全文获取类型
收费全文 | 1407篇 |
免费 | 229篇 |
国内免费 | 257篇 |
专业分类
化学 | 985篇 |
晶体学 | 21篇 |
力学 | 104篇 |
综合类 | 19篇 |
数学 | 178篇 |
物理学 | 586篇 |
出版年
2024年 | 1篇 |
2023年 | 12篇 |
2022年 | 25篇 |
2021年 | 26篇 |
2020年 | 61篇 |
2019年 | 47篇 |
2018年 | 49篇 |
2017年 | 43篇 |
2016年 | 57篇 |
2015年 | 61篇 |
2014年 | 85篇 |
2013年 | 116篇 |
2012年 | 79篇 |
2011年 | 81篇 |
2010年 | 72篇 |
2009年 | 96篇 |
2008年 | 88篇 |
2007年 | 96篇 |
2006年 | 98篇 |
2005年 | 70篇 |
2004年 | 78篇 |
2003年 | 67篇 |
2002年 | 53篇 |
2001年 | 46篇 |
2000年 | 57篇 |
1999年 | 41篇 |
1998年 | 41篇 |
1997年 | 33篇 |
1996年 | 38篇 |
1995年 | 28篇 |
1994年 | 17篇 |
1993年 | 19篇 |
1992年 | 22篇 |
1991年 | 19篇 |
1990年 | 5篇 |
1989年 | 14篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 4篇 |
1985年 | 10篇 |
1984年 | 6篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有1893条查询结果,搜索用时 31 毫秒
951.
A multireference configuration interaction (MRCI) study has been carried out on the LiCl molecule. The potential energy has been calculated over a wide range of internuclear separation for the 21 low-lying electronic states of the LiCl molecule dissociating into Li (2S, 2P, 3S)+Cl(2P). The (4)1Σ+, (3)1Π, 1-33Σ+, 1-33Π, 1,3Δ, 1,3Σ-, (5)1Σ+, (4)3Σ+, (4)1Π, (4)3Π excited states are studied for the first time in theory. Molecular spectroscopic constants (Re, De, ωe, ωeχe, Be and αe) have been derived for the 9 bound states (X1Σ+, (3)1Σ+, (2)1Σ+, 1,3Δ, 1,3Σ-, (4)1Π, (4)3Π) with a regular shape, and the spectroscopic constants of ground states X1Σ+ are in good agreement with available experimental and theoretical values. The relative differences between experimental values and our values for Re, De, ωe, ωeχe, Be and αe are 1.02%, 0.60%, 1.72%, 9.46%, 2.0%, and 0.75%, respectively. Moreover, vibrational levels of 9 bound states, which have not been investigated experimentally, are computed. 相似文献
952.
在(S)-氨基丙醇及(R)-氨基丙醇手性臂的作用下, (M/P)-4,4'-二甲氧基-5,6,5',6'-二次甲二氧基-2-甲酸酯-2'-甲酰氨联苯经构型转化, 制备了光学纯轴手性联苯化合物(P, S)-3a 和(M, R)-3a. 测定了(P, S)-3a 的晶体结构及CD 光谱. 结果表明, 化合物(P, S)-3a 晶体属单斜晶系, P2(1)空间群, 晶胞参数为a=12.122(2) Å, b=8.9911(18) Å, c=12.779(3) Å, β=112.38(3)°, 在晶体中存在两组分子间氢键相互作用, 一组氢键由羟基氢与另一分子酰胺基团的羰基氧组成O-H…O,另一组由酰胺基团的NH 与另一分子酰胺基团的羰基氧构成N-H…O, 每一分子通过四个氢键与另外两个分子相连,构成棒状结构. 由CD 光谱确定了(M, R)-3b 的立体构型. 此外, 由(P, S)-3a 合成了轴手性化合物(P)-2,2'-二羟甲基-4,4'-二甲氧基-5,6,5',6'-二亚甲二氧基-联苯(6a). 相似文献
953.
Evolution of infrared spectra and optical emission spectra in hydrogenated silicon thin films prepared by VHF-PECVD 下载免费PDF全文
A series of hydrogenated silicon thin films with varying silane concentrations have been deposited by using very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) method. The deposition process and the silicon thin films are studied by using optical emission spectroscopy (OES) and Fourier transfer infrared (FTIR) spectroscopy, respectively. The results show that when the silane concentration changes from 10% to 1%, the peak frequency of the Si—H stretching mode shifts from 2000 cm - 1 to 2100 cm - 1, while the peak frequency of the Si—H wagging—rocking mode shifts from 650 cm - 1 to 620 cm - 1. At the same time the SiH*/Hα intensity ratio in the plasma decreases gradually. The evolution of the infrared spectra and the optical emission spectra demonstrates a morphological phase transition from amorphous silicon (a-Si:H) to microcrystalline silicon (μc-Si:H). The structural evolution and the μc-Si:H formation have been analyzed based on the variation of Hα and SiH* intensities in the plasma. The role of oxygen impurity during the plasma process and in the silicon films is also discussed in this study. 相似文献
954.
955.
以十甲基环五硅氧烷(D5)和氧气(O2)作为反应气体,采用电子回旋共振等离子体化学气相沉积(ECR-CVD)方法制备了k=2.62的SiCOH薄膜.研究了O2掺杂对薄膜结构与电学性能的影响.结果表明,采用O2掺杂可以在保持较低介电常数的前提下极大地降低薄膜的漏电流,提高薄膜的绝缘性能,这与薄膜中Si-O立体鼠笼、Si-OH结构含量的提高有关.
关键词:
SiCOH薄膜
2掺杂')" href="#">O2掺杂
介电性能
键结构 相似文献
956.
K. Koc 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2009,53(1):9-14
Lifetime of 2s22p 2P3/2 level along with fine structure separations for the lowest 2P doublet of boron-like ions, from Z=14 to Z=30 have been calculated using multireference relativistic configuration interaction
(MR RCI) method based on the no-pair Dirac-Coulomb-Breit (DCB) Hamiltonian. Analytic basis sets of Gaussian-type functions
are employed to expand the upper and lower components of the Dirac four-spinors in the matrix Dirac-Fock self-consistent field
(DF SCF) and relativistic MR CI procedures. Error with respect to variation of basis set parameters was estimated. QED corrections
were estimated by evaluation of electron self-energy and vacuum polarisation. 相似文献
957.
Nagao Kobayashi Prof. Dr. Fumiko Narita Kazuyuki Ishii Prof. Dr. Atsuya Muranaka Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(39):10173-10181
Oxovanadium(IV) phthalocyanines (VOPcs) with a single‐handed rotation have been prepared, and their right‐ and left‐handed enantiomers resolved on a chiral HPLC column. These enantiomers gave circular dichroism (CD) spectra of opposite signs; the correlation between the CD sign and conformation was obtained by time‐dependent density functional theory (TDDFT) calculations: an enantiomer showing a negative sign in the Q band was suggested to be the right‐handed conformer viewing from the axial oxygen side, whereas that giving a positive CD sign was assigned to the left‐handed conformer. Although silicon phthalocyanines (SiPcs) with two different alkoxy axial ligands have been resolved similarly, the absence of a meaningful CD difference probably reflects the flat character of the SiPc plane compared to the VOPc plane. Changes in the Q‐band CD, depending on the relative orientation of the peripheral substituents, have been worked out theoretically and the origin of the chiroptical properties is discussed. 相似文献
958.
Dagmar C. Kapeller Dr. Friedrich Hammerschmidt Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(23):5729-5739
The smaller the better : Enantiopure aminomethyllithiums, such as depicted, the smallest of their kind, have been prepared and tested for their micro‐ and macroscopic configurational stabilities, which are delicately influenced by R1 and R2. The species with R1=R2=Bn is macroscopically configurationally stable below ?95 °C.
959.
分子水平上的激发态理论研究能够用来解释分子材料的多色发光机理. 采用单组态相互作用方法, 计算研究了四种二配位的Au(I)配合物Ph3PAuCl, Ph3PAuBr, Ph3AsAuCl和Ph3AsAuBr的分子结构限制的三重激发态(T1a)和分子结构松弛的三重激发态(T1b)的分子结构与光物理性质. 由于θ(PAuX)/θ(AsAuX)从180°扭曲到120°左右, T1b态的能量在单线态组态相互作用(CIS)水平上降低了0.805-1.124 eV, 在密度泛函理论(DFT)水平上降低了0.820-0.947 eV. 自然键轨道电荷布居数分析表明, 在T1a态中两个单电子主要分布在一个苯基上, 而在T1b态中两个单电子分布在PAuX/AsAuX 上. 因此, 在晶体中观察到的较高能磷光归属于T1a态的苯基之间的3π*→1π电子跃迁, 而较低能磷光主要起源于T1b态的Au 的3σ*→1σ电子跃迁. 相似文献
960.
为了理解gemini表面活性剂柔性烷基联接链在自组织过程中的特殊作用,我们合成了三种gemini表面活性剂烷基-a,w-二(二-十二烷基甲基溴化铵)(记为2C12-s-2C12×2Br (s=3, 6, 8))。2C12-s-2C12×2Br在水表面构成铺展膜后,由于每个分子带有4根烷烃链,它们形成了稠密的烷烃尾链层。增强的烷烃尾链与联接链间的疏水相互作用促使联接链弯曲朝向空气一端,可发生弯曲的联接链长度要小于吸附在水溶液表面上的gemini表面活性剂C12-s-C12×2Br,后者每个分子只有2根烷烃链。由此可见,增强的烷烃尾链与联接链间的疏水相互作用可以有效地促进联接链的弯曲。 相似文献