全文获取类型
收费全文 | 49398篇 |
免费 | 8065篇 |
国内免费 | 6226篇 |
专业分类
化学 | 36355篇 |
晶体学 | 713篇 |
力学 | 2724篇 |
综合类 | 404篇 |
数学 | 5789篇 |
物理学 | 17704篇 |
出版年
2024年 | 94篇 |
2023年 | 895篇 |
2022年 | 1180篇 |
2021年 | 1626篇 |
2020年 | 1951篇 |
2019年 | 2021篇 |
2018年 | 1645篇 |
2017年 | 1616篇 |
2016年 | 2313篇 |
2015年 | 2287篇 |
2014年 | 2817篇 |
2013年 | 3637篇 |
2012年 | 4600篇 |
2011年 | 4803篇 |
2010年 | 3434篇 |
2009年 | 3243篇 |
2008年 | 3488篇 |
2007年 | 3217篇 |
2006年 | 2992篇 |
2005年 | 2440篇 |
2004年 | 1794篇 |
2003年 | 1388篇 |
2002年 | 1321篇 |
2001年 | 1079篇 |
2000年 | 989篇 |
1999年 | 981篇 |
1998年 | 762篇 |
1997年 | 731篇 |
1996年 | 699篇 |
1995年 | 603篇 |
1994年 | 535篇 |
1993年 | 394篇 |
1992年 | 376篇 |
1991年 | 330篇 |
1990年 | 286篇 |
1989年 | 226篇 |
1988年 | 161篇 |
1987年 | 156篇 |
1986年 | 141篇 |
1985年 | 130篇 |
1984年 | 64篇 |
1983年 | 60篇 |
1982年 | 53篇 |
1981年 | 24篇 |
1980年 | 17篇 |
1979年 | 17篇 |
1978年 | 8篇 |
1976年 | 10篇 |
1975年 | 11篇 |
1957年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Density functional method (DFT) (B3p86) of Gaussian98 has been used to optimize the structure of the Tc_2 molecule. The result shows that the ground state for Tc_2 molecule is an 11-multiple state and its electronic configuration is {}^{11}Σ_g^-, which shows the spin polarization effect of Tc_2 molecule of a transition metal element for the first time. Meanwhile, we have not found any spin pollution because the wavefunction of the ground state does not mingle with wavefunctions of higher energy states. So, that the ground state for Tc_2 molecule is an 11-multiple state is indicative of the spin polarization effect of Tc_2 molecule of a transition metal element: that is, there exist 10 parallel spin electrons. The non-conjugated electron is greatest in number. These electrons occupy different spacious tracks, so that the energy of Tc_2 molecule is minimized. It can be concluded that the effect of parallel spin of the Tc_2 molecule is larger than the effect of the conjugated molecule, which is obviously related to the effect of electron d delocalization. In addition, the Murrell--Sorbie potential functions with the parameters for the ground state {}^{11}Σ_g^- and other states of Tc_2 molecule are derived. Dissociation energy D_e for the ground state of T_{c2} molecule is 2.266eV, equilibrium bond length R_e is 0.2841nm, vibration frequency ω_e is 178.52cm^{-1}. Its force constants f_2, f_3, and f_4 are 0.9200aJ·nm^{-2}, --3.5700aJ·nm^{-3}, 11.2748aJ·nm^{-4} respectively. The other spectroscopic data for the ground state of Tc_2 molecule ω_eχ_e, B_e, α_e are 0.5523cm^{-1}, 0.0426cm^{-1}, 1.6331×10^{-4}cm^{-1} respectively. 相似文献
112.
113.
Zhang Xinliang Huang Dexiu Sun Junqiang Liu Deming 《Optical and Quantum Electronics》2004,36(7):627-634
A simple scheme for single to multi-channel wavelength conversion based on cross-gain modulation of amplified spontaneous emission (ASE) spectrum in semiconductor optical amplifier (SOA) is described. Single to 16-channel wavelength conversion at 10 Gb/s is first demonstrated without any additional probe lights, the modulation information carried by input signal could be converted into arbitrary many channels if only the demultiplexer with enough channels is exploited. Output performance and pattern effects are investigated experimentally. 相似文献
114.
115.
Mid-infrared generation based on a periodically poled LiNbO3 optical parametric oscillator 下载免费PDF全文
We report a nanosecond Nd:YVO_4-pumped optical parametric oscillator (OPO) based on periodically poled LiNbO_3 (PPLN). Tuning is achieved in this experiment by varying the temperature and period of the PPLN. The design of double-pass singly resonant oscillator (DSRO) and confocal cavity enables the OPO threshold to be lowered considerably, resulting in a simple, compact, all-solid-state configuration with the mid-infrared idler powers of up to 466mW at 3.41μm. 相似文献
116.
Nickel complexes 1–4 ligated with 2,9-disubstituted-1,10-phenanthroline were synthesized and characterized by FT-IR spectra and elemental analysis.
The molecular structure of complex 2 was confirmed by X-ray crystal diffraction analysis. Activated with methylaluminoxane (MAO), those complexes showed moderate
activities for ethylene oligomerization.
Published in Kinetika i Kataliz, 2007, Vol. 48, No. 5, pp. 710–714.
This article was submitted by the authors in English. 相似文献
117.
Pengbo Jiang Tiexin Cheng Yan Liu Xianghao Cui Yingli Bi Kaiji Zhen 《Reaction Kinetics and Catalysis Letters》2004,82(1):49-56
The novel phase transfer catalysts S-8 [4-(dimethyloctylammonium) propansultan] and DB-X [1,4-bis(triethylmethylammonium)benzene dibromide] were synthesized and employed for high conversion synthesis of dichlorocyclopropane from various olefins.This revised version was published online in December 2005 with corrections to the Cover Date. 相似文献
118.
119.
This Note addresses the derivation of an improved On-Surface Radiation Condition for the numerical solution of the exterior Helmholtz equation at high-frequencies. This condition is built as an approximation of the Neumann-to-Dirichlet map by using a local regularization of its principal classical symbol in the gliding zone for modelling the creeping waves. The numerical simulation of this pseudodifferential operator is efficiently realized with a linear cost according to the dimension of the boundary element approximation space using suitable complex Padé approximants. A numerical example is provided. To cite this article: X. Antoine et al., C. R. Acad. Sci. Paris, Ser. I 340 (2005). 相似文献
120.
L. C. Lew Yan Voon C. Galeriu M. Willatzen 《Physica E: Low-dimensional Systems and Nanostructures》2003,18(4):547-549
We argue that the two-dimensional elliptic quantum dot problem with finite barrier cannot be exactly solved, contrary to a recent assertion (van den Broek and Peeters, Physica E 11 (2001) 345. We also prove it explicitly by numerically calculating the correct energy spectrum. 相似文献