全文获取类型
收费全文 | 545篇 |
免费 | 26篇 |
国内免费 | 5篇 |
专业分类
化学 | 390篇 |
晶体学 | 3篇 |
力学 | 2篇 |
数学 | 38篇 |
物理学 | 143篇 |
出版年
2022年 | 7篇 |
2021年 | 6篇 |
2020年 | 12篇 |
2019年 | 10篇 |
2018年 | 4篇 |
2017年 | 4篇 |
2016年 | 25篇 |
2015年 | 13篇 |
2014年 | 24篇 |
2013年 | 37篇 |
2012年 | 35篇 |
2011年 | 38篇 |
2010年 | 23篇 |
2009年 | 17篇 |
2008年 | 33篇 |
2007年 | 27篇 |
2006年 | 29篇 |
2005年 | 18篇 |
2004年 | 32篇 |
2003年 | 19篇 |
2002年 | 12篇 |
2001年 | 9篇 |
2000年 | 11篇 |
1999年 | 7篇 |
1998年 | 6篇 |
1997年 | 5篇 |
1996年 | 9篇 |
1995年 | 12篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 5篇 |
1989年 | 2篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1985年 | 3篇 |
1984年 | 3篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1975年 | 5篇 |
1973年 | 2篇 |
1938年 | 2篇 |
1929年 | 2篇 |
1927年 | 1篇 |
1903年 | 1篇 |
排序方式: 共有576条查询结果,搜索用时 15 毫秒
81.
82.
We introduce a new implementation of time-dependent density-functional theory which allows the entire spectrum of a molecule or extended system to be computed with a numerical effort comparable to that of a single standard ground-state calculation. This method is particularly well suited for large systems and/or large basis sets, such as plane waves or real-space grids. By using a superoperator formulation of linearized time-dependent density-functional theory, we first represent the dynamical polarizability of an interacting-electron system as an off-diagonal matrix element of the resolvent of the Liouvillian superoperator. One-electron operators and density matrices are treated using a representation borrowed from time-independent density-functional perturbation theory, which permits us to avoid the calculation of unoccupied Kohn-Sham orbitals. The resolvent of the Liouvillian is evaluated through a newly developed algorithm based on the nonsymmetric Lanczos method. Each step of the Lanczos recursion essentially requires twice as many operations as a single step of the iterative diagonalization of the unperturbed Kohn-Sham Hamiltonian. Suitable extrapolation of the Lanczos coefficients allows for a dramatic reduction of the number of Lanczos steps necessary to obtain well converged spectra, bringing such number down to hundreds (or a few thousands, at worst) in typical plane-wave pseudopotential applications. The resulting numerical workload is only a few times larger than that needed by a ground-state Kohn-Sham calculation for a same system. Our method is demonstrated with the calculation of the spectra of benzene, C(60) fullerene, and of chlorophyll a. 相似文献
83.
Leonori F Petrucci R Segoloni E Bergeat A Hickson KM Balucani N Casavecchia P 《The journal of physical chemistry. A》2008,112(7):1363-1379
A detailed investigation of the dynamics of the reactions of ground- and excited-state carbon atoms, C(3P) and C(1D), with acetylene is reported over a wide collision energy range (3.6-49.1 kJ mol-1) using the crossed molecular beam (CMB) scattering technique with electron ionization mass spectrometric detection and time-of-flight (TOF) analysis. We have exploited the capability of (a) generating continuous intense supersonic beams of C(3P, 1D), (b) crossing the two reactant beams at different intersection angles (45, 90, and 135 degrees ) to attain a wide range of collision energies, and (c) tuning the energy of the ionizing electrons to low values (soft ionization) to suppress interferences from dissociative ionization processes. From angular and TOF distribution measurements of products at m/z=37 and 36, the primary reaction products of the C(3P) and C(1D) reactions with C2H2 have been identified to be cyclic (c)-C3H + H, linear (l)-C3H + H, and C3 + H2. From the data analysis, product angular and translational energy distributions in the center-of-mass (CM) system for both the linear and cyclic C3H isomers as well as the C3 product from C(3P) and for l/c-C3H and C3 from C(1D) have been derived as a function of collision energy from 3.6 to 49.1 kJ mol-1. The cyclic/linear C3H ratio and the C3/(C3 + c/l-C3H) branching ratios for the C(3P) reaction have been determined as a function of collision energy. The present findings have been compared with those from previous CMB studies using pulsed beams; here, a marked contrast is noted in the CM angular distributions for both C3H- and C3-forming channels from C(3P) and their trend with collision energy. Consequently, the interpretation of the reaction dynamics derived in the present work contradicts that previously proposed from the pulsed CMB studies. The results have been discussed in the light of the available theoretical information on the relevant triplet and singlet C3H2 ab initio potential energy surfaces (PESs). In particular, the branching ratios for the C(3P) + C2H2 reaction have been compared with the available theoretical predictions (approximate quantum scattering calculations and quasiclassical trajectory calculations on ab initio triplet PESs and, very recent, statistical calculations on ab initio triplet PESs as well as on ab initio triplet/singlet PESs including nonadiabatic effects, that is, intersystem crossing). While the experimental branching ratios have been corroborated by the statistical predictions, strong disagreement has been found with the results of the dynamical calculations. The astrophysical implications of the present results have been noted. 相似文献
84.
Paquet V Volmer AA Carreira EM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(8):2465-2481
Novel cationic amphotericin B derivatives as highly potent antifungal agents are reported. These semi-synthetic derivatives of amphotericin B were elaborated through a series of modifications both on the nitrogen atom of the mycosamine and on the C-16 carboxylic acid moiety. The antifungal activity of the new conjugates was tested against Saccharomyces cerevisiae and also against nine different strains of Candida albicans and Candida glabrata, including an amphotericin resistant strain. High potency was observed in the case of polyamine derivatives bearing two 3-aminopropyl chains on the mycosamine. The evaluation of the biological properties also included the determination of the hemolytic activity of the compounds by measuring the EH50 values. 相似文献
85.
Yu SC Borchert A Kuhn H Ivanov I 《Chemistry (Weinheim an der Bergstrasse, Germany)》2008,14(23):7066-7071
Starting from low toxic salicyloylglycine, a new seleninic acid anhydride 7 that lacks SeN or SeO non-bonded interactions was synthesized. This compound exhibits a fourfold higher glutathione peroxidase-like (GPx-like) activity than ebselen and inhibits plant and mammalian 12/15-lipoxygenases at lower micromolar concentrations. Because of these pharmacological properties, 7 may constitute a new lead compound for the development of anti-inflammatory low-molecular-weight seleno-organic compounds. Analyzing the redox products of 7 with glutathione (GSH) and tBuOOH, we identified three potential catalytic cycles (A, B, C) of GPx-like activity that are interconnected by key metabolites. To study the relative contribution of these cycles to the catalytic activity, we prepared selected reaction intermediates and found that the activity of seleninic acid anhydride 7 and of the corresponding diselenide 11 and selenol 14 compounds were in the same range. In contrast, the GPx-like activity of monoselenide 9 was more than one order of magnitude lower. These data suggested that cycles A and B may constitute the major routes of GPx-like activity of 7, whereas cycle C may not significantly contribute to catalysis. 相似文献
86.
Ronny Klæboe Astrid H. Amundsen Christian Madshus Karin M. Norén-Cosgriff Iiris Turunen-Rindel 《Applied Acoustics》2016
Rock blasting may cause disturbances, fear, and annoyance in residential and community areas affected by such activities. These community reactions can be quite strong, even when the blasting activities and the resulting vibrations are unlikely to cause physical damage to building foundations or buildings. A socio-vibrational survey was undertaken to assess residential reactions to blasting activities. Vibration velocities were obtained for 520 respondent dwellings located in seven study areas, and compared to the residents’ assessments of environment quality. Even at low vibration values, many people report annoyance. Exposure–effect relationships with acceptable statistical error bands were obtained. The level of annoyance from long-term blasting activities (quarry blasting) was not higher than from finite periods of more intense blasting activities (road and rail tunnels). Providing information in advance of the blasting activities, can reduce community reactions. Self-reported sensitivity to vibrations was associated with significantly increased annoyance. Sensitivity to vibrations was uncorrelated with exposure to vibrations. Sensitivity to noise and sensitivity to vibration were moderately correlated. 相似文献
87.
88.
Gemma E Hulme AN Jahnke A Jin L Lyon M Müller RM Uhrín D 《Chemical communications (Cambridge, England)》2007,(26):2686-2688
Efficient functionalisation of the non-reducing end of uronic acid derivatives and glycosaminoglycan-derived disaccharides using peptide coupling has been achieved, mediated by the water-soluble agent DMT-MM. 相似文献
89.
The present work has for the first time described and verified a theoretical model of the analytical extraction process electro-membrane extraction (EME), where target analytes are extracted from an aqueous sample, through a thin layer of 2-nitrophenyl octylether immobilized as a supported liquid membrane (SLM) in the pores in the wall of a porous hollow fibre, and into an acceptor solution present inside the lumen of the hollow fibre by the application of an electrical potential difference. The mathematical model was based on the Nernst-Planck equation, and described the flux over the SLM. The model demonstrated that the magnitude of the electrical potential difference, the ion balance of the system, and the absolute temperature influenced the flux of analyte across the SLM. These conclusions were verified by experimental data with five basic drugs. The flux was strongly dependent of the potential difference over the SLM, and increased potential difference resulted in an increase in the flux. The ion balance, defined as the sum of ions in the donor solution divided by the sum of ions in the acceptor solution, was shown to influence the flux, and high ionic concentration in the acceptor solution relative to the sample solution was advantageous for high flux. Different temperatures also led to changes in the flux in the EME system. 相似文献
90.
I. Peter W. von Oertzen H.G. Bohlen A. Gadea B. Gebauer J. Gerl M. Kaspar I. Kozhoukharov T. Kröll M. Rejmund C. Schlegel S. Thummerer H.J. Wollersheim 《The European Physical Journal A - Hadrons and Nuclei》1999,4(4):313-317
One and two neutron transfer has been measured in the heaviest asymmetric nuclear system with semi magic nuclei showing superfluid
properties, in 206Pb+118Sn collisions at an energy well below the Coulomb barrier with scattering orbits covering the largest angles. Particle-γ coincidence
techniques using 5 Euroball-Cluster detectors (EB) combined in a set-up with the Heidelberg-Darmstadt NaI-Crystal Ball (CB)
have been used. Transfer channels are identified with EB via their known γ-decays of the lowest excited states. Using the
unique feature of the set-up with the CB, transfer to well defined final states with known quantum numbers (without feeding)
are selected using the high efficiency multiplicity filter of the CB (no second γ-ray). The data are analysed using the semiclassical approach and transfer probabilities are obtained. The enhancement
for the two-neutron transfer populating the low lying superfluid 2+ state in 120Sn (and 116Sn), while the Pb-branch is in the groundstate is deduced by comparison with the strongest single neutron transfer transition.
Large enhancements (EF ≃ 103) are observed. This is the first direct measurement of enhancement for a heavy nuclear binary system with experimentally
separated levels suggesting a strong contribution from superfluid pair transfer.
Received: 18 December 1998 / Revised version: 23 February 1999 相似文献