全文获取类型
收费全文 | 185304篇 |
免费 | 2675篇 |
国内免费 | 823篇 |
专业分类
化学 | 102039篇 |
晶体学 | 2528篇 |
力学 | 7910篇 |
综合类 | 7篇 |
数学 | 20501篇 |
物理学 | 55817篇 |
出版年
2021年 | 1109篇 |
2020年 | 1259篇 |
2019年 | 1282篇 |
2018年 | 1276篇 |
2017年 | 1282篇 |
2016年 | 2582篇 |
2015年 | 2184篇 |
2014年 | 2758篇 |
2013年 | 8332篇 |
2012年 | 6887篇 |
2011年 | 8759篇 |
2010年 | 5300篇 |
2009年 | 5397篇 |
2008年 | 8060篇 |
2007年 | 8062篇 |
2006年 | 7772篇 |
2005年 | 7369篇 |
2004年 | 6593篇 |
2003年 | 5653篇 |
2002年 | 5480篇 |
2001年 | 6211篇 |
2000年 | 4736篇 |
1999年 | 3635篇 |
1998年 | 2777篇 |
1997年 | 2808篇 |
1996年 | 2806篇 |
1995年 | 2437篇 |
1994年 | 2327篇 |
1993年 | 2173篇 |
1992年 | 2492篇 |
1991年 | 2447篇 |
1990年 | 2156篇 |
1989年 | 2122篇 |
1988年 | 2154篇 |
1987年 | 2098篇 |
1986年 | 1977篇 |
1985年 | 3013篇 |
1984年 | 2963篇 |
1983年 | 2378篇 |
1982年 | 2625篇 |
1981年 | 2461篇 |
1980年 | 2468篇 |
1979年 | 2379篇 |
1978年 | 2445篇 |
1977年 | 2412篇 |
1976年 | 2307篇 |
1975年 | 2257篇 |
1974年 | 2157篇 |
1973年 | 2233篇 |
1972年 | 1253篇 |
排序方式: 共有10000条查询结果,搜索用时 217 毫秒
101.
102.
Richard F. W. Bader 《International journal of quantum chemistry》1994,49(3):299-308
The physics of a system is determined by a variation of the action integral, i.e., by a variation of the space–time volume integral of the Lagrange function. If one demands that the properties of an atom in a molecule be derived from physics, the atom must generate its own space–time volume, requiring that its boundaries be defined in real space. The variations in the action are related to the actions of generators of infinitesimal unitary transformations. In the general case, the action integral is altered by generators acting in both the spacelike and timelike surface bounding the space–time volume, whereas for a total isolated system, the physics is totally determined by their action in just the spacelike surfaces at the two time endpoints. It is shown and illustrated for a one-dimensional system that the definition of an atom corresponds to the possibility of choosing a subsystem in such a way that the contributions to the change in action resulting from the evolution in time of its spatial boundaries vanishes identically. The properties of these subsystems and of the total system of which they are a part are, therefore, determined by one and the same action principle. This choice of subsystem corresponds to the possibility of augmenting the Lagrange function by the divergence of the gradient of the electron density a step that, while leaving the equations of motion unchanged, modifies the generating operators in the required manner. © 1994 John Wiley & Sons, Inc. 相似文献
103.
104.
105.
106.
A. J. Maroulis P. D. Akrivos C. P. Hadjiantoniou-Maroulis 《Journal of heterocyclic chemistry》1993,30(4):913-916
Oxidation of the title compounds yields, besides the reported isoimides 3 and/or the amides 4 , also the imides 5 . The observed product dichotomy is considered as the result of an intramolecular nucleophilic attack on the aroyl group, of the pressumed zwitterionic intermediate 2 , by O or N present in the ambident N-aroylimine site of 2 . The results of AM1 calculations agree with the product studies and both permit the formulation of a set of rules correlating structure and selectivity. 相似文献
107.
J.V.P. de OliveiraA.V. Cardona M.T. VilhenaR.C. Barros 《Journal of Quantitative Spectroscopy & Radiative Transfer》2002,73(1):55-62
We describe a semi-analytical numerical method for coherent isotropic scattering time-dependent radiative transfer problems in slab geometry. This numerical method is based on a combination of two classes of numerical methods: the spectral methods and the Laplace transform (LTSN) methods applied to the radiative transfer equation in the discrete ordinates (SN) formulation. The basic idea is to use the essence of the spectral methods and expand the intensity of radiation in a truncated series of Laguerre polynomials in the time variable and then solve recursively the resulting set of “time-independent” SN problems by using the LTSN method. We show some numerical experiments for a typical model problem. 相似文献
108.
C. Daum L. Hertzberger W. Hoogland R. Jongerius S. Peters P. Van Deurzen V. Chabaud A. Gonzalez-Arroyo B. Hyams H. Tiecke P. Weilhammer A. Dwurazny G. Polok M. Rozanska K. Rybicki M. Turala J. Turnau G. Ascoli H. Backer G. Blanar M. Cerrada H. Dietl J. Gallivan M. Glaubmann R. Klanner E. Lorenz G. Lütjens G. Lutz W. Männer U. Stierlin I. Blakey M. Bowler R. Cashmore J. Loken W. Spalding G. Thompson B. Alper C. Damerell A. Gillman C. Hardwick M. Hotchkiss F. Wickens ACCMOR Collaboration 《Zeitschrift fur Physik C Particles and Fields》1981,10(2):95-100
Muon electron pairs were detected in an Al multiplate spark chamber, exposed to a neutrino beam from the CERN PS. The leptons were not accompanied by other particles, except occasionally by protons. The background came mainly from muon associated π0 production, with one decay gamma lost. It was determined empirically, together with the small contribution from υ e reactions. For electron energies above 2 GeV the background is 5.7±1.5 events, whereas 18 (μe)-candidates have been observed. Hence the effect is established, with a rate of about 10?4 as compared to the muonic reactions above 3 GeV. Charm creation as the origin of this (μe)-production process is excluded; heavy neutral lepton production does not fit the kinematics observed. Instead the events are compatible with the two-body decay of an object with variable invariant mass of order 1 GeV, possibly resulting from axion interactions. 相似文献
109.
110.