全文获取类型
收费全文 | 789篇 |
免费 | 22篇 |
国内免费 | 2篇 |
专业分类
化学 | 601篇 |
晶体学 | 14篇 |
力学 | 4篇 |
数学 | 76篇 |
物理学 | 118篇 |
出版年
2023年 | 4篇 |
2022年 | 8篇 |
2021年 | 22篇 |
2020年 | 30篇 |
2019年 | 18篇 |
2018年 | 10篇 |
2017年 | 6篇 |
2016年 | 17篇 |
2015年 | 16篇 |
2014年 | 8篇 |
2013年 | 41篇 |
2012年 | 39篇 |
2011年 | 53篇 |
2010年 | 35篇 |
2009年 | 15篇 |
2008年 | 51篇 |
2007年 | 37篇 |
2006年 | 52篇 |
2005年 | 35篇 |
2004年 | 38篇 |
2003年 | 28篇 |
2002年 | 27篇 |
2001年 | 10篇 |
2000年 | 8篇 |
1999年 | 5篇 |
1998年 | 6篇 |
1997年 | 10篇 |
1996年 | 10篇 |
1995年 | 7篇 |
1994年 | 4篇 |
1993年 | 5篇 |
1992年 | 4篇 |
1991年 | 3篇 |
1989年 | 10篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 9篇 |
1983年 | 17篇 |
1982年 | 15篇 |
1981年 | 10篇 |
1980年 | 9篇 |
1979年 | 9篇 |
1978年 | 11篇 |
1977年 | 7篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1974年 | 3篇 |
1971年 | 2篇 |
排序方式: 共有813条查询结果,搜索用时 15 毫秒
201.
Photoinduced Rydberg ionization (PIRI) spectra of the second excited electronic state of benzonitrile cation were recorded via the origin and 6a1 and 6b1 vibrational levels of the cation ground electronic state. This B<--X transition was verified to be a forbidden 2B2<--2B1 transition with an origin at 17,225 cm-1 above the ground ionic state. By the use of vibronic coupling calculations, as well as symmetry analysis and comparison of the PIRI spectra via different ground vibrational levels, a nearly complete assignment of the vibrational structure was made, and the vibrational frequencies of the B 2B2 state of benzonitrile cation were obtained based on the assignments. Comparisons of the experimental spectra with simulations from the vibronic structure calculations are also used to validate the theoretical procedures used in the simulations. 相似文献
202.
A method is described for the use of electronic structure and Franck-Condon factor programs in the calculation of the vibrational intensities in forbidden electronic transitions. Using the B 2B2-X 2B1 electronic transition of benzonitrile cation as a test case, transition moments were calculated using the symmetry adapted cluster/configuration interaction method at various points along the normal mode displacements of the molecule, from which transition moment derivatives were obtained. The transition moments were found to vary almost linearly with respect to the normal mode displacements. Using these, along with Franck-Condon factors, an expansion of the transition moment with respect to the normal coordinates provides a measure of vibrational intensities, including the effects of geometry change and Duschinsky rotation [Acta Physicochim. URSS 7, 551 (1937)]. Second order terms in the moment expansion are calculated, and it is determined that they must be included if the intensity of combination bands is to be properly obtained. 相似文献
203.
204.
A room acoustic diffuser breaks up reflected wavefronts, and this can be achieved by presenting a spatially varying surface impedance. In hybrid surfaces, varying impedance is achieved by patches of absorption and reflection, giving reflection coefficients nominally of 0 and 1. These surfaces are hybrids, absorbing some of the incident sound while diffusing any reflected energy. A problem with planar hybrid surfaces is that specular energy is only removed by absorption. By exploiting interference, by reflecting waves out-of-phase with the specular energy, it is possible to diminish the specular energy further. This can be achieved by using a diffuser based on a ternary sequence that nominally has reflection coefficients of 0, -1, and +1. Ternary sequences are therefore a way of forming hybrid absorber-diffusers that achieve better scattering performance without additional absorption. This paper discusses methods for making ternary sequence diffusers, including giving sequence generation methods. It presents prediction results based on Fourier and boundary element method models to examine the performance. While ternary diffusers have better performance than unipolar binary diffusers at most frequencies, there are frequencies at which the performances are the same. This can be overcome by forming diffusers from four-level, quadriphase sequences. 相似文献
205.
We report the observation of levels in the state of CH2 via optical–optical double resonance spectroscopy. Direct transitions between the lowest singlet state and the state are allowed by symmetry, but weak because they correspond to a two electron excitation in the single configuration approximation to the electronic wavefunction. The observed transitions involve sequential single photon absorptions at visible and near infrared wavelengths using state intermediate levels. Recent ab initio results (S.N. Yurchenko et al., J. Mol. Spectrosc. 208 (2001), 136) predicted the positions of some of the levels which are confirmed by the present results. The new spectra provide accurate energies for rotational levels in the , l = 0 level of the state. 相似文献
206.
Okuda M Suzumoto Y Iwahori K Kang S Uchida M Douglas T Yamashita I 《Chemical communications (Cambridge, England)》2010,46(46):8797-8799
We report here, for the first time, a biotemplated synthesis of uniform CdSe nanoparticle (4.1 ± 0.5 nm) and a fabrication of two-dimensional CdSe nanoparticles (over one micrometre) with nanometric gaps by cage-like protein, Listeria-Dps. 相似文献
207.
Benzimidazoles are the organic compounds investigated in this work. The experimental determination of the pKa values of protonated benzimidazoles in water is a challenge because of their low solubility. In addition, some derivatives are involved in tautomeric equilibria which increase the complexity of the theoretical pKa determinations. In the present study, different approaches are considered to develop a methodology for the accurate prediction of aqueous pKa values of protonated benzimidazoles at 298.15 K. We have considered different reaction schemes for approximating the acid dissociation equilibrium; two distinct equations are used for the calculation of pKa values, and a number of levels of theory and empirical corrections are applied in the process of working toward this aim. The best correlations between the experimental and calculated data are obtained at the B3LYP/6-31+G(d,p)-PCM(opt) level of theory. The predictive capabilities of the methodologies attempted are tested with two compounds that were not included in the set of benzimidazoles initially investigated. The direct calculations differ significantly from the expected values, but the pKa values calculated using the correlation equations are very similar and in reasonable agreement with the expected pKa values. 相似文献
208.
209.
Gardner T 《The Journal of the Acoustical Society of America》2003,113(3):1368-1378
The presence of occluded gas in inland lakes, harbor muds, and surficial marine sediments is well documented. Surficial gassy sediments cause underlying beds to be acoustically impenetrable to seismic surveys; therefore, the modeling of signal loss arising from mudline reflection and transmission absorption is of particular interest. The Anderson and Hampton [J. Acoust. Soc. Am. 67, 1890-1903 (1980)] model for attenuation in gassy sediments was evaluated against the physical and acoustical properties of eight laboratory silty clay soils containing different amounts of occluded gas in bubbles of 0.2- to 1.8-mm diameter. The model was shown to give good agreement with measured data over the lower frequencies of bubble resonance and above resonance. It did not agree with measured data at frequencies below resonance, for which the model did not simulate the bulk properties of the gassy soils. The Mackenzie [J. Acoust. Soc. Am. 32, 221-231 (1960)] model for reflection loss was also examined for the gassy soils. The maximum reflection losses of 6 dB, at a grazing angle of 40 degrees, does not wholly support speculation by Levin [Geophysics 27, 35-47 (1962)] of highly reflective pressure-release boundaries arising from substantial reflection and absorption losses in gassy sediments. It was found that mudlines formed from sediments with significant occluded gas may be successfully penetrated, although the substantial absorption loss arising from signal transmission through the sediment prevents penetration of the surficial layers to much beyond a meter in depth. 相似文献
210.
Nanoscale patches, created by nanografting a maleimide-terminated thiol into a self-assembled monolayer, were elaborated by sequential chemical reactions. Each stage in the nanofabrication was followed by atomic force microscopy (AFM), providing a controlled approach to the fabrication of novel three-dimensional (3D) surface nanostructures. 相似文献