全文获取类型
收费全文 | 1229篇 |
免费 | 46篇 |
国内免费 | 3篇 |
专业分类
化学 | 919篇 |
晶体学 | 15篇 |
力学 | 14篇 |
数学 | 122篇 |
物理学 | 208篇 |
出版年
2023年 | 5篇 |
2022年 | 13篇 |
2021年 | 19篇 |
2020年 | 9篇 |
2019年 | 21篇 |
2018年 | 17篇 |
2017年 | 14篇 |
2016年 | 28篇 |
2015年 | 21篇 |
2014年 | 17篇 |
2013年 | 57篇 |
2012年 | 70篇 |
2011年 | 68篇 |
2010年 | 46篇 |
2009年 | 61篇 |
2008年 | 79篇 |
2007年 | 82篇 |
2006年 | 83篇 |
2005年 | 72篇 |
2004年 | 57篇 |
2003年 | 60篇 |
2002年 | 44篇 |
2001年 | 19篇 |
2000年 | 18篇 |
1999年 | 16篇 |
1998年 | 18篇 |
1997年 | 8篇 |
1996年 | 18篇 |
1995年 | 14篇 |
1994年 | 18篇 |
1993年 | 15篇 |
1992年 | 15篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 8篇 |
1988年 | 12篇 |
1987年 | 8篇 |
1986年 | 10篇 |
1985年 | 15篇 |
1984年 | 19篇 |
1982年 | 8篇 |
1981年 | 12篇 |
1980年 | 11篇 |
1979年 | 11篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 4篇 |
1974年 | 5篇 |
1973年 | 4篇 |
排序方式: 共有1278条查询结果,搜索用时 15 毫秒
131.
Ohta K Yamada S Kamada K Slepkov AD Hegmann FA Tykwinski RR Shirtcliff LD Haley MM Sałek P Gel'mukhanov F Ågren H 《The journal of physical chemistry. A》2011,115(2):105-117
The two-photon absorption (TPA) properties of four TPEB [tetrakis(phenylethynyl)benzene] derivatives (TD, para, ortho, and meta) with different donor/acceptor substitution patterns have been investigated experimentally by the femtosecond open-aperture Z-scan method and theoretically by the time-dependent density-functional theory (TDDFT) method. The four compounds show relatively large TPA cross sections, and the all-donor substituted species (TD) displays the largest TPA cross-section σ(2) = 520 ± 30 GM. On the basis of the calculated electronic structure, TD shows no TPA band in the lower energy region of the spectrum because the transition density is concentrated on particular transitions due to the high symmetry of the molecular structure. The centrosymmetric donor-acceptor TPEB para shows excitations resulting from transitions centered on D-π-D and A-π-A moieties, as well as transition between the D-π-D and A-π-A moieties; this accounts for the broad nature of the TPA bands for this compound. Calculations for two noncentrosymmetric TPEBs (ortho and meta) reveal that the diminished TPA intensities of higher-energy bands result from destructive interference between the dipolar and three-state terms. The molecular orbitals (MOs) of the TPEBs are derivable with linear combinations of the MOs of the two crossing BPEB [bis(phenylethynyl)benzene] derivatives. Overall, the characteristics of the experimental spectra are well-described based on the theoretical analysis. 相似文献
132.
Understanding the effect of codoping on the properties of photonic glasses is important for improving their properties. The effect of codoping on the ligand field around Cu(2+) ions in a sodium borate glass is examined using optical absorption spectroscopy, continuous-wave electron paramagnetic resonance, and three-pulse electron-spin-echo envelope-modulation. Glass with a composition of 0.1CuO·5Na(2)O·95B(2)O(3) was codoped with 2 mol % of Al(3+), Si(4+), P(5+), Zr(4+), or La(3+) oxide. Three codoping effects are found: strengthening the ligand field, as observed for Zr-codoping, which induces a large blue shift of the optical absorption peak of Cu(2+); weakening the ligand field, as observed for P-codoping, which causes a red shift of the Cu(2+) absorption peak; and almost no effect on the ligand field, which is observed for Al-, Si-, and La-codoping. Coordination structure models based on local charge neutrality are proposed for the codoped glasses. The mechanism of the codoping effect is revealed by elucidating the local structure around Cu(2+). 相似文献
133.
Hirohisa Yokota Jun-ichi Igarashi Yoshitaka Satoda Satoru Ohuchi Yutaka Sasaki 《Optical Review》2002,9(3):94-99
A new all-fiber optical add-drop multiplexer (ADM) using a pair of fiber grating couplers (FGCs) was proposed and first demonstrated. It was theoretically and experimentally shown that both drop and add efficiencies could be maximized in the proposed ADM. It was also clarified that the crosstalk between dropped and added signals could be improved in the proposed ADM compared with that in the single FGC. Drop and add operations in the ADM for 3-channel WDM signals with 1.6 nm wavelength spacing were confirmed. The ADM with signal amplification ability, which consists of a pair of FGCs, an EDF, and a pump-LD, was proposed and demonstrated. It was confirmed that the dropped and transmitted optical signals could be amplified in the ADM, and would be able to expand fiber routing spans. 相似文献
134.
135.
136.
Yoichiro Hoshina Satoru Ikegami Kyoko Fujimoto Akihiko Okuyama Tatsuhiro Harada Susan James Peter G. Griffiths Zemin Wu Michael Lilly Andrew M. Bray 《Tetrahedron letters》2005,46(52):9035-9038
2,3-Diphenylpropionic acid library for VLA-4 antagonist was synthesized on solid-phase. Comparison of the two synthetic routes via an orthogonal generation of two aromatic amino functional groups are discussed. From this work, several compounds were identified as potent VLA-4 antagonists. 相似文献
137.
138.
In 1′, 1‴-dibutylbiferrocenium triiodide crystals A show temperature-independent trapped-valence state, while crystals B show
a valence-delocalization of the mixed-valence state. The mixed-valence states of both crystals are discussed in relation to
their crystal structures.
Address after October 1991: Department of Chemistry, Faculty of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima
724, Japan 相似文献
139.
Kouichi Tsuji Tetsuya Emoto Yosuke Nishida Eiichiro Tamaki Yoshikuni Kikutani Akihide Hibara Takehiko Kitamori 《Analytical sciences》2005,21(7):799-803
Grazing-exit x-ray fluorescence (GE-XRF) and micro x-ray fluorescence (micro-XRF) methods were applied to chemical microchips as a detection method. Since an energy-dispersive x-ray detector was used, the simultaneous detection of multiple elements was possible. An analyzing region was especially designed on the microchip so that a sample solution could be dried and concentrated in a suitable area corresponding to the size of the primary x-ray beam. Finally, it was confirmed that both analytical methods could be combined well for use with a microchip. In GE-XRF, the background intensity in the XRF spectrum was reduced at grazing-exit angles. In addition, a good relationship between the x-ray fluorescence intensities and the concentrations of standard solutions that were introduced into the microchip was obtained. This indicates that the GE-XRF method is feasible for trace elemental analysis in chemical microchip systems. In micro-XRF, an attempt was made to concentrate and dry the analyte within a small analyzing region. The preliminary results indicated that the micro-XRF method could be applied for the analysis of microchips. 相似文献
140.
Norikazu Nakayama Satoru Kijima Satoshi Itoh Toyoharu Ohata Akira Ishibashi Yoshifumi Mori 《Optical Review》1995,2(3):167-170
Green light-emitting diodes (LEDs) were fabricated employing a ZnCdSe/ZnSSe triple quantum-well (TQW) active region surrounded by ZnMgSSe cladding layers grown on an n-type (100) GaAs substrate by molecular beam epitaxy (MBE). A 3.5 mW pure green emission was observed for the surface-emitting LED device at a peak wavelength of 513.3 nm (2.415 eV) with a spectral half-width of 11.7 nm (55 meV) under a 20 mA (4.6 V) direct current at room temperature (25°C). These correspond to an external quantum efficiency of 7.2%, a power conversion efficiency of 3.8%, a luminous current efficiency of 66 lm/A, and a luminous efficiency of 14 lm/W. 相似文献