全文获取类型
收费全文 | 8508篇 |
免费 | 253篇 |
国内免费 | 62篇 |
专业分类
化学 | 5773篇 |
晶体学 | 84篇 |
力学 | 250篇 |
综合类 | 1篇 |
数学 | 1313篇 |
物理学 | 1402篇 |
出版年
2022年 | 69篇 |
2021年 | 81篇 |
2020年 | 155篇 |
2019年 | 112篇 |
2018年 | 84篇 |
2017年 | 94篇 |
2016年 | 186篇 |
2015年 | 151篇 |
2014年 | 194篇 |
2013年 | 395篇 |
2012年 | 456篇 |
2011年 | 556篇 |
2010年 | 281篇 |
2009年 | 223篇 |
2008年 | 493篇 |
2007年 | 456篇 |
2006年 | 443篇 |
2005年 | 447篇 |
2004年 | 428篇 |
2003年 | 285篇 |
2002年 | 308篇 |
2001年 | 89篇 |
2000年 | 84篇 |
1999年 | 64篇 |
1998年 | 79篇 |
1997年 | 99篇 |
1996年 | 139篇 |
1995年 | 85篇 |
1994年 | 69篇 |
1993年 | 96篇 |
1992年 | 68篇 |
1991年 | 81篇 |
1990年 | 77篇 |
1989年 | 58篇 |
1988年 | 71篇 |
1987年 | 61篇 |
1986年 | 54篇 |
1985年 | 113篇 |
1984年 | 131篇 |
1983年 | 91篇 |
1982年 | 113篇 |
1981年 | 108篇 |
1980年 | 92篇 |
1979年 | 82篇 |
1978年 | 108篇 |
1977年 | 93篇 |
1976年 | 80篇 |
1975年 | 67篇 |
1974年 | 67篇 |
1973年 | 58篇 |
排序方式: 共有8823条查询结果,搜索用时 31 毫秒
161.
Bogumil Zelent Józef Kuśba Ignacy Gryczynski Michael L. Johnson Joseph R. Lakowicz 《Journal of fluorescence》1993,3(3):199-207
We examined the time-dependent intensity decays ofN-acetyl-l-tryptophanamide (NATA) when collisionally quenched by acrylamide in propylene glycol over a range of temperatures. The intensity decays of NATA became increasingly heterogeneous in the presence of acrylamide. The NATA intensity decays were not consistent with the Collins-Kimball radiation boundary condition (RBC) model for quenching. The steady-state Stern-Volmer plots show significant upward curvature, and quenching of NATA by acrylamide was observed even in vitrified propylene glycol, where translational diffusion cannot occur during the lifetime of the excited state. These frequencydomain and steady-state data indicate a through-space quenching interaction between NATA and acrylamide, and the results are consistent with a rate constant for quenching that depends exponentially on the fluorophore-quencher separation distance. The exponential distance-dependent rate of quenching also explains the upward curvature of the Stern-Volmer plot, and the steady-state data aid in determining the interaction distance between NATA and acrylamide. These results suggest that the distance-dependent quenching rates need to be considered in the interpretation of acrylamide quenching of proteins. 相似文献
162.
Summary In earlier works, the gauge theorem was proved for additive functionals of Brownian motion of the form
0
t
q(B
s
)ds, whereq is a function in the Kato class. Subsequently, the theorem was extended to additive functionals with Revuz measures in the Kato class. We prove that the gauge theorem holds for a large class of additive functionals of zero energy which are, in general, of unbounded variation. These additive functionals may not be semi-martingales, but correspond to a collection of distributions that belong to the Kato class in a suitable sense. Our gauge theorem generalizes the earlier versions of the gauge theorem.Research supported in part by NSA grant MDA-92-H-30324 相似文献
163.
Ingo Althöfer Gautam Das David Dobkin Deborah Joseph José Soares 《Discrete and Computational Geometry》1993,9(1):81-100
Given a graphG, a subgraphG' is at-spanner ofG if, for everyu,v V, the distance fromu tov inG' is at mostt times longer than the distance inG. In this paper we give a simple algorithm for constructing sparse spanners for arbitrary weighted graphs. We then apply this algorithm to obtain specific results for planar graphs and Euclidean graphs. We discuss the optimality of our results and present several nearly matching lower bounds.The work of G. Das and D. Joseph was supported by NSF PYI Grant DCR-8402375. The work of D. Dobkin was supported by NSF Grant CCR-8700917. The work of J. Soares was supported by CNPq proc 203039/87.4 (Brazil) and NSF Grant CCR-9014562. This research was accomplished while G. Das was a student at the University of Wisconsin-Madison. A preliminary version was presented at the Second Scandinavian Workshop on Algorithm Theory, Bergen, Norway, 1990, under the title Generating Sparse Spanners for Weighted Graphs, and proceedings appear in the series Lecture Notes in Computer Science, Springer-Verlag. The preliminary version also appears as Princeton University Technical Report CS-TR-261-90, and as University of Wisconsin-Madison Computer Sciences Technical Report 882. 相似文献
164.
Mohamed Brakta Denis Sinou Michel Becchi Joseph Banoub 《Journal of mass spectrometry : JMS》1992,27(5):621-624
A series of anomeric unsaturated C-glycosides were studied by negative ion mass spectrometry following keV ion bombardment The compounds bearing an acidic hydrogen show a prominent [M ? H]? peak; in the other cases, the [M ? H]? peak is of very low intensity. A retro-Diels–Alder fragmentation and the abstraction of a hydrogen atom from the aglycon residue are observed in all cases. 相似文献
165.
[reaction: see text] In this Letter, we describe a general method for preparing the dianions of cyclopropene carboxylic acids, and we show that their subsequent reactions with electrophiles provide a general means for selectively introducing diverse types of functional groups. This provides a general method for the synthesis of chiral 1,2-disubstituted cyclopropenes, and opens new avenues for the enantioselective preparation of cyclopropenes. 相似文献
166.
Ray K Badugu R Lakowicz JR 《The journal of physical chemistry. C, Nanomaterials and interfaces》2007,111(19):7091-7097
Metal-Enhanced Fluorescence (MEF) has become an important method in biomedical sensing. In this paper, we present the distance-dependent MEF of sulforhodamine B (SRB) monolayer on silver island films (SIFs). SRB is electrostatically incorporated into the Langmuir-Blodgett (LB) layers of octadecylamine (ODA) deposited on glass and SIFs substrates. The distances between SRB and SIFs or glass surfaces are controlled by depositing a varied number of inert stearic acid (SA) spacer layers. SRB is incorporated into positively charged LB layers of ODA by immersing the ODA deposited substrates into aqueous solution of SRB. Dye incorporated ODA layers with 10 nm separation distance from the SIFs surface show maximum metal-enhanced fluorescence intensity; ~7-fold increase in intensity as compared to that from the glass surface. The corresponding enhancement factor is reduced with increasing or decreasing the probe distance from the SIFs surface. Additionally, SRB on SIF surfaces show reduced lifetimes. We observed the shortest lifetime from the SRB with 5 nm distance from the SIF surfaces and the lifetime increased consistently with increasing the distances between the fluorophore and the SIFs surface. These observed spectral changes, increase in fluorescence intensity and decreased fluorescence lifetimes, are in accordance with the expected effects due to near-field interactions between the silver nanoparticles and fluorophores. We have also analyzed the complex fluorescence heterogeneous decays on metallic nanostructured surfaces using continuous distributions of decay times. The decay-time distributions appear to be sensitive to the distance between the metal and fluorophore and represent the underlying heterogeneity of the samples. The present systematic study provides significant information on the effect of fluorophore distance on the metal-enhanced fluorescence phenomenon. 相似文献
167.
David A. Place Gerald P. Ferrara Joseph J. Harland James C. Dabrowiak 《Journal of heterocyclic chemistry》1980,17(3):439-443
Using a metal template reaction, a series of [14]azaannulenes formed from a β-diketone and an aromatic diamine in the presence of nickel(II) have been synthesized. Attempted cyclizations with bulky β-diketones and certain substituted diamines showed that the cyclization is sensitive to steric effects. At least one of the nickel complexes readily reacts with a series of nucleophilic reagents to give methine-substituted products. 相似文献
168.
In this Letter, it is demonstrated that the unusual reactivity of cyclopropenes can increase the scope and utility of intermolecular Pauson-Khand reactions. The well-defined chiral environment of cyclopropenes has a powerful influence on the diastereoselectivity of the reactions and leads to the production of a single cyclopentenone in each of the described cases. The cyclopropane ring strongly influences the stereochemistry of reactions at the enone, and the three-membered ring can subsequently be cleaved under mild conditions. [reaction: see text] 相似文献
169.
The cyclen based aromatic diaza-15-crown-5 and 18-crown-6 ether conjugates 1Tb-4Tb were designed as luminescent switches for sodium and potassium where the delayed Tb(III) emission, occurring as line-like emission bands between 490-622 nm, was 'switched on' upon recognition of these ions in pH 7.4 buffered water solution. 相似文献
170.
Naphthoquinone , a fully functionalized model for intermediates in ansamycin synthesis, was prepared by the intramolecular Claisen condensation of benzofuran followed by oxidation of the novel tricyclic compound . 相似文献