全文获取类型
收费全文 | 24400篇 |
免费 | 1184篇 |
国内免费 | 1463篇 |
专业分类
化学 | 17323篇 |
晶体学 | 205篇 |
力学 | 475篇 |
综合类 | 11篇 |
数学 | 3857篇 |
物理学 | 5176篇 |
出版年
2023年 | 174篇 |
2022年 | 294篇 |
2021年 | 510篇 |
2020年 | 392篇 |
2019年 | 413篇 |
2018年 | 426篇 |
2017年 | 446篇 |
2016年 | 981篇 |
2015年 | 755篇 |
2014年 | 854篇 |
2013年 | 1921篇 |
2012年 | 1747篇 |
2011年 | 1909篇 |
2010年 | 1119篇 |
2009年 | 1287篇 |
2008年 | 1278篇 |
2007年 | 1194篇 |
2006年 | 1013篇 |
2005年 | 1000篇 |
2004年 | 1282篇 |
2003年 | 938篇 |
2002年 | 874篇 |
2001年 | 473篇 |
2000年 | 395篇 |
1999年 | 329篇 |
1998年 | 303篇 |
1997年 | 278篇 |
1996年 | 283篇 |
1995年 | 253篇 |
1994年 | 197篇 |
1993年 | 209篇 |
1992年 | 186篇 |
1991年 | 145篇 |
1990年 | 133篇 |
1989年 | 125篇 |
1988年 | 130篇 |
1987年 | 126篇 |
1986年 | 131篇 |
1985年 | 177篇 |
1984年 | 170篇 |
1983年 | 140篇 |
1982年 | 132篇 |
1981年 | 131篇 |
1980年 | 133篇 |
1979年 | 110篇 |
1978年 | 110篇 |
1977年 | 114篇 |
1976年 | 92篇 |
1974年 | 95篇 |
1973年 | 89篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
41.
The dynamics of multiphoton transitions in a two-level spin system excited by transverse microwave and longitudinal RF fields with the frequencies ωmw and ωrf, respectively, is analyzed. The effective time-independent Hamiltonian describing the “dressed” spin states of the “spin + bichromatic field” system is obtained by using the Krylov-Bogoliubov-Mitropolsky averaging method. The direct detection of the time behavior of the spin system by the method of nonstationary nutations makes it possible to identify the multiphoton transitions for resonances ω0 = ωmw + rωrf (ω0 is the central frequency of the EPR line, r = 1, 2), to measure the amplitudes of the effective fields of these transitions, and to determine the features generated by the inhomogeneous broadening of the EPR line. It is shown that the Bloch-Siegert shifts for multiphoton resonances at the inhomogeneous broadening of spectral lines reduce only the nutation amplitude but do not change their frequencies. 相似文献
42.
(S)-2-[2-(p-Tolylsulfinyl)phenyl]acetaldehyde reacts with different O-silylated ketenethioacetals in the presence of Yb(OTf)3 yielding β-hydroxythioesters in high yields and diastereoselectivities. The obtained compounds were readily transformed into β-hydroxyacids and their corresponding diols. These Mukaiyama aldol reactions are a direct evidence of the ability of the sulfinyl group to control 1,5- and 1,6-asymmetric induction processes. 相似文献
43.
We analyze the dynamics of an AdS5 braneworld with matter fields when gravity is allowed to deviate from the Einstein form on the brane. We consider exact five-dimensional warped solutions which are associated with conformal bulk fields of weight –4 and describe on the brane the following three dynamics: those of inhomogeneous dust, of generalized dark radiation, and of homogeneous polytropic dark energy. We show that, with modified gravity on the brane, the existence of such dynamical geometries requires the presence of non-conformal matter fields confined to the brane. 相似文献
44.
Dušan Hemzal 《Czechoslovak Journal of Physics》2006,56(12):1467-1479
In this article, general idea of focusing is studied within the framework of optics extension into general relativity (covariant optics). In a configuration of static spacetime, the general, mathematically rigorous treatment of rays, wavefronts and caustics
of spherical symmetry is presented, particularly with regard to problems of obtaining them within general relativity. An original
result is the aberration formulation to covariant optics, whose application is given in this paper; a particular solution
of Einstein equations is finally chosen to provide concrete, exact results of cluster focal length and its aberration structure.
In this way, a gravitational lensing situation is shown to be a true lens. 相似文献
45.
46.
A simple model consisting of two electronic levels and one vibrational mode (phonon) was theoretically studied. The electronic-vibrational interaction was linear in the vibrational displacement. The vibrational mode was taken in the harmonic approximation and was attached to the thermal bath formed by the ambient environment. The kinetic constants of the vibrational dissipation were of the second order in the vibrational-bath coupling and were taken in the Markovian limit. Although, depending on the parameters of the model, different curves of the non-radiative vibration-induced excited state decay were obtained, in general, three time intervals, corresponding to different physical behaviour, were found. In the short-time interval, small oscillations superimposed on the excited state decay were observed. They were determined by the vibrational frequency and influenced by electronic-vibrational coupling. In the middle-time interval, almost quasi-exponential decay was detected; its rate constant increased with stronger electronic-vibrational interaction and speed of vibrational relaxation. In the long-time interval, the decay was very slow and, under special conditions, even an asymptotic non-zero excited state population was observed. Its value increased with the strength of the off-diagonal electron-vibrational coupling. Links of the parameters of the model with quantum chemical terms were estimated. 相似文献
47.
Monika Goikoetxea María J. Barandiaran José M. Asua 《Journal of polymer science. Part A, Polymer chemistry》2007,45(24):5838-5846
The mechanisms involved in the formation of n‐butanol during the synthesis of butyl acrylate containing latices were investigated. The experimental results showed that neither the hydrolysis of butyl acrylate nor of the ester bond in the butyl acrylate segments of the polymer played a major role in the formation of n‐butanol, which was mainly generated from the polymer backbone, by transfer reactions to polymer chain followed by cyclization. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5838–5846, 2007 相似文献
48.
49.
50.
We investigate some topological properties of a normal functorH introduced earlier by Radul which is some functorial compactification of the Hartman-Mycielski construction HM. We prove
that the pair (H X, HMY) is homeomorphic to the pair (Q, σ) for each nondegenerated metrizable compactumX and each denseσ-compact subsetY. 相似文献