全文获取类型
收费全文 | 5689篇 |
免费 | 90篇 |
国内免费 | 724篇 |
专业分类
化学 | 4363篇 |
晶体学 | 10篇 |
力学 | 8篇 |
综合类 | 1篇 |
物理学 | 2121篇 |
出版年
2024年 | 11篇 |
2023年 | 116篇 |
2022年 | 93篇 |
2021年 | 100篇 |
2020年 | 89篇 |
2019年 | 148篇 |
2018年 | 116篇 |
2017年 | 174篇 |
2016年 | 158篇 |
2015年 | 152篇 |
2014年 | 214篇 |
2013年 | 279篇 |
2012年 | 488篇 |
2011年 | 484篇 |
2010年 | 338篇 |
2009年 | 473篇 |
2008年 | 455篇 |
2007年 | 467篇 |
2006年 | 422篇 |
2005年 | 294篇 |
2004年 | 246篇 |
2003年 | 157篇 |
2002年 | 152篇 |
2001年 | 109篇 |
2000年 | 89篇 |
1999年 | 114篇 |
1998年 | 80篇 |
1997年 | 94篇 |
1996年 | 48篇 |
1995年 | 49篇 |
1994年 | 51篇 |
1993年 | 48篇 |
1992年 | 49篇 |
1991年 | 38篇 |
1990年 | 29篇 |
1989年 | 13篇 |
1988年 | 11篇 |
1987年 | 17篇 |
1986年 | 9篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 1篇 |
1982年 | 6篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有6503条查询结果,搜索用时 15 毫秒
921.
Chemical effects on the K shell fluorescence yields of Fe, Mn, Co, Cr and Cu compounds were investigated. Samples were excited
using 59.5 keV energy photons from a241 Am radioisotope source. K X-rays emitted by samples were counted by a Si(Li) detector with a resolution 160 eV at 5.9 keV.
Chemical effects on the K shell fluorescence yields (ωk) for Fe, Mn, Co, Cr and Cu compounds were observed. The values are
compared with theoretical, semiempirical fit and experimental ones for the pure elements. 相似文献
922.
Wenjie Xie Yee-Loy Lam Yuen-Chuen Chan Siu-Chung Tam Jianhui Gu Feng Zhou Hongru Yang Gang Zhao 《Optics Communications》1999,170(4-6):265-268
We implement active feedback control in a Q-switched diode-pumped Nd:YVO4 laser by monitoring the fluorescence intensity from the laser crystal. When the initial inversion level indicated by the detected fluorescence has reached a predetermined value, Q-switching is initiated. The Q-switched pulse energy is stabler with our feedback scheme based on the fluorescence intensity than that with the conventional Q-switching when pumping source is not stable. 相似文献
923.
Summary 1,3- and 1,4-Bis-dipyrrin-benzenes and their various zinc and difluoroboryl chelates are prepared. Their absorption and fluorescence spectra are compared with those of the corresponding mono-dipyrrin derivatives. The conformations of these compounds are analyzed using the ring current model and absorption spectra. Dipole-dipole type transfer of excitation energy is discussed to represent a main photophysical channel in these compounds. 相似文献
924.
Jun Zhe Min Toshimasa Toyooka Hiroki Kawanishi Takeshi Fukushima Masaru Kato 《Analytica chimica acta》2005,550(1-2):173-181
Several fluorescent asparaginyl-N-acetyl-d-glucosamines (Asn-GlcNAcs), i.e., DBD-Asn-GlcNAc, NBD-Asn-GlcNAc, NDA-Asn-GlcNAc, PS-Asn-GlcNAc, FITC-Asn-GlcNAc, DMEQ-Asn-GlcNAc and DBD-PZ-Boc-Asn-GlcNAc, were synthesized as the acceptors for the resolution of oligosaccharides in glycopeptides. The resolution of oligosaccharides is based on the transglycosylation reaction with end-β-N-acetylglucosaminidase (Endo-M), isolated from Mucor hiemalis. The synthesis of each fluorescent acceptor was carried out in a one-pot reaction of Asn-GlcNAc and the corresponding fluorescent tagging reagent. The transglycosylation reaction using Endo-M proceeds at RT in neutral phosphate buffer (pH 6.0) and reached maxima at around 30 min. When Fmoc-Asn-GlcNAc (acceptor) was enzymatically reacted with Disialo-Asn (donor) in the presence of Endo-M, the ratio of Disialo-Asn-Fmoc (transglycosylation product) was almost comparable with the decreasing ratio of Fmoc-Asn-GlcNAc. Therefore, the transglycosylation activity of Endo-M from Disialo-Asn (donor) and fluorescent-Asn-GlcNAc (acceptor) was calculated from the decreasing ratio of fluorescent-Asn-GlcNAc. The order was NDA-Asn-GlcNAc > DBD-Asn-GlcNAc Fmoc-Asn-GlcNAc > NBD-Asn-GlcNAc DMEQ-Asn-GlcNAc > DNS-Asn-GlcNAc > PS-Asn-GlcNAc > FITC-Asn-GlcNAc. On the other hand, the activity with a fluorescent acceptor (DBD-PZ-Boc-Asn-GlcNAc), labeled to a carboxylic acid group in the Asn residue, was the strongest among the synthesized acceptors. 相似文献
925.
926.
This work presents a simple, fast and sensitive method for the preconcentration and quantification of graphene quantum dots (GQDs) in aqueous samples. GQDs are considered an object of analysis (analyte) not an analytical tool which is the most frequent situation in Analytical Nanoscience and Nanotechnology. This approach is based on the preconcentration of graphene quantum dots on an anion exchange sorbent by solid phase extraction and their subsequent elution prior fluorimetric analysis of the solution containing graphene quantum dots. Parameters of the extraction procedure such as sample volume, type of solvent, sample pH, sample flow rate and elution conditions were investigated in order to achieve extraction efficiency. The limits of detection and quantification were 7.5 μg L−1 and 25 μg L−1, respectively. The precision for 200 μg L−1, expressed as %RSD, was 2.8%. Recoveries percentages between 86.9 and 103.9% were obtained for two different concentration levels. Interferences from other nanoparticles were studied and no significant changes were observed at the concentration levels tested. Consequently, the optimized procedure has great potential to be applied to the determination of graphene quantum dots at trace levels in drinking and environmental waters. 相似文献
927.
A novel fluorogenic receptor 1 was prepared by the reaction of thiosemicarbazide with 1-naphthaldehyde. Based on in situ-formed 1-Fe(III) complexes having the specific binding affinity for F− anions, this sensory system allows rapid recognition and quantitative detection of fluoride in neutral aqueous media in an ‘off-on’ fashion. The fluoride measurement method not only exhibits a low detection limit but also has strong anti-interference ability to common coexisting ions, as evidenced by competitive experiments. 相似文献
928.
Pradeep P. KapadiaJohn C. Widen Mackenzie A. MagnusDale C. Swenson F. Christopher Pigge 《Tetrahedron letters》2011,52(19):2519-2522
Two tetrapyridyl-substituted tetraphenylethylenes have been prepared via Suzuki coupling between tetrabromo tetraphenylethylene and 3- or 4-pyridine boronic acid. Both compounds exhibit aggregation-induced emission as determined by solid state fluorescence spectroscopy and solution phase fluorescence measurements performed in aqueous/organic solvent mixtures. Solution phase fluorescence was also found to be switchable as a function of pH. 3-Pyridyl-substituted tetraphenylethylene has been structurally characterized by X-ray crystallography. 相似文献
929.
Na’il Saleh Yaseen A. Al-SoudLeena Al-Kaabi Indrajit GhoshWerner M. Nau 《Tetrahedron letters》2011,52(41):5249-5254
A new coumarin derivative containing benzothiazole and piperazine substituents was synthesized. Preferential inclusion of the benzothiazole group, over the coumarin and piperazine groups, inside the cavity of the molecular container cucurbit[7]uril (CB7) was evidenced by using optical and NMR techniques. The binding constant of the new complex with CB7 is higher in its protonated forms (e.g., K = 2.8 × 106 M−1) than in its neutral forms, which led to an increase in the pKa value associated with protonation of the aza nitrogen on the benzothiazole ring of ca. 2.5 units. Such CB7-induced protonation disabled the photoinduced electron transfer (PET) in the included molecule, enhancing its coumarin fluorescence up to ca. 45-fold (pH 3.5, 410 nm). The results are discussed in the context of designing sensitive analytical tools for reversible monitoring of optically inactive analytes by competitive displacement experiments. 相似文献
930.
Pablo Wessig Robert WawrzinekKristian Möllnitz Elvira FeldbuschUwe Schilde 《Tetrahedron letters》2011,52(46):6192-6195
We report on synthesis and photophysical properties of a new class of fluorescent dyes. They are characterized by large Stokes-shifts, long fluorescence lifetimes in organic solvents and a pronounced dependency of the fluorescence lifetime on the solvent polarity. Also worthy of note is the high bleaching stability. To provide access to biochemical and medical applications a series of derivatives were prepared, which exhibit specific reactivity towards different biologically relevant functional groups (carboxylic acids, amines, maleimides, N-hydroxysuccinimide esters). Furthermore, two alkynes were prepared, which could be used in ‘Click’ chemistry. 相似文献