全文获取类型
收费全文 | 2956篇 |
免费 | 138篇 |
国内免费 | 9篇 |
专业分类
化学 | 2123篇 |
晶体学 | 3篇 |
力学 | 46篇 |
数学 | 427篇 |
物理学 | 504篇 |
出版年
2023年 | 26篇 |
2022年 | 23篇 |
2021年 | 51篇 |
2020年 | 59篇 |
2019年 | 68篇 |
2018年 | 37篇 |
2017年 | 32篇 |
2016年 | 89篇 |
2015年 | 113篇 |
2014年 | 107篇 |
2013年 | 166篇 |
2012年 | 229篇 |
2011年 | 221篇 |
2010年 | 127篇 |
2009年 | 117篇 |
2008年 | 150篇 |
2007年 | 161篇 |
2006年 | 174篇 |
2005年 | 150篇 |
2004年 | 170篇 |
2003年 | 121篇 |
2002年 | 96篇 |
2001年 | 56篇 |
2000年 | 32篇 |
1999年 | 37篇 |
1998年 | 41篇 |
1997年 | 39篇 |
1996年 | 25篇 |
1995年 | 38篇 |
1994年 | 39篇 |
1993年 | 19篇 |
1992年 | 25篇 |
1991年 | 22篇 |
1990年 | 12篇 |
1989年 | 19篇 |
1988年 | 10篇 |
1987年 | 8篇 |
1985年 | 7篇 |
1984年 | 7篇 |
1982年 | 10篇 |
1980年 | 6篇 |
1979年 | 24篇 |
1978年 | 14篇 |
1977年 | 12篇 |
1976年 | 8篇 |
1974年 | 6篇 |
1973年 | 12篇 |
1970年 | 6篇 |
1964年 | 6篇 |
1957年 | 6篇 |
排序方式: 共有3103条查询结果,搜索用时 15 毫秒
11.
The trisilanol 1,3,5‐(HOi‐Bu2Si)3C6H3 ( 7 ), prepared in three steps from 1,3,5‐tribromobenzene via the intermediates 1,3,5‐(Hi‐Bu2Si)3C6H3 ( 8 ) and 1,3,5‐(Cli‐Bu2Si)3C6H3 ( 9 ) forms an equimolar complex with trans‐bis(4‐pyridyl)ethylene (bpe), 7 ·bpe, whose structure was investigated by X‐ray crystallography. The hydrogen‐bonded network features a number of SiO? H(H)Si and SiO? H hydrogen bridges. Evidence was found for cooperative strengthening within the sequential hydrogen bonds. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
12.
We demonstrate significant enhancement of four-wave mixing in coherently driven mercury isotopes to generate vacuum-ultraviolet radiation at 125 nm. The enhancement is accomplished by preparation of the mercury atoms in a state of maximum coherence, i.e. maximum nonlinear-optical polarization, driven by Stark-chirped rapid adiabatic passage (SCRAP). In this technique, a pump laser at 313 nm excites the two-photon transition between the ground state 6s21S0 and the target state 7s 1S0 in mercury. A strong, off-resonant radiation field at 1064 nm generates dynamic Stark shifts. These Stark shifts serve to induce a rapid adiabatic passage process on the two-photon transition. During the process a coherent superposition of the two states is established, which enhances the nonlinear-optical polarization in the medium to the maximum possible value. The maximum coherence permits efficient four-wave mixing of a pump laser and an additional probe laser at 626 nm. The efficiency is further enhanced, as the SCRAP process allows to stimulate the complete set of different mercury isotopes to participate in the frequency conversion process. This enlarges the effective atomic density of the medium. Thus, we observe the generation of vacuum-ultraviolet radiation at 125 nm enhanced by more than one order of magnitude with respect to conventional frequency conversion. Parallel to the frequency conversion process, we monitored the evolution of the population in the medium by laser-induced fluorescence. These data demonstrate efficient coherent population transfer by SCRAP. 相似文献
13.
14.
Frank Abild-Pedersen Ole Lytken Jakob Engbk Gunver Nielsen Ib Chorkendorff Jens K. Nrskov 《Surface science》2005,590(2-3):127-137
We have, theoretically and experimentally, investigated the dissociation of methane on the terraces and steps of a Ni(1 1 1) surface. Using Density Functional Theory (DFT) total energy calculations combined with Ultra High Vacuum (UHV) experiments, we find that the steps exhibit a higher activity than the terraces. We have, furthermore, investigated how carbon and sulfur present on the surface will deactivate the steps, leaving only the terraces active. We find the intrinsic sticking probabilities of methane on the steps and terraces at 500 K to be 2.8 × 10−7 for the steps and 2.1 × 10−9 for the terraces, in complete agreement with our calculated difference in activation energy of 17 kJ/mol. 相似文献
15.
16.
17.
The sound propagation into a courtyard shielded from direct exposure is predicted using an equivalent sources approach. The problem is simplified into that of a two-dimensional city canyon. A set of equivalent sources are used to couple the free half-space above the canyon to the cavity inside the canyon. Atmospheric turbulence causes an increase in the expected value of the sound pressure level compared to a homogeneous case. The level increase is estimated using a von Kármán turbulence model and the mutual coherences of all equivalent sources' contributions. For low frequencies the increase is negligible, but at 1.6 kHz it reaches 2-5 dB for the geometries and turbulence parameters used here. A comparison with a ray-based model shows reasonably good agreement. 相似文献
18.
19.
Merkx M Kopp DA Sazinsky MH Blazyk JL Müller J Lippard SJ 《Angewandte Chemie (International ed. in English)》2001,40(15):2725
The cover picture shows in the background the whole cell of a methanotrophic bacterium on which are superimposed components of methane monooxygenase (the structure of the hydroxylase component (top), one of the two four-helix bundles that house the catalytic diiron centers (left)) and a schematic diagram of the catalytic cycle by which the enzyme converts dioxygen and methane into methanol and water. More about this unusual enzyme system is reported by Lippard et al. on p. 2782 ff. 相似文献
20.