全文获取类型
收费全文 | 773篇 |
免费 | 14篇 |
国内免费 | 6篇 |
专业分类
化学 | 406篇 |
晶体学 | 3篇 |
力学 | 8篇 |
数学 | 129篇 |
物理学 | 247篇 |
出版年
2021年 | 9篇 |
2020年 | 8篇 |
2019年 | 4篇 |
2018年 | 5篇 |
2017年 | 5篇 |
2016年 | 11篇 |
2015年 | 8篇 |
2014年 | 9篇 |
2013年 | 26篇 |
2012年 | 47篇 |
2011年 | 29篇 |
2010年 | 19篇 |
2009年 | 15篇 |
2008年 | 32篇 |
2007年 | 61篇 |
2006年 | 39篇 |
2005年 | 36篇 |
2004年 | 36篇 |
2003年 | 31篇 |
2002年 | 23篇 |
2001年 | 28篇 |
2000年 | 21篇 |
1999年 | 10篇 |
1997年 | 8篇 |
1996年 | 13篇 |
1995年 | 21篇 |
1994年 | 12篇 |
1993年 | 17篇 |
1992年 | 10篇 |
1991年 | 9篇 |
1990年 | 21篇 |
1989年 | 16篇 |
1988年 | 5篇 |
1987年 | 14篇 |
1986年 | 10篇 |
1985年 | 16篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 12篇 |
1978年 | 4篇 |
1977年 | 12篇 |
1976年 | 6篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1973年 | 7篇 |
1972年 | 3篇 |
1966年 | 3篇 |
排序方式: 共有793条查询结果,搜索用时 15 毫秒
91.
Rebane A Drobizhev M Makarov NS Beuerman E Haley JE Krein DM Burke AR Flikkema JL Cooper TM 《The journal of physical chemistry. A》2011,115(17):4255-4262
We investigate two-photon absorption (2PA) in a series of fluorenyl-based 9,9-diethyl-2-ethynyl-7-((4-R-phenyl)ethynyl)-9,9a-dihydro-4aH-fluorene chromophores with R being various electron donating (ED) and electron withdrawing (EW) groups. We use wavelength-tunable femtosecond laser pulses to measure the 2PA cross sections in the lowest dipole-allowed transition and show that the substituents with stronger ED or EW character enhance the peak 2PA cross section (up to σ(2) ~ 60-80 GM) while the neutral substituents lead to smaller cross sections, σ(2) < 10 GM. We apply two-level approximation to establish a quantitative relation between the 2PA in the pure electronic transition (0-0) and the corresponding change of the permanent electric dipole moment upon the excitation (Δμ). This relation is elucidated by comparing Δμ values obtained from the 2PA measurements with quantum-chemical calculations and with measurements of solvatochromic shifts in a series of solvents. We show that the calculated Δμ correlate well with the values obtained from the 2PA spectroscopy. The Δμ values obtained from the solvatochromic shifts agree well with the above two methods for the chromophores with neutral or weak EW or ED substituents. On the other hand, stronger EW or ED end groups give much larger Stokes shifts, which lead to an overestimation of the Δμ values. We tentatively attribute this effect to the excitation-induced electronic density change occurring predominantly at the substituent side of the molecule, which causes the effective point dipole associated with the Δμ to interact more strongly with the surrounding solvent. 相似文献
92.
We present a model for the surgery admission planning problem, and a meta-heuristic algorithm for solving it. The problem
involves assigning operating rooms and dates to a set of elective surgeries, as well as scheduling the surgeries of each day
and room. Simultaneously, a schedule is created for each surgeon to avoid double bookings. The presented algorithm uses simple
Relocate and Two-Exchange neighbourhoods, governed by an iterated local search framework. The problem’s search space associated
with these move operators is analysed for three typical fitness surfaces, representing different compromises between patient
waiting time, surgeon overtime, and waiting time for children in the morning on the day of surgery. The analysis shows that
for the same problem instances, the different objectives give fitness surfaces with quite different characteristics. We present
computational results for a set of benchmarks that are based on the admission planning problem in a chosen Norwegian hospital. 相似文献
93.
94.
Wavelength-scanning interferometers can separate the multiple interference signals generated by several reflecting surfaces
in the frequency space. Optical thickness variations of a three-layer object and its top surface shape were measured simultaneously
by a wavelength-scanning Fizeau interferometer. An efficient sampling window for the phase detection has been derived that
can compensate for the frequency detuning of the interference signal and suppress the cross talk from noise components in
other frequencies. From the experimental results, the refractive index variation of a glass plate was calculated with an accuracy
of ∼1 ppm, which is equivalent to several tens of nanometers in optical thickness variations.
The text was submitted by the authors in English. 相似文献
95.
Angular distributions for the 163Dy(t,p) and 177Hf(t,p) reactions were measured using 17 MeV tritons from the McMaster University Tandem Van de Graaff accelerator. Reaction products were analyzed with a magnetic spectrograph and detected with photographic emulsions. Favored L=0 transitions confirmed assignments of the 5/2−[523] band in 165Dy and 7/2−[514] band in 179Hf. Additional L=0 transitions in each nuclide identified previously unknown 5/2− levels in 165Dy and 7/2− ones in 179Hf. Overall trends of L=0 strengths support the existence of subshell closures at neutron numbers 98 and 108. On the basis of a relatively strong L=2 transition, the Kπ=11/2− γ-vibration based on the 7/2−[514] state is identified at 1689 keV in 179Hf, about 440 keV above its previously-assigned Kπ=3/2− partner. 相似文献
96.
A Tabu-Search Hyperheuristic for Timetabling and Rostering 总被引:4,自引:0,他引:4
Hyperheuristics can be defined to be heuristics which choose between heuristics in order to solve a given optimisation problem. The main motivation behind the development of such approaches is the goal of developing automated scheduling methods which are not restricted to one problem. In this paper we report the investigation of a hyperheuristic approach and evaluate it on various instances of two distinct timetabling and rostering problems. In the framework of our hyperheuristic approach, heuristics compete using rules based on the principles of reinforcement learning. A tabu list of heuristics is also maintained which prevents certain heuristics from being chosen at certain times during the search. We demonstrate that this tabu-search hyperheuristic is an easily re-usable method which can produce solutions of at least acceptable quality across a variety of problems and instances. In effect the proposed method is capable of producing solutions that are competitive with those obtained using state-of-the-art problem-specific techniques for the problems studied here, but is fundamentally more general than those techniques. 相似文献
97.
Richard N. Butler Denise C. Grogan Luke A. Burke 《Journal of heterocyclic chemistry》1997,34(6):1825-1827
An unusual inverted V-shaped Hammett plot has been observed for substituent effects at the N-terminus of 1,2,3-triazolium-1-imide 1,3-dipoles in cycloaddition reactions. Theoretical studies (3-21G and 6-31G) suggest strong resonance interactions separately stabilizing both ends of the dipole. 相似文献
98.
James O. Jensen Pallassana N. Krishnan Luke A. Burke 《Journal of Molecular Structure》1996,370(2-3):245-252
Twenty-two structures with the empirical formula H3CNO are presented. These structures are examined since they have the same formula as the H2O…HCN complex. The H2O…HCN potential energy surface is of interest to chemists studying the water catalyzed polymerization of HCN. Structures, thermodynamics, and vibrational spectra are examined. 相似文献
99.
In this paper we prove that p-spaces and ?ech-complete spaces with Gδ-diagonals can be characterized by a familiar function extension property. 相似文献
100.
Murray D. Burke 《Journal of multivariate analysis》1977,7(4):491-511
The recently developed strong approximation methods are discussed and applied to the problem of testing whether two independent multivariate samples come from the same population and whether the components of the observations are independent. The usual Cramér-von Mises statistic, as well as one based on the difference between the sum of the two multivariate EDF's and twice the product of the marginal EDF's of one, are studied. A fairly sensitive integral statistic is also discussed. Consistency and some asymptotic power properties are explored. Emphasis is placed on explication of the strong approximation methodology. 相似文献