全文获取类型
收费全文 | 496448篇 |
免费 | 3188篇 |
国内免费 | 908篇 |
专业分类
化学 | 242385篇 |
晶体学 | 7679篇 |
力学 | 25938篇 |
综合类 | 11篇 |
数学 | 67077篇 |
物理学 | 157454篇 |
出版年
2020年 | 4675篇 |
2019年 | 5460篇 |
2018年 | 7852篇 |
2017年 | 8064篇 |
2016年 | 10657篇 |
2015年 | 5404篇 |
2014年 | 9695篇 |
2013年 | 20282篇 |
2012年 | 15996篇 |
2011年 | 19034篇 |
2010年 | 14900篇 |
2009年 | 14836篇 |
2008年 | 18160篇 |
2007年 | 18058篇 |
2006年 | 16568篇 |
2005年 | 14618篇 |
2004年 | 13778篇 |
2003年 | 12648篇 |
2002年 | 12830篇 |
2001年 | 13083篇 |
2000年 | 10363篇 |
1999年 | 7835篇 |
1998年 | 7179篇 |
1997年 | 7058篇 |
1996年 | 6551篇 |
1995年 | 5921篇 |
1994年 | 6082篇 |
1993年 | 5820篇 |
1992年 | 6231篇 |
1991年 | 6549篇 |
1990年 | 6425篇 |
1989年 | 6321篇 |
1988年 | 6196篇 |
1987年 | 6167篇 |
1986年 | 5726篇 |
1985年 | 7244篇 |
1984年 | 7659篇 |
1983年 | 6497篇 |
1982年 | 6735篇 |
1981年 | 6438篇 |
1980年 | 6242篇 |
1979年 | 6584篇 |
1978年 | 6943篇 |
1977年 | 6939篇 |
1976年 | 7138篇 |
1975年 | 6611篇 |
1974年 | 6517篇 |
1973年 | 6792篇 |
1972年 | 5247篇 |
1971年 | 4563篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
M. Di Valentin A. Bisol G. Agostini G. Giacometti D. Carbonera 《Applied magnetic resonance》2006,30(3-4):555-576
A comparative electron paramagnetic resonance (EPR) study has been performed on a series of structurally related molecular triads which undergo photoinduced electron transfer and differ one from the other in terms of the acceptor or donor moieties. The molecular triads, C-P-C60, TTF-P-C60 and C-P-PF, share the same free-base, tetraarylporphyrin (P) as the primary electron donor, which after light excitation initiates the electron transfer process, but differ either in terms of the electron acceptor (fullerene derivative, C60, versus fluorinated free-base porphyrin, PF), or in terms of the final electron donor (carotenoid polyene, C, versus tetrathiafulvalene, TTF). All these molecular triads can be considered artificial photosynthetic reaction centers in their ability to mimic several key properties of the reaction center primary photochemistry. Photoinduced charge separation and recombination have been followed by time-resolved EPR in a glass of 2-methyltetrahydrofuran and in the nematic phase of the uniaxial liquid crystal E-7. All the triads undergo photoinduced electron transfer, with the generation of charge-separated states in both the low-dielectric environment of the 2-methyl-tetrahydrofuran glass and in anisotropic E-7 medium. Different photochemical pathways have been recognized depending on the specific donor and acceptor moieties constituting the molecular triads. In the presence of the tetrathiafulvalene electron donor singlet- and triplet-initiated electron transfer routes are concurrently active. Recombination to the low-lying carotenoid triplet state occurs in the carotene-based triads, while singlet recombination is the only active route for the TTF-P-C60 triad, where a low-lying triplet state is lacking. Long-lived charge separation has been observed in the case of TTF-P-C60: about 8 μs for the singlet-born radical pair in the glassy isotropic matrix and about 7 μs for the triplet-born radical pair in the nematic phase of E-7. For all the molecular triads, a weak exchange interaction (J?1 G) between the electrons in the final spin-correlated radical pair has been evaluated by simulation of the EPR spectra, providing evidence for superexchange electronic interactions mediated by the tetraarylporphyrin bridge. 相似文献
112.
Yu. V. Brezhnev 《Theoretical and Mathematical Physics》2002,133(3):1657-1662
We construct Dubrovin equations and trace formulas for finite-band operators with trigonal curves and present some examples. 相似文献
113.
We establish sufficient conditions for the existence and uniqueness of a periodic solution of a system of linear differential equations with a small parameter and a degenerate matrix of coefficients of derivatives in the case of a multiple spectrum of a boundary matrix pencil. We construct asymptotics of this solution. 相似文献
114.
In this Note, we study a 2×2 system of evolution equations with some codimension 3 crossing. We derive two conditions of non-degeneracy. We focus on one of them and reduce our system to some Landau–Zener's type system. Using this reduction, we describe the energy transfer at the crossing by Landau–Zener formula for 2-scales semi-classical measures. To cite this article: C. Fermanian Kammerer, P. Gérard, C. R. Acad. Sci. Paris, Ser. I 335 (2002) 915–920. 相似文献
115.
We obtain upper bounds for the tail distribution of the first nonnegative sum of a random walk and for the moments of the overshoot over an arbitrary nonnegative level if the expectation of jumps is positive and close to zero. In addition, we find an estimate for the expectation of the first ladder epoch. 相似文献
116.
117.
By using the method of generalized conjugation problems, we propose a numerical scheme for investigation of the redistribution of temperature stresses in a piecewise-homogeneous cylindrical shell caused by a longitudinal crack. This scheme is based on systems of integral equations (some of them are singular) to determine the unknown jumps of integral characteristics of the disturbed temperature field and displacements and their derivatives on the line of a crack and on the interface as well as the derivatives of these integral characteristics with respect to the longitudinal coordinate at the interface. 相似文献
118.
We investigate the linear stability of the Bickley jet in the framework of the beta-plane approximation. Because singular inviscid neutral modes exist in the retrograde case , it is necessary to add viscosity to interpret them. One of these modes was found in closed form by Howard and Drazin [1] . However, its critical point is at the center of the jet and it was therefore not possible for these authors to ascertain the relationship of this mode to the stability problem or to discuss how to continue the eigenfunction across the singularity.
The viscous critical layer problem associated with this singularity is considerably more difficult than the usual one (which leads to integrals of the Airy function) because and, consequently, a second-order turning point is involved. Our analysis shows that the Howard–Drazin mode is degenerate in the domain where it is valid as a limit of the viscous problem (wavenumber α2 ≤ 9/2 ), that is, it corresponds to both an odd and an even mode. This conclusion is confirmed by direct numerical solution of the Orr–Sommerfeld equation which shows, in addition, that viscosity is destabilizing along portions of the stability boundary. For a retrograde jet, instability is found to occur beyond the inviscid critical value of β, that is, in the region where the flow would be stable according to the Rayleigh–Kuo condition. 相似文献
The viscous critical layer problem associated with this singularity is considerably more difficult than the usual one (which leads to integrals of the Airy function) because and, consequently, a second-order turning point is involved. Our analysis shows that the Howard–Drazin mode is degenerate in the domain where it is valid as a limit of the viscous problem (wavenumber α
119.
Ricardo Celorrio Vı́ctor Domínguez Francisco-Javier Sayas 《Comptes Rendus Mathematique》2002,334(10):923-926
In this work we study the solution of Laplace's equation in a domain with holes by an iteration consisting of splitting the problem in an exterior one, around the holes, plus an interior problem in the unholed domain. We show the existence of a decomposition of the solution when the exterior problem is represented by means of a single-layer protential. Also, for the three-dimensional case and with some adjustments for the two-dimensional case, we prove convergence of the method by writing the iteration as a Jacobi iteration for an operator equation and studying the spectrum of the iteration operator. To cite this article: R. Celorrio et al., C. R. Acad. Sci. Paris, Ser. I 334 (2002) 923–926. 相似文献
120.