全文获取类型
收费全文 | 1914篇 |
免费 | 15篇 |
国内免费 | 11篇 |
专业分类
化学 | 1111篇 |
晶体学 | 15篇 |
力学 | 65篇 |
数学 | 442篇 |
物理学 | 307篇 |
出版年
2020年 | 16篇 |
2019年 | 16篇 |
2016年 | 18篇 |
2015年 | 14篇 |
2014年 | 23篇 |
2013年 | 101篇 |
2012年 | 43篇 |
2011年 | 56篇 |
2010年 | 43篇 |
2009年 | 54篇 |
2008年 | 60篇 |
2007年 | 57篇 |
2006年 | 83篇 |
2005年 | 71篇 |
2004年 | 55篇 |
2003年 | 63篇 |
2002年 | 47篇 |
2001年 | 43篇 |
2000年 | 48篇 |
1999年 | 30篇 |
1998年 | 45篇 |
1997年 | 38篇 |
1996年 | 50篇 |
1995年 | 26篇 |
1994年 | 35篇 |
1993年 | 26篇 |
1992年 | 30篇 |
1991年 | 29篇 |
1990年 | 20篇 |
1989年 | 24篇 |
1988年 | 21篇 |
1987年 | 22篇 |
1986年 | 17篇 |
1985年 | 38篇 |
1984年 | 35篇 |
1983年 | 33篇 |
1982年 | 30篇 |
1981年 | 17篇 |
1980年 | 46篇 |
1979年 | 19篇 |
1978年 | 34篇 |
1977年 | 41篇 |
1976年 | 27篇 |
1975年 | 20篇 |
1974年 | 39篇 |
1973年 | 34篇 |
1972年 | 21篇 |
1971年 | 21篇 |
1969年 | 29篇 |
1968年 | 15篇 |
排序方式: 共有1940条查询结果,搜索用时 15 毫秒
131.
P.H. Cannington Norman S. Ham 《Journal of Electron Spectroscopy and Related Phenomena》1983,31(2):175-179
Band intensifies in the He(I) and HE(II) photoelectron spectra of some simple organic molecules (methanol, methylamine, formaldehyde, acetaldehyde, formic acid, acetic acid, methyl formate, ethylene, butadiene and methyl isothiocyanate) have been measured. The relative band intensity ratios for a number of ionizations from n and π orbitals are presented. 相似文献
132.
133.
The molecular structures and the electronic spectra of the geometric isomers of a model carotenoid polyene, 6,11-dimethylhexadecaheptaene, were calculated. It was concluded that solvent effects and conformational isomerization must be taken into account in order to satisfactorily explain the observed spectra. Molecular structures were calculated using molecular mechanics (MM2), and electronic spectra using the VESCF-MO-CI method including all singly-excited configurations. A method based on the calculated and observed spectra of simple linear polyenes was devised to estimate the solvent effects. 相似文献
134.
A criticism sometimes made of dynamic programming is that in deterministic problems, optimal decisions are calculated which are never needed, as the decisions relate to states which never arise. In this paper we describe how some of these "redundant" calculations have been used, in a certain problem, to derive a working rule of general validity.The problem concerns the installation and replacement of an item of electrical equipment at a large number of sites, to meet a known load at each site which increases exponentially at a known rate. 相似文献
135.
This paper was originally presented under the title "Decision" at the Third Operational Research Conference of the Universities of Lancaster and Sussex held at Windermere in April 1968. In it, the question of what is meant by "improved decision-making" is examined and the ways in which operational research can and does contribute to this are discussed. In the course of this the "American School" of Decision Analysis is reviewed. The paper concludes with some suggestions on computational procedures. This paper was written as a vehicle for discussion. In order to stimulate this, the authors have been deliberately provocative and presented an extreme point of view. The paper which follows this is a condensed version of some of the discussion which took place at the Conference. The authors would welcome further contributions. 相似文献
136.
Timms PL Norman NC Pardoe JA Mackie ID Hinchley SL Parsons S Rankin DW 《Dalton transactions (Cambridge, England : 2003)》2005,(3):607-616
The structure of B8F12 has been shown by gas electron diffraction and computational methods (up to MP2/6-31+G*) to have the same highly asymmetric form observed in crystalline phases. The structure can be regarded as derived from a central B2 group, bridged by two BF2 groups to give a central B4 core that is folded, not planar, and with a very short bond [164.3 pm calculated, 164.2(19) pm experimental] along the fold line. There are also four terminal BF2 groups. One of the other four bonds in the core is consistently 20-30 pm longer than the others. This asymmetry has been attributed to many intra-molecular B...F interactions, particularly those between core boron atoms and fluorines of the terminal BF2 groups. Calculations for the chloro analogue lead to a structure similar to that for B8F12, but with the long core bond extended so that one of the bridging BCl2 groups may now be regarded as terminal. With bromine as the halogen the structure changes again, with one bromine atom taking up a bridging position. With iodine, this process continues further, and there are three bridging iodine atoms. However, in this case this is not the lowest energy structure, and instead a loosely associated dimer of B4I6 is preferred. In all these cases, and particularly with the heavier halogens, there are huge differences between the results obtained with different computational methods. 相似文献
137.
Bishop AI Nieminen TA Heckenberg NR Rubinsztein-Dunlop H 《Physical review letters》2004,92(19):198104
We demonstrate an optical system that can apply and accurately measure the torque exerted by the trapping beam on a rotating birefringent probe particle. This allows the viscosity and surface effects within liquid media to be measured quantitatively on a micron-size scale using a trapped rotating spherical probe particle. We use the system to measure the viscosity inside a prototype cellular structure. 相似文献
138.
139.
140.
Yang VX Munce N Pekar J Gordon ML Lo S Marcon NE Wilson BC Vitkin IA 《Optics letters》2004,29(15):1754-1756
High-resolution optical coherence tomography demands a large detector bandwidth and a high numerical aperture for real-time imaging, which is difficult to achieve over a large imaging depth. To resolve these conflicting requirements we propose a novel multifocus fiber-based optical coherence tomography system with a micromachined array tip. We demonstrate the fabrication of a prototype four-channel tip that maintains a 9-14-microm spot diameter with more than 500 microm of imaging depth. Images of a resolution target and a human tooth were obtained with this tip by use of a four-channel cascaded Michelson fiber-optic interferometer, scanned simultaneously at 8 kHz with geometric power distribution across the four channels. 相似文献