全文获取类型
收费全文 | 262861篇 |
免费 | 2032篇 |
国内免费 | 544篇 |
专业分类
化学 | 135276篇 |
晶体学 | 4261篇 |
力学 | 13086篇 |
综合类 | 11篇 |
数学 | 29219篇 |
物理学 | 83584篇 |
出版年
2021年 | 2573篇 |
2020年 | 2705篇 |
2019年 | 3308篇 |
2018年 | 4748篇 |
2017年 | 4792篇 |
2016年 | 6317篇 |
2015年 | 3129篇 |
2014年 | 5590篇 |
2013年 | 11202篇 |
2012年 | 9077篇 |
2011年 | 10599篇 |
2010年 | 8427篇 |
2009年 | 8615篇 |
2008年 | 9959篇 |
2007年 | 9796篇 |
2006年 | 8814篇 |
2005年 | 7768篇 |
2004年 | 7355篇 |
2003年 | 6750篇 |
2002年 | 6743篇 |
2001年 | 7290篇 |
2000年 | 5417篇 |
1999年 | 4136篇 |
1998年 | 3630篇 |
1997年 | 3588篇 |
1996年 | 3277篇 |
1995年 | 2881篇 |
1994年 | 2956篇 |
1993年 | 2899篇 |
1992年 | 3208篇 |
1991年 | 3359篇 |
1990年 | 3203篇 |
1989年 | 3344篇 |
1988年 | 3170篇 |
1987年 | 3126篇 |
1986年 | 2996篇 |
1985年 | 3712篇 |
1984年 | 3881篇 |
1983年 | 3242篇 |
1982年 | 3263篇 |
1981年 | 3151篇 |
1980年 | 2931篇 |
1979年 | 3311篇 |
1978年 | 3435篇 |
1977年 | 3614篇 |
1976年 | 3507篇 |
1975年 | 3254篇 |
1974年 | 3236篇 |
1973年 | 3325篇 |
1972年 | 2528篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The results of optical study and quantum-chemical simulation of a conjugated organic system, 2-cyclooctylamino-5-nitropyridine (COANP)-fullerene, performed to determine its potential for application as a limiter of visible and near-IR laser radiation and as a material for diffraction elements in systems for reversible recording of optical information, are presented. Complexation between a COANP molecule and fullerene is considered as one of the main mechanisms responsible for the corresponding properties of this system. For the first time, nonlinear optical characteristics of COANP-C60 and COANP-C70 systems are comparatively studied and the intermolecular interaction between a COANP molecule and fullerene is analyzed on the quantum-chemical level. 相似文献
992.
O. V. Andreeva V. G. Bespalov A. L. Pyait V. N. Sizov A. S. Cherkasov 《Optics and Spectroscopy》2004,96(2):149-156
The results of a complex investigation of the recording of dynamic holograms by pulsed radiation with a duration of 20 ns and 300 ps in fullerene-containing media based on porous glasses and polymethyl methacrylate matrices are reported. Spectral analysis of these media in the range 300–700 nm was performed. The efficiency of hologram recording was found to relate with the occurrence of a band at 330 nm in the absorption spectrum of a medium. It is shown that dynamic holograms can be recorded in the fullerene-containing media by nanosecond and picosecond pulses. In the latter case, holograms arise mainly due to a change in the electronic polarizability of fullerene molecules, which is almost inertialess. The results obtained can be used in the development of ultrafast switching devices based on dynamic holograms. 相似文献
993.
N. D. Kundikova V. G. Nikolaev N. R. Sadykov M. O. Sadykova 《Optics and Spectroscopy》2004,96(2):281-284
The Magnus effect in multimode fibers with triangular and parabolic refractive index profiles (RIPs) is simulated in the geometrical optics approximation. The calculations confirm the linear relation between the angle of rotation Δ? and the fiber length z. The results of calculations for a spiral path with constant radius are compared with the analytical solution obtained. For a fiber with a parabolic RIP, the value of Δ? obtained in this work is one-half the result obtained in the wave approach. 相似文献
994.
The diffraction of a plane wave incident on a thick hologram at the first and second Bragg angles in the process of nonlinear recording is analyzed using the coupled-wave equations. It is shown that, in this case, the two-wave approximation with the use of effective modulation amplitudes of the refractive index, n 1ef and n 2ef, can be applied. Analytical dependences of n 1ef and n 2ef on the modulation amplitudes of the refractive index n 1 and n 2 at the fundamental and doubled spatial frequencies are obtained and the criteria of their applicability are determined. The dependences found are used to calculate the parameters of gratings recorded in a photopolymeric material. 相似文献
995.
Computer processing of electronic absorption spectra of rare-earth elements in inorganic laser media
Algorithms and program modules are developed for calculation of the Judd-Ofelt parameters on modern computers. These programs can be used to perform numerical analysis of the energy absorption by rareearth ions in laser media. 相似文献
996.
N. A. Borisevich S. L. Dziarzhytski V. A. Povedailo D. L. Yakovlev 《Optics and Spectroscopy》2004,96(6):819-823
The spectra of the fluorescence excitation within the rotational contours of the bands of the pure electronic long-wavelength S 0-S 1 transitions of jet-cooled indole and carbazole molecules and their complexes with water are measured. For the carbazole-water complex, a contour with three maxima is registered, which is possibly related to the occurrence of two isomers, differing in a slight displacement of hydrogen between the nitrogen atom of the imine group of carbazole and the oxygen atom of the water molecule. The degrees of polarization of integral fluorescence upon excitation within the rotational contours of the S 0-S 1 electronic transition bands of the above molecules and their complexes with water are determined for the first time. The coincidence of the calculated (7.7%) and measured (7.3%) values of the degree of polarization upon excitation in the rotational Q branch of the b L 1-A electronic transition of indole confirms the accepted intramolecular orientation of the transition dipole moment at an angle of 38.3° with respect to the principal axis of inertia A. Upon excitation of indole, its complex with water, and carbazole into the P and R branches, the measured and calculated degrees of polarization are also close to each other and amount to 2–3%. This confirms the occurrence of contributions to the fluorescence polarization due to the rotations of the indole molecules around the principal axes of inertia A and C. 相似文献
997.
D. G. Gusev M. G. Martemyanov I. V. Soboleva T. V. Dolgova A. A. Fedyanin O. A. Aktsipetrov 《JETP Letters》2004,80(10):633-637
The resonance features of the third-harmonic generation have been observed in 1D coupled microcavities consisting of three Bragg reflectors and two identical half-wave layers of mesoporous silicon. The third-harmonic intensity increases by a factor of about 103 in the resonance of fundamental radiation with each of the modes of coupled microcavities. It has been shown that the resonance positions in the angular spectra of the third-harmonic intensity depend on the coupling between microcavities that is determined by the transmission of the intermediate Bragg reflector. In the framework of the transfer-matrix method with nonlinear sources, it has been shown that the basic mechanism of the enhancement of the third-harmonic generation in coupled microcavities based on porous silicon is the constructive interference of the partial third-harmonic waves that are generated by near-surface layers. 相似文献
998.
S. N. Molotkov 《JETP Letters》2004,80(8):563-567
A “collective” attack on the key is considered, and its connection with the classical capacity of a quantum communication channel is analyzed. It has been shown that the allowable error probability for legitimate users to which the secure key can be extracted is less than half the value for “translucent” eavesdropping and individual measurements. 相似文献
999.
A scheme of nuclear excitation by the ionic X-ray lines in laser plasma using two femtosecond laser pulses is proposed. The first pulse produces plasma with a given degree of ionization, allowing the X-ray line energies of the target ions to be tuned to resonance with the nuclear transition, while the second pulse generates hot electrons that are necessary for X-ray generation. 相似文献
1000.
The properties of high-energy ion beams (beamlets) observed in the boundary layer of the plasma sheet of the Earth’s magnetotail during short time intervals (1–2 min) have been considered. Beamlets are induced by nonlinear impulse accelerating processes occurring in the current sheet of the far regions of the geomagnetic tail. Then, moving toward the Earth along the magnetic field lines, they are detected in the magnetotail (in the plasma sheet boundary layer) and in the high-latitude part of the auroral zone in the form of short bursts of high-energy ions (with energies of several tens of keVs). The size of the localization region of the beamlets in the magnetotail and auroral zone has been determined by the epoch-superposition method, and it has been shown that beamlets are concentrated in a narrow region near the plasma sheet boundary, whose latitude size is no more than 0.8δ. This conclusion corroborates the theoretical prediction that the nonadiabatic resonant acceleration of ions occurs in a spatially localized region near the separatrix separating the open magnetic field lines and closed field lines, which contain the hot and isotropic plasmas of the plasma sheet. Based on the CLUSTER multisatellite measurements, the spatial structure of beamlets is analyzed and it has been found that the Alfvén wave arises due to the excitation of fire-hose instability at the instant of the exit of the ion beam from the current sheet to the high-latitude region of the far tail of the Earth’s magnetosphere. The longitudinal (along the magnetic field) and transverse sizes of a beamlet are estimated. It has been found that the beamlet is a dynamic plasma structure whose longitudinal size is several hundred times larger than its transverse size. 相似文献