全文获取类型
收费全文 | 496篇 |
免费 | 43篇 |
国内免费 | 1篇 |
专业分类
化学 | 438篇 |
晶体学 | 2篇 |
力学 | 5篇 |
数学 | 29篇 |
物理学 | 66篇 |
出版年
2023年 | 8篇 |
2022年 | 7篇 |
2021年 | 10篇 |
2020年 | 31篇 |
2019年 | 10篇 |
2018年 | 5篇 |
2017年 | 4篇 |
2016年 | 29篇 |
2015年 | 27篇 |
2014年 | 19篇 |
2013年 | 27篇 |
2012年 | 36篇 |
2011年 | 37篇 |
2010年 | 27篇 |
2009年 | 18篇 |
2008年 | 44篇 |
2007年 | 36篇 |
2006年 | 34篇 |
2005年 | 36篇 |
2004年 | 17篇 |
2003年 | 12篇 |
2002年 | 10篇 |
2001年 | 5篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1985年 | 5篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1943年 | 2篇 |
1938年 | 2篇 |
1935年 | 1篇 |
1934年 | 1篇 |
1933年 | 5篇 |
排序方式: 共有540条查询结果,搜索用时 203 毫秒
521.
Dieter Schumann Emil Frese Klaus Praefcke Werner Knfel 《Journal of mass spectrometry : JMS》1975,10(7):527-536
The mass spectral fragmentation patterns of some diarylmethylene-bis-xanthates have been investigated and are compared with those for the 1,1,α,α-dibenzyl-thioacetals 1,1-diphenylmethane. The formation of a sulphenium cation is characteristic for the bis-xanthates. Their subsequent fragmentation is similar to that of thioesters. Also noteworthy is the appearance of ions having thio-ketone-like structures, except in the case of 1,1,α,α-diphenyl thioacetal 1,1-diphenylmethane. This is compared with the pyrolytic behaviour. The intensity of various metastable decompositions leading to the [benzophenone]+. ion in the first field free region depends mostly on the excess thermal energy. 相似文献
522.
Hisashi Yamada Kenji Iso Makoto Saito Hirohiko Hirasawa Natalie Fellows Hisashi Masui Kenji Fujito James S. Speck Steven P. DenBaars Shuji Nakamura 《固体物理学:研究快报》2008,2(2):89-91
InGaN/GaN‐based light emitting diodes (LEDs) grown on m ‐plane, a ‐plane and off‐axis between m ‐ and a ‐plane GaN bulk substrates were investigated. A smooth surface was obtained when a ‐plane substrate was applied; however, large amounts of defects were observed. Photoluminescence measurements of the LEDs with a well thickness of 2.5 nm revealed that all the LEDs showed the peak emission wavelength at 389 nm. The PL intensity of the a ‐plane LED is one order of magnitude lower than that of the m ‐plane LED. The a ‐plane LEDs showed significant lower electroluminescence output powers than m ‐plane LEDs, suggesting that excitons are trapped by the defects, which act as non‐radiative recombination centers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
523.
Frese D Ueberholz B Kuhr S Alt W Schrader D Gomer V Meschede D 《Physical review letters》2000,85(18):3777-3780
We describe a simple experimental technique which allows us to store a small and deterministic number of neutral atoms in an optical dipole trap. The desired atom number is prepared in a magneto-optical trap overlapped with a single focused Nd:YAG laser beam. Dipole trap loading efficiency of 100% and storage times of about one minute have been achieved. We have also prepared atoms in a certain hyperfine state and demonstrated the feasibility of a state-selective detection via resonance fluorescence at the level of a few neutral atoms. A spin relaxation time of the polarized sample of 4.2+/-0.7 s has been measured. Possible applications are briefly discussed. 相似文献
524.
Matrix components are known to significantly alter the ionization of a target analyte in ESI-based measurements particularly when working with complex biological samples. This issue however may be alleviated by extracting the analyte of interest from the original sample into a relatively simple matrix compatible with ESI mass-spectrometric analysis. In this article, we report a microfluidic device that enables such extraction of small peptide molecules into an ESI-compatible solvent stream significantly improving both the sensitivity and reproducibility of the measurements. The reported device realizes this analyte extraction capability based on the free-flow zone electrophoretic fractionation process using a set of internal electrodes placed across the width of the analysis channel. Employing lateral electric fields and separation distances of 75 V/cm and 600 µm, respectively, efficient extraction of the model peptide human angiotensin II was demonstrated allowing a reduction in its detection limit by one to three orders of magnitude using the ESI-MS method. The noted result was obtained in our experiments both for a relatively simple specimen comprising DNA strands and angiotensin II as well as for human serum samples spiked with the same model peptide. 相似文献
525.
Victor Hugo Catricala Fernandes Natalie de Barros Viera Luiza Beatriz Lima Zanini Aline de Fátima Silva Paula Pio de Oliveira Salem Marisi Gomes Soares Karen de Jesus Nicácio Ana Claudia Chagas de Paula Luciano Sindra Virtuoso Tiago Branquinho Oliveira Eliane de Oliveira Silva Danielle Ferreira Dias Daniela Aparecida Chagas-Paula 《Photochemistry and photobiology》2020,96(1):14-20
There is some evidence in the literature of the photocyclization reaction of Tagitinin C ( 1 ) to Tagitinin F ( 2 ). Compound 2 has high pharmacological potential, but it is not easy to obtain, while compound 1 is easily obtained from a widespread plant, Tithonia diversifolia. Among different reaction conditions monitored, one was found that allowed the cyclization of 1 into 2 in <15 min in a photo-dependent reaction. Scaling-up the photocyclization of the pure compound 1 into 2 demonstrated 100% yield, and the isolation of 2 from a UV-irradiated extract was eight-fold higher than the quantity isolated from the non-UV-irradiated extract. We were also able to better understand the process of photoconversion and determine methods to isolate and quantify these compounds, which are known for their important antitumoral activities among other important pharmacological properties. 相似文献
526.
The complexes [(dpdpm)Ni(2-NO3)2] (1), [(dpdpm)Ni(2-NO3)(1-NO3)(CH3CN)] (2), [(dpdpm)2Ni(1-NO3)(H2O)]NO3 (3), and [(dpdpm)2Ni(H2O)2][NO3]2 (4) (dpdpm = diphenyl(dipyrazolyl)methane, Ph2C(C3N2H3)2), have been prepared and characterized by IR and UV-vis-NIR spectroscopy and X-ray diffraction studies. X-ray studies have confirmed that complexes 1-4 all adopt variously distorted octahedral structures in the solid state, the largest distortions arising from the small bite-angle of the bidentate nitrate ligand in 1 and 2. Magnetic moment measurements indicate that these solids are paramagnetic with two unpaired electrons. The solution 1H NMR data show that the paramagnetism is maintained in solution. Absorption spectra of 1-4 show three main bands in the region of 350-1000 nm representing spin allowed (d-d) transitions from the ground state 3A2g to the excited states 3T2g, 3T1g(3F), and 3T1g(3P). A weak shoulder was also detected at about 700-800 nm in most spectra, representing spin-forbidden transitions 3A2g 1Eg. A comparison of the crystal field parameters 10Dq and B for 1-4 to the corresponding values for related complexes indicated that these parameters are fairly insensitive to structural variations within this family of complexes. The 10Dq/B ratios show greater variations, but no clear correlations are apparent between 10Dq/B and such structural features as the nature of ligator atoms (N:O ratio), the bonding mode of the nitrate ligand, or the overall charge. Complexes 1 (green) and 2 (blue) interconvert as a function of temperature (solutions and solid samples), concentration of CH3CN (solutions), or CH3CN vapor pressure (solid samples). 相似文献
527.
We introduce a formalism for handling general spaces of hierarchical tilings, a category that includes substitution tilings, Bratteli–Vershik systems, S-adic transformations, and multi-dimensional cut-and-stack transformations. We explore ergodic, spectral and topological properties of these spaces. We show that familiar properties of substitution tilings carry over under appropriate assumptions, and give counter-examples where these assumptions are not met. For instance, we exhibit a minimal tiling space that is not uniquely ergodic, with one ergodic measure having pure point spectrum and another ergodic measure having mixed spectrum. We also exhibit a 2-dimensional tiling space that has pure point measure-theoretic spectrum but is topologically weakly mixing. 相似文献
528.
Natalie Aisbett 《Discrete and Computational Geometry》2014,51(2):323-336
For any flag nestohedron, we define a flag simplicial complex whose f-vector is the γ-vector of the nestohedron. This proves that the γ-vector of any flag nestohedron satisfies the Frankl–Füredi–Kalai inequalities, partially solving a conjecture by Nevo and Petersen (Discrete Comput. Geom. 45:503–521, 2010). We also compare these complexes to those defined by Nevo and Petersen (Discrete Comput. Geom. 45:503–521, 2010) for particular flag nestohedra. 相似文献
529.
Klaas Vantournhout Thomas NeffHans Feldmeier Natalie JachowiczJan Ryckebusch 《Progress in Particle and Nuclear Physics》2011,66(2):271-275
We present a novel method to study the dynamics of bulk fermion systems such as the neutron star’s crust. By introducing periodic boundary conditions into Fermionic Molecular Dynamics, it becomes possible to examine the long-range many-body correlations induced by antisymmetrisation in bulk fermion systems. The presented technique treats the spins and the fermionic nature of the nucleons explicitly and permits investigating the dynamics of the system. Despite the increased complexity related to the periodic boundary conditions, the proposed formalism remains computationally feasible. 相似文献
530.
Caputo CB Vukotic VN Sirizzotti NM Loeb SJ 《Chemical communications (Cambridge, England)》2011,47(30):8545-8547
A new tetradentate, pyridine ligand with a rigid tetrahedral core can be prepared in good yield by a cross-coupling methodology. Two metal organic framework structures of Cu(II) with PtS-type topology having a carbon atom as the tetrahedral node have been characterized utilising this ligand. 相似文献