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   4178篇
  免费   732篇
  国内免费   475篇
化学   4842篇
晶体学   28篇
力学   9篇
综合类   24篇
数学   47篇
物理学   435篇
  2024年   11篇
  2023年   60篇
  2022年   210篇
  2021年   239篇
  2020年   315篇
  2019年   204篇
  2018年   153篇
  2017年   170篇
  2016年   276篇
  2015年   266篇
  2014年   329篇
  2013年   420篇
  2012年   310篇
  2011年   288篇
  2010年   234篇
  2009年   260篇
  2008年   246篇
  2007年   233篇
  2006年   197篇
  2005年   183篇
  2004年   135篇
  2003年   133篇
  2002年   76篇
  2001年   53篇
  2000年   59篇
  1999年   45篇
  1998年   43篇
  1997年   35篇
  1996年   25篇
  1995年   36篇
  1994年   30篇
  1993年   19篇
  1992年   11篇
  1991年   10篇
  1990年   4篇
  1989年   9篇
  1988年   10篇
  1987年   8篇
  1986年   6篇
  1985年   11篇
  1984年   8篇
  1983年   3篇
  1982年   2篇
  1981年   4篇
  1980年   1篇
  1977年   1篇
  1976年   1篇
  1974年   1篇
  1972年   1篇
  1970年   1篇
排序方式: 共有5385条查询结果,搜索用时 15 毫秒
1.
Pyrrolopyrrole aza‐BODIPY (PPAB) developed in our recent study from diketopyrrolopyrrole by titanium tetrachloride‐mediated Schiff‐base formation reaction with heteroaromatic amines is a highly potential chromophore due to its intense absorption and fluorescence in the visible region and high fluorescence quantum yield, which is greater than 0.8. To control the absorption and fluorescence of PPAB, particularly in the near‐infrared (NIR) region, further molecular design was performed using DFT calculations. This results in the postulation that the HOMO–LUMO gap of PPAB is perturbed by the heteroaromatic moieties and the aryl‐substituents. Based on this molecular design, a series of new PPAB molecules was synthesized, in which the largest redshifts of the absorption and fluorescence maxima up to 803 and 850 nm, respectively, were achieved for a PPAB consisting of benzothiazole rings and terthienyl substituents. In contrast to the sharp absorption of PPAB, a PPAB dimer, which was prepared by a cross‐coupling reaction of PPAB monomers, exhibited panchromatic absorption across the UV/Vis/NIR regions. With this series of PPAB chromophores in hand, a potential application of PPAB as an optoelectronic material was investigated. After identifying a suitable PPAB molecule for application in organic photovoltaic cells based on evaluation using time‐resolved microwave conductivity measurements, a maximized power conversion efficiency of 1.27 % was achieved.  相似文献   
2.
Small amounts of certain halogenated compounds are found to have, at most, only a slight enhancing effect on the radiolytic decay rates of added poly-unsaturated compounds in polyethylene, but significantly increase the elastic modulus at 433 K (melt modulus) obtained thereby. Experiments with model chlorine-containing additives suggest that this increase is due to a more random distribution of polymer and monomer mediated crosslinks in the polymer, that it does not result from a significant increase in crosslinking and that it is mediated by chlorine atoms, in a similar manner to radiolytic hydrogen atoms, through facilitation of long range free radical migration. Although low molecular weight chloro-paraffins inhibit radiolytically induced growth of melt modulus in monomer containing polyethylenes, even very small additions of chlorinated polyethylenes, which form a separate phase, increase the melt modulus. This again indicates that the active species is the chlorine radical.  相似文献   
3.
The sub-millisecond protonation dynamics of the chromophore in S65T mutant form of the green fluorescent protein (GFP) was tracked after a rapid pH jump following laser-induced proton release from the caged photolabile compoundo-nitrobenzaldehyde. Following a jump in pH from 8 to 5 (which is achieved within 2 μs), the fluorescence of S65T GFP decreased as a single exponential with a time constant of ∼90 μs. This decay is interpreted as the conversion of the deprotonated fluorescent GFP chromophore to a protonated non-fluorescent species. The protonation kinetics showed dependence on the bulk viscosity of the solvent, and therefore implicates bulk solvent-controlled protein dynamics in the protonation process. The protonation is proposed to be a sequential process involving two steps: (a) proton transfer from solvent to the chromophore, and (b) internal structural rearrangements to stabilize a protonated chromophore. The possible implications of these observations to protein dynamics in general is discussed  相似文献   
4.
We employ an agent‐based model to show that memory and the absence of an a priori best strategy are sufficient for self‐segregation and clustering to emerge in a complex adaptive system with discrete agents that do not compete over a limited resource nor contend in a winner‐take‐all scenario. An agent starts from a corner of a two‐dimensional lattice and aims to reach a randomly selected site in the opposite side within the shortest possible time. The agent is isolated during the course of its journey and does not interact with other agents. Time‐bound obstacles appear at random lattice locations and the agent must decide whether to challenge or evade any obstacle blocking its path. The agent is capable of adapting a strategy in dealing with an obstacle. We analyze the dependence of strategy‐retention time with strategy for both memory‐based and memory‐less agents. We derive the equality spectrum to establish the environmental conditions that favor the existence of an a priori best strategy. We found that memory‐less agents do not polarize into two opposite strategy‐retention time distributions nor cluster toward a center distribution. © 2004 Wiley Periodicals, Inc. Complexity 9: 41–46, 2004  相似文献   
5.
吴涛  邓佩珍 《光学学报》1997,17(7):66-869
测量了Cr^4+,YAG、Cr^4+,Mg2SiO4晶体在室温和液氮温度下的荧光光谱,吸收光谱和激发态寿命,讨论了温度变化时,两种晶体中Cr^4+近红外辐射积分强度变化与激光发态寿命变化的关系,得出结论:在77K ̄300K范围内,Cr^4+的^3T2能级荧光辐射截面本身受温度影响不大,Cr^4+辐射荧光的变化,主要是由无辐射弛豫速率随温度变化而引起的。  相似文献   
6.
Applied Biochemistry and Biotechnology -  相似文献   
7.
You-Sheng Chen 《Tetrahedron》2006,62(40):9410-9416
The major tautomer of several triketone derivatives in organic and aqueous solutions has been determined. Their solvent- and base-sensitive properties have been applied in the design of a polarity-sensitive fluorescent probe and an acidichromic colorant, respectively. The regioselective acetylation and methylation of 2-acyldimedone, 3-acyl-4-hydroxycoumarin, and 2-acyl-1,3-indandione have also been investigated. The results indicated that acetylation and methylation of the first two occurred specifically at endocyclic enolic oxygens, whereas for the latter they occurred at exocyclic enolic oxygen.  相似文献   
8.
9.
荧光波长对共焦显微镜成像特性的影响   总被引:7,自引:0,他引:7  
导出了共焦显微镜中不同荧光波长情况下的荧光功率传输函数、三维脉冲响应函数(3D-PSF)和三维光学传递函数(3D-OTF)。结果表明,不同的荧光波长对共焦显微镜的空间截止频率、分辨率、光学传递函数存在明显的影响。当激发波长与荧光波长的比值下降到一定程度时,可以看到明显的失锥现象。  相似文献   
10.
Lately, scientists have explored approaches to developing fluorescent and/or bioluminescent indicators to pinpoint cellular processes in single living cells. These analytical methods have become a key technology for visualizing and detecting what was otherwise unseen in live cells. The target signaling included second messengers, protein phosphorylations, protein–protein interactions, and protein localizations.  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