-phase. Comparison of the two synthetic routes via an orthogonal generation of two aromatic amino functional groups are discussed. From this work, several compounds were identified as potent VLA-4 antagonists.  相似文献   
137.
138.
In 1′, 1‴-dibutylbiferrocenium triiodide crystals A show temperature-independent trapped-valence state, while crystals B show a valence-delocalization of the mixed-valence state. The mixed-valence states of both crystals are discussed in relation to their crystal structures. Address after October 1991: Department of Chemistry, Faculty of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima 724, Japan  相似文献   
139.
Grazing-exit x-ray fluorescence (GE-XRF) and micro x-ray fluorescence (micro-XRF) methods were applied to chemical microchips as a detection method. Since an energy-dispersive x-ray detector was used, the simultaneous detection of multiple elements was possible. An analyzing region was especially designed on the microchip so that a sample solution could be dried and concentrated in a suitable area corresponding to the size of the primary x-ray beam. Finally, it was confirmed that both analytical methods could be combined well for use with a microchip. In GE-XRF, the background intensity in the XRF spectrum was reduced at grazing-exit angles. In addition, a good relationship between the x-ray fluorescence intensities and the concentrations of standard solutions that were introduced into the microchip was obtained. This indicates that the GE-XRF method is feasible for trace elemental analysis in chemical microchip systems. In micro-XRF, an attempt was made to concentrate and dry the analyte within a small analyzing region. The preliminary results indicated that the micro-XRF method could be applied for the analysis of microchips.  相似文献   
140.
Green light-emitting diodes (LEDs) were fabricated employing a ZnCdSe/ZnSSe triple quantum-well (TQW) active region surrounded by ZnMgSSe cladding layers grown on an n-type (100) GaAs substrate by molecular beam epitaxy (MBE). A 3.5 mW pure green emission was observed for the surface-emitting LED device at a peak wavelength of 513.3 nm (2.415 eV) with a spectral half-width of 11.7 nm (55 meV) under a 20 mA (4.6 V) direct current at room temperature (25°C). These correspond to an external quantum efficiency of 7.2%, a power conversion efficiency of 3.8%, a luminous current efficiency of 66 lm/A, and a luminous efficiency of 14 lm/W.  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