全文获取类型
收费全文 | 1017篇 |
免费 | 21篇 |
国内免费 | 6篇 |
专业分类
化学 | 770篇 |
晶体学 | 9篇 |
力学 | 6篇 |
数学 | 111篇 |
物理学 | 148篇 |
出版年
2023年 | 6篇 |
2022年 | 10篇 |
2021年 | 16篇 |
2020年 | 16篇 |
2019年 | 16篇 |
2018年 | 10篇 |
2017年 | 9篇 |
2016年 | 26篇 |
2015年 | 18篇 |
2014年 | 23篇 |
2013年 | 59篇 |
2012年 | 53篇 |
2011年 | 70篇 |
2010年 | 36篇 |
2009年 | 35篇 |
2008年 | 73篇 |
2007年 | 75篇 |
2006年 | 66篇 |
2005年 | 62篇 |
2004年 | 56篇 |
2003年 | 40篇 |
2002年 | 28篇 |
2001年 | 14篇 |
2000年 | 20篇 |
1999年 | 14篇 |
1998年 | 4篇 |
1997年 | 7篇 |
1996年 | 19篇 |
1995年 | 17篇 |
1994年 | 10篇 |
1993年 | 10篇 |
1992年 | 10篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 11篇 |
1988年 | 14篇 |
1987年 | 16篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 2篇 |
1975年 | 4篇 |
1957年 | 1篇 |
排序方式: 共有1044条查询结果,搜索用时 15 毫秒
121.
Ground-state rotational energy distributions of N2 molecules produced in pure and He-seeded supersonic expansions have been determined by measurements of the N+2 first negative band rotational line intensities produced by 800 eV electron impact on cooled pure and He-seeded N2 supersonic beams. Sufficient spectral resolution was employed to resolve completely both P and R branches of the first negative bands. Rotational state distributions were obtained to much higher values of J than in previous investigations. The data show that at 800 eV, the electric dipole selection rule, |ΔJ| = 1, is consistent with the observed N+2 emission bands and that the rotational energy distributions produced in the cooled, supersonic beam are non-Boltzmann with a large population in the first few rotational states followed by a long, high-energy fail to quite high J values. 相似文献
122.
In the Langevin formalism, the delicate balance maintained between the fluctuations in the system and their corresponding dissipation may be upset by the presence of a secondary, space-dependent stochastic force, particularly in the low-friction regime. In prior work, the latter was dissipated self-consistently through an additional uniform (mean-field) friction [T. Shepherd and R. Hernandez, J. Chem. Phys. 115, 2430 (2001).] An alternative approach to ensure that equipartition is satisfied relies on the use of a space-dependent friction while ignoring nonlocal correlations. The approach is evaluated with respect to its ability to maintain constant temperature for two simple one-dimensional, stochastic potentials of mean force wherein the friction can be evaluated explicitly when there is no memory in the barriers. The use of a space-dependent friction is capable of providing qualitatively similar results to those obtained previously, but in extreme cases deviations from equipartition may be observed due to the neglect of the memory effects present in the stochastic potentials. 相似文献
123.
A 3 x 14 matrix of substituted N-aryl-1,8-naphthalimides was synthesized for the evaluation and discovery of dual fluorescence. Because of their unique photophysical properties, these dual fluorescent systems represent an exception to the widely studied TICT (Twisted Internal Charge Transfer) fluorescent dyes or tautomeric benzofluorescein class of two-color dyes. The matrix library was designed to investigate the effects of heterocycles, particularly pi-excessive and pi-deficient systems. Of the 42 compounds surveyed, five displayed well-resolved two-color emission in solvents as nonpolar as hexane. Based on the observed trends in fluorescence lambda(max) and quantum yield, a new model is proposed that predicts LW and SW emission for these systems. In addition, this model provides potential design features for the synthesis of new dual fluorescent species. 相似文献
124.
125.
126.
Marshall LJ Williams JR Almond MJ Atkinson SD Cook SR Matthews W Mortimore JL 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(1-2):233-241
Three ochre samples (A (orange-red in colour), B (red) and C (purple)) from Clearwell Caves, (Gloucestershire, UK) have been examined using an integrated analytical methodology based on the techniques of IR and diffuse reflectance UV-visible-NIR spectroscopy, X-ray diffraction, elemental analysis by ICP-AES and particle size analysis. It is shown that the chromophore in each case is haematite. The differences in colour may be accounted for by (i) different mineralogical and chemical composition in the case of the orange ochre, where higher levels of dolomite and copper are seen and (ii) an unusual particle size distribution in the case of the purple ochre. When the purple ochre was ground to give the same particle size distribution as the red ochre then the colours of the two samples became indistinguishable. An analysis has now been completed of a range of ochre samples with colours from yellow to purple from the important site of Clearwell Caves. 相似文献
127.
B. Lionel Funt Wendy Severs Arnold Glasel 《Journal of polymer science. Part A, Polymer chemistry》1976,14(11):2763-2771
An electroanalytical technique has been utilized in a new method for the study of reactive intermediates in polymerization reactions. A ring-disk electrode system generated persistent carbocation radicals whose stability decreased in the order: 1,3,6,8-tetraphenylpyrene (TPP), rubrene (Ru), 9,10-diphenylanthracene (DPA), and 9,10-dimethylanthracene (DMA). Radical cations from these parent compounds flowed to a collecting ring which was controlled potentiostatically to reduce unreacted cations. When styrene or isobytyl vinyl ether was added to the solution, the concentration of carbocation radicals reaching the electrode was reduced. Current collection efficiencies N were determined as a function of rotation speed ω for each monomer concentration. Plots of N?1 as ω?1 in the absence of monomer show no dependence on ω (indicative of stable intermediates), but a linear dependence is found with each concentration of monomer. This indicates a first-order dependence on radical cation concentration. The rate constants show a trend in cation reactivities which is in agreement with that obtained by other methods. This technique, however, extends the range of investigation to a much shorter time scale. 相似文献
128.
129.
Andrew S. Kende Michel Benechie Dennis P. Curran Pawel Fludzinski Wendy Swenson Jon Clardy 《Tetrahedron letters》1979,20(47):4513-4516
The condensation of certain ketone and ester enolates with trichloroethylene proceeds with surprising ease to yield dichlorovinylation products. The stereochemistry for one such product is established by X-ray, and subsequent transformations of these initial products to ethinyl or ω-chloroethinyl derivatives is described. 相似文献
130.