全文获取类型
收费全文 | 42421篇 |
免费 | 2060篇 |
国内免费 | 205篇 |
专业分类
化学 | 21101篇 |
晶体学 | 344篇 |
力学 | 1064篇 |
数学 | 3267篇 |
物理学 | 7969篇 |
无线电 | 10941篇 |
出版年
2024年 | 60篇 |
2023年 | 439篇 |
2022年 | 550篇 |
2021年 | 1005篇 |
2020年 | 807篇 |
2019年 | 866篇 |
2018年 | 785篇 |
2017年 | 747篇 |
2016年 | 1325篇 |
2015年 | 1062篇 |
2014年 | 1500篇 |
2013年 | 2480篇 |
2012年 | 2785篇 |
2011年 | 3056篇 |
2010年 | 2006篇 |
2009年 | 2030篇 |
2008年 | 2795篇 |
2007年 | 2527篇 |
2006年 | 2468篇 |
2005年 | 2186篇 |
2004年 | 1975篇 |
2003年 | 1686篇 |
2002年 | 1524篇 |
2001年 | 1055篇 |
2000年 | 964篇 |
1999年 | 737篇 |
1998年 | 539篇 |
1997年 | 496篇 |
1996年 | 530篇 |
1995年 | 395篇 |
1994年 | 377篇 |
1993年 | 367篇 |
1992年 | 308篇 |
1991年 | 272篇 |
1990年 | 243篇 |
1989年 | 185篇 |
1988年 | 165篇 |
1987年 | 126篇 |
1986年 | 81篇 |
1985年 | 128篇 |
1984年 | 95篇 |
1983年 | 92篇 |
1982年 | 99篇 |
1981年 | 99篇 |
1980年 | 72篇 |
1979年 | 80篇 |
1978年 | 82篇 |
1977年 | 60篇 |
1976年 | 65篇 |
1975年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
The nuclear coordinate dependence of electronic transtion moments has been investigated for the purpose of finding new interpretations of deuterium isotope effects on spectral intensities and radiative decay rates in orbitally forbidden electronic transitions. By using “AO following nuclei” wavefunctions as the building block for the electronic wavefunction in the adiabatic BO vibronic wavefunction, the spin-free hamiltonian is diagonalized to generate eigenfunctions and eigen-energies. It is found that the electronic transtion moments based on these eigenfunctions show dependences upon the vibrational modes which are not directly involved in vibronic coupling. This leads to interpretations of the deuterium isotope effects in T1 → S0 radiative transitions of aromatic hydrocarbons and S0 → S1 absorption in pyrazine which are not based on the conventional Herzberg—Teller or non-BO coupling. 相似文献
942.
Xudong?Hu Gui-Bing?Zhao S.?V.?B.?Janardhan Garikipati Kim?Nicholas Stanislaw?F.?Legowski Maciej?RadoszEmail author 《Plasma Chemistry and Plasma Processing》2005,25(4):351-370
Laser-induced fluorescence (LIF) is an effective in-situ probe for NO concentrations below 300 ppm in a non-thermal plasma reactor. A new method has been developed to measure in-situ NO concentration in the reactor discharge region using a long-time—on the order of seconds—averaged fluorescence detection. This method, for quantifying NO concentration in a nonthermal plasma reactor, is simpler than a short-time—on the order of nanoseconds—fluorescence detection. For accurate measurement based on the new method, the LIF intensity must be close to the corona-induced fluorescence (CIF) intensity; the CIF intensity serves as a guide in selecting the LIF intensity. We find that a kinetic model proposed earlier works for two-tube reactors and represents the NO concentration in the middle of the reactor, which verifies the assumption of gas plug flow. 相似文献
943.
Interaction of electronically excited TCNB (tetracyanobenzone) and ground-state MB (methylated benzenes) leads to the formation of fluorescent exciplexes in the vapor phase. Studies of exciplex fluorescence as a function of excess vibrational energy in TCNB and of temperature and pressure of added buffer gas, lead to the conclusion that the low-frequency inter-molecular vibrations play a very important role in the radiationless deactivation of exciplexes. The results also indicate that collisional relaxation of low-frequency intermolecular vibrations proceeds much more rapidly than that of high-frequency intramolecular vibrations. 相似文献
944.
Hwang JY Choi HS Seo JS La HJ Kim DS Jeon HS Jeon MK Lee DH Gong YD 《The Journal of organic chemistry》2005,70(24):10151-10154
[reaction: see text] A library containing 1200 analogues of 2,6-difunctionalized 2-methyl-2H-1-benzopyran was constructed by using a solid-phase synthesis protocol. Polymer-bound 6-amido-, 6-sulfonamido-, and 6-uredo-functionalized 2-hydroxymethyl-2-methylbenzopyrans 10 were prepared as part of a first-generation diversification step by employing reactions of respective acid halides, sulfonyl chlorides, and isocyanates with the amine precursor 7. Transformations of the resin-bound intermediates 10 by reactions with alkyl and acid halides were then used to produce a diverse series of 2,6-difunctionalized 2-methyl-2H-1-benzopyran analogues 12 and 14. 相似文献
945.
The collision dynamics of the NO+Ne system is investigated in a molecular beam scattering experiment at a collision energy of 1055 cm(-1). Employing resonance enhanced multiphoton ionization of NO, we measured state-resolved integral and differential cross sections for the excitation to various levels of both spin-orbit manifolds. The dependence of the scattered intensity on the laser polarization is used to extract differential quadrupole moments for the collision induced angular momentum alignment. The set of cross section data is compared with results of a full quantum mechanical close coupling calculation using the set of ab initio potential energy surfaces of Alexander et al. [J. Chem. Phys. 114, 5588 (2001)]. In previous work, it was found that the positions and rotational substructures for the lowest bend-stretch vibrational states derived from these surfaces agree very well with the observed spectrum of the NO-Ne complex. For the same potential, we find that the calculated cross sections show a less satisfactory agreement with the experimental data. While the overall Jf dependence and magnitude of the integral and differential cross sections are in good agreement, noticeable discrepancies exist for the angle dependence of the differential cross sections. In general, the calculated rotational rainbow structures are shifted towards larger scattering angles indicating that the anisotropy of the potential is overestimated in the fit to the ab initio points or in the ab initio calculation itself. For most states, we find the measured alignment moments to be in excellent agreement with the results of the calculation as well as with predictions of sudden models. Significant deviations from the sudden models are observed only for those fine-structure changing collisions which are dominated by forward scattering. Results of the full quantum calculation confirm the deviations for these states. 相似文献
946.
Yoshihisa Kurasawa Tomoyoshi Hosaka Kazue Ikeda Yuko Matsumoto Aiko Ishikura Atsushi Takada Ho Sik Kim Yoshihisa Okamoto 《Journal of heterocyclic chemistry》1994,31(2):527-533
The 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 9–11 were synthesized by the reaction of the quinoxalines 6–8 with various p-substituted benzenediazonium salts. Compounds 9–11 showed the tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in dimethyl sulfoxide media. The substituent effect on the tautomer ratios of A to B was studied by the nmr spectroscopy to clarify that the presence of the ester group R2 on the hydrazone carbon and electron-donating p-substituent R1 on the side chain benzene ring exhibited a tendency to increase the ratios of the tautomer A . 相似文献
947.
A new Co(II) complex with tridentate Schiff base ligand with a N,N,O donor set has been synthesized and characterized by single X-ray technique and spectroscopic techniques. The thermochemical properties have been studied. X-ray structure analysis of the title compound Co(II)L2 [HL = 2-methyl-6-(2′-hydroxybenzylideneamino) pyridine] shows that the polyhedron around Co(II) is a distorted tetrahedron, and the self-assembly via intermolecular π-π interactions leads to a three-dimensional network. Crystal data for the title complex C26H22CoN4O2: Monoclinic, space group Ia, a = 11.741(7) Å, b = 8.149(5) Å, c = 22.764(16) Å, β = 98.530(12)°, V = 2154(2) Å3, Z = 4. 相似文献
948.
949.
950.
Kim SY Saxena A Kwak G Fujiki M Kawakami Y 《Chemical communications (Cambridge, England)》2004,(5):538-539
Cooperative amplification of the C-F...Si weak interaction between side chains and the main chain was found to afford rigid rodlike helical polysilanes with a preferential screw sense. 相似文献