全文获取类型
收费全文 | 2401篇 |
免费 | 64篇 |
国内免费 | 2篇 |
专业分类
化学 | 1380篇 |
晶体学 | 28篇 |
力学 | 66篇 |
数学 | 209篇 |
物理学 | 784篇 |
出版年
2021年 | 17篇 |
2020年 | 20篇 |
2019年 | 20篇 |
2016年 | 65篇 |
2015年 | 42篇 |
2014年 | 38篇 |
2013年 | 92篇 |
2012年 | 69篇 |
2011年 | 85篇 |
2010年 | 71篇 |
2009年 | 58篇 |
2008年 | 95篇 |
2007年 | 99篇 |
2006年 | 91篇 |
2005年 | 59篇 |
2004年 | 63篇 |
2003年 | 51篇 |
2002年 | 57篇 |
2001年 | 66篇 |
2000年 | 55篇 |
1999年 | 53篇 |
1998年 | 28篇 |
1997年 | 31篇 |
1996年 | 39篇 |
1995年 | 36篇 |
1994年 | 31篇 |
1993年 | 30篇 |
1992年 | 55篇 |
1991年 | 29篇 |
1990年 | 48篇 |
1989年 | 31篇 |
1988年 | 39篇 |
1987年 | 23篇 |
1986年 | 35篇 |
1985年 | 45篇 |
1984年 | 34篇 |
1983年 | 31篇 |
1982年 | 30篇 |
1981年 | 29篇 |
1980年 | 44篇 |
1979年 | 36篇 |
1978年 | 27篇 |
1977年 | 40篇 |
1976年 | 25篇 |
1975年 | 29篇 |
1974年 | 23篇 |
1973年 | 22篇 |
1971年 | 23篇 |
1967年 | 26篇 |
1966年 | 21篇 |
排序方式: 共有2467条查询结果,搜索用时 0 毫秒
11.
Hyperfine structures of transitions in the a 3Πu ← X1Σg system of Na2 have been accurately measured by Doppler-free laser excitation spectroscopy in a collimated molecular beam. The hyperfine patterns are quantitatively explained and the reduced matrix elements of the magnetic dipole hf interaction determined from a least-squares fit to the measured line positions. 相似文献
12.
Using the level crossing technique the ratios and absolute values of the hyperfine structure (hfs) constants of the levelsz 4F9/2 andz 4F7/2 of the configuration 3d 74s4p of Co I were measured:z 4 F 9/2: ¦A¦=(811±12)MHz; ¦B¦=(48±93) MHz;B/A=?0.06±0.11 A>0; B<0z 4 F 7/2: ¦A¦ = (659 ±11)MHz; ¦B¦=(33±84)MHz;B/A=?0.05±0.13 A>0; B<0. In addition the hfs constants of three other excited levels of Co I could be determined by optical methods:z 4 F 9/2:A=525±26 MHz;B=200 MHzy 4 F 9/2:A=300±30 MHz;B=?500 MHzy 4 G 11/2:A=315±20 MHz;B=400 MHz. The experimental results are compared with known experimental and also with theoretical values which where calculated using the parametric potential method. 相似文献
13.
Abstract— A systematic comprehensive consideration of the emission spectroscopy, emission lifetimes, transient spectroscopy, as well as quantum yields of fluorescence, triplet occupation and photoisomeriz-ation is given for a wide variety of polyene derivatives including retinyl and longer, as well as shorter chainlength homologs. Absorption spectral properties and the results and significance of theoretical calculations are also included. Chainlength, solvent and temperature effects on state order and photophysical as well as photoisomerization properties are evaluated. The mechanism for the primary light step in vision is considered in light of photophysical and photoisomerization data on model visual pigments and rhodopsin. 相似文献
14.
15.
We report new measurements of the absolute electron-impact double ionization cross sections for Ar and Kr and of the ratios of double-to-single ionization for impact energies from threshold to 200 eV using the crossed electron-beam — fast-atom-beam technique. The work was motivated by the recently highlighted spread of about 30% in the Ar2+/Ar+ ionization cross section ratios obtained by several groups using different experimental techniques. Such a spread is inconsistent with statistical uncertainties of typically 3% or less that were quoted for the various reported ratios. A similar situation exists for Kr where the spread among the recently published Kr2+/Kr+ ionization cross section ratios is about 15%. We made an attempt to identify all potential systematic errors inherent to the fast-beam technique that could affect the measurement of cross section ratios with special emphasis on those systematic errors that could influence the detection of singly and doubly charged product ions differently. We found Ar2+/Ar+ and Kr2+/Kr+ cross section ratios of, respectively 0.066 ±0.007 and 0.087 ±0.008 at 100 eV which confirm earlier measurements using the same experimental technique. The error limits on cross sections ratios measured in our fast-beam apparatus were determined to be at least ±9% for cross section ratios of multiple-to-single ionization for the same target atom and at least ±10% for ratios of single ionization cross sections for different target species. Our error limits are dominated by systematic uncertainties of the apparatus which do not cancel when cross section ratios are measured, since the ratios are obtained under similar, but not identical experimental conditions. 相似文献
16.
17.
Alkylidynephosphanes and -arsanes. I [P ≡ C? S]?[Li(dme)3]+ – Synthesis and Structure O,O′-Diethyl thiocarbonate and bis(tetrahydrofuran)-lithium bis(trimethylsilyl)phosphanide dissolved in 1,2-dimethoxyethane, react below 0°C to give ethoxy trimethylsilane and tris(1,2-dimethoxyethane-O,O′)lithium 2λ3-phosphaethynylsulfanide – [P≡C? S]? [Li(dme)3]+ – ( 1a ). Apart from bis(trimethylsilyl)sulfane or carbon oxide sulfide, dark red concentrated solutions of λ3-phosphaalkyne 1 are also obtained from reactions of carbon disulfide with bis(tetrahydrofuran)-lithium bis(trimethylsilyl)phosphanide or with the homologous lithoxy-methylidynephosphane ( 2 ) [1]. The ir spectrum shows two absorptions at 1762 and 747 cm?1 characteristic for the P≡C and C? S stretching vibrations. The nmr parameters {δ(31P) ? 121.3; δ(13C) 190.8 ppm; 1JCP 18.2 Hz} resemble much more values of diorganylamino-2λ3-phosphaalkynes than those of bis(1,2-dimethoxyethane-O,O′)lithoxy-methylidyne-phosphane ( 2a ). As found by an X-ray structure analysis (P21/c; a = 1192.6(16); b = 1239.1(19); c = 1414.8(26) pm; β = 105.91(13)° at ?100 ± 3°C; Z = 4 formula units; wR = 0.064) of pale yellow crystals (mp. + 16°C) isolated from the reaction with O,O′-diethyl thiocarbonate, the solid is built up of separate [P≡C? S]? and [Li(dme)3]+ ions. Typical bond lengths and angles are: P≡C 155.5(11); C? S 162.0(11); Li? O 206.4(17) to 220.3(20) pm; P≡C? S 178.9(7)°. 相似文献
18.
Kenn Freedman Ralph S. Becker † Dieter Hannak Ernst Bayer 《Photochemistry and photobiology》1986,43(3):291-295
Abstract— The 11-cis and all-trans isomers of a series of poly(ethylene glycol)-oligopeptide - Schiff bases as models for rhodopsin were synthesized and studied. Absorption data for certain of the PEG-peptide Schiff bases demonstrated that no intramolecular hydrogen-bonding (or protonation) occurs between the Schiff base and an acidic amino acid residue, as was previously thought. Photoisomerization of the 11-cis protonated and unprotonated Schiff bases were examined using both steady state and laser flash techniques. Also with 355 nm excitation (and additionally 532 nm in one case), an approximate 40% increase in quantum yield of isomerization (φ) occurred for all protonated PEG-peptide Schiff bases compared to the H+ -n-butylamine counterparts (in methanol). In one case, a > 100% increase in φ was found in dichloromethane. These data show that PEG-oligopeptide Schiff bases are still further improved models for rhodopsin compared to their n-butylamine analogs. 相似文献
19.
20.
Christoph R. Becker Lothar R. Schad Walter J. Lorenz 《Magnetic resonance imaging》1994,12(8):1167-1174
A new method for the ultrafast generation of diffusion-weighted images is reported. The technique combines a quick echo split NMR imaging sequence with the principle of Stejskal and Tanner. It allows to determine the diffusion constant with nearly the same accuracy as the conventional spin-echo technique, requiring only a fraction of the time. The determined values for water doped with 1 g Cu(NO3)2 per liter of H2O and pure acetone were Dwater = (1.95 ± 0.02) × 10−9 m2/s and Dacetone = (4.05 ± 0.02) × 10−9 m2/s at 18.5°C. They are in good agreement both with literature and our own reference measurements using a diffusion-weighted spin-echo sequence. In addition, the temperature dependence of Dwater was measured in the range of 18.5–45.9°C and a good correspondence with reported data was found. 相似文献