全文获取类型
收费全文 | 845篇 |
免费 | 31篇 |
国内免费 | 10篇 |
专业分类
化学 | 492篇 |
晶体学 | 2篇 |
力学 | 26篇 |
数学 | 255篇 |
物理学 | 111篇 |
出版年
2023年 | 9篇 |
2022年 | 8篇 |
2021年 | 14篇 |
2020年 | 10篇 |
2019年 | 18篇 |
2018年 | 19篇 |
2017年 | 20篇 |
2016年 | 24篇 |
2015年 | 31篇 |
2014年 | 25篇 |
2013年 | 68篇 |
2012年 | 43篇 |
2011年 | 41篇 |
2010年 | 55篇 |
2009年 | 43篇 |
2008年 | 60篇 |
2007年 | 55篇 |
2006年 | 48篇 |
2005年 | 43篇 |
2004年 | 43篇 |
2003年 | 36篇 |
2002年 | 27篇 |
2001年 | 21篇 |
2000年 | 13篇 |
1999年 | 13篇 |
1998年 | 14篇 |
1997年 | 6篇 |
1996年 | 13篇 |
1995年 | 4篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 2篇 |
1991年 | 10篇 |
1990年 | 4篇 |
1988年 | 5篇 |
1986年 | 4篇 |
1984年 | 2篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 3篇 |
1970年 | 1篇 |
1968年 | 1篇 |
1966年 | 2篇 |
1937年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有886条查询结果,搜索用时 15 毫秒
91.
Several different models have been proposed to explain the origin of the complex anti-hole features observed in hole-burned (HB) spectra of excitonically coupled systems such as photosynthetic complexes. This lack of consensus presents a serious constraint on the interpretation of HB spectra and the underlying electronic structures of these systems. To resolve this problem we present results of modeling studies of non-resonant HB spectra taking uncorrelated excitation energy transfer and excitonic interactions into account. Simplified analytical results are compared with Monte Carlo simulations in which excitonic interactions are explicitly taken into account in order to disentangle a number of distinct effects. It is shown that these effects can accurately account for both hole shapes and the broad anti-hole structure observed in excitonically coupled systems. We argue that these models will provide a necessary framework for probing the electronic structure of these systems via HB spectroscopy. 相似文献
92.
Peter Ferenc Peter Solár Ján Kleban Jaromír Mikeš Peter Fedoročko 《Journal of photochemistry and photobiology. B, Biology》2010,98(1):25-34
Presented experiment considers combination of genistein and photodynamic therapy with hypericin with a view to achieve higher therapeutic outcome in human breast adenocarcinoma cell lines MCF-7 and MDA-MB-231, both identified in our conditions as photodynamic therapy resistant. Since genistein is known to suppress Bcl-2 expression, we predicted that photodynamic therapy with hypericin might benefit from mutual therapeutic combination. In line with our expectations, combined treatment led to down-regulation of Bcl-2 and up-regulation of Bax in both cell lines as well as to suppression of Akt and Erk1/2 phosphorylation induced by photoactivated hypericin in MCF-7 cells. Although Akt and Erk1/2 phosphorylation was not stimulated by photodynamic therapy with hypericin in MDA-MB-231 cells, it was effectively suppressed in combination. Variations in cell death signaling favoring apoptosis were indeed accompanied by cell cycle arrest in G2/M-phase, activation of caspase-7, PARP cleavage and increased occurrence of cells with apoptotic morphology of nucleus. All these events corresponded with suppression of proliferation and significantly lowered clonogenic ability of treated cells. In conclusion, our results indicate that pre-treatment with tyrosine kinase inhibitor genistein may significantly improve the effectiveness of photodynamic therapy with hypericin in MCF-7 and MDA-MB-231 breast cancer cells. 相似文献
93.
We have studied the potential degradation of poly(lactic acid)-based fabrics treated with commercial softeners and stored under two sets of conditions for one year. Initial wet-processing caused a fall in molecular weight of about 28%, irrespective of after-treatment. Storage at 40 °C and 80% RH produced further degradation which, with few exceptions, was aggravated by the presence of softeners. Ultimately, all samples degraded beyond the point of commercial usefulness. No clear distinction could be made between the effects of softeners having differing compositions. In contrast, fabrics stored under milder conditions of 23 °C and 50% RH showed no significant time-dependent polymer degradation, irrespective of the treatment applied. There were slight changes in tensile properties and some evidence of physical structural effects having occurred, which we attribute to physical aging. However, we do not believe these to be so serious as to call into question the long-term viability of PLA-based textile products. 相似文献
94.
95.
Min Liu Mike Ronk Da Ren Judy Ostovic Nina Cauchon Zhaohui Sunny Zhou Janet Cheetham 《Journal of chromatography. A》2010,1217(22):3598-3611
A hydrophilic interaction liquid chromatography (HILIC) method was used to separate a commonly used pharmaceutical starting material, 4-aminomethylpyridine (4-AMP), and its degradants. The structures of the major degradants were characterized and elucidated without prior isolation by accurate mass measurement, MS/MS analysis and on-line hydrogen/deuterium (H/D) exchange experiments. The mass spectra obtained from H/D exchange experiments are particularly useful to differentiate structural isomers, to elucidate the fragmentation pathways, and to aid in structure elucidation in the absence of MS/MS fragmentation information. The impact of deuterium oxide and temperature on HILIC separation has also been explored here. The integration of H/D exchange with HILIC has been described here for the first time and has been demonstrated to be a powerful structure elucidation tool via the study of degradants in 4-AMP. 相似文献
96.
Zeidler A Salmon PS Fischer HE Neuefeind JC Simonson JM Lemmel H Rauch H Markland TE 《Physical review letters》2011,107(14):145501
The method of oxygen isotope substitution in neutron diffraction is introduced as a site specific structural probe. It is employed to measure the structure of light versus heavy water, thus circumventing the assumption of isomorphism between H and D as used in more traditional neutron diffraction methods. The intramolecular and intermolecular O-H and O-D pair correlations are in excellent agreement with path integral molecular dynamics simulations, both techniques showing a difference of ?0.5% between the O-H and O-D intramolecular bond distances. The results support the validity of a competing quantum effects model for water in which its structural and dynamical properties are governed by an offset between intramolecular and intermolecular quantum contributions. 相似文献
97.
Computer-generated holograms displayed by phase-modulating spatial light modulators have become a well-established tool for beam shaping purposes in holographic optical tweezers. Still, the generation of light intensity patterns with high spatial symmetry and simultaneously without interfering ghost traps is a challenge. We have implemented an iterative Fourier transform algorithm that is capable of controlling these ghost traps and demonstrate the benefit of this approach in the experiment. 相似文献
98.
The synthesis of a series of 3-desoxyanthocyanidins via a BF3·Et2O-mediated aldol condensation between a substituted acetophenone and salicylaldehyde derivatives is described. Using this new synthetic methodology the synthesis of tricetanidin, a natural pigment isolated from black tea, was accomplished. 相似文献
99.
Lower and upper bounds are given for the the number of permutations of length n generated by two stacks in series, two stacks in parallel, and a general deque. 相似文献
100.