全文获取类型
收费全文 | 1869篇 |
免费 | 55篇 |
国内免费 | 10篇 |
专业分类
化学 | 1116篇 |
晶体学 | 15篇 |
力学 | 48篇 |
数学 | 430篇 |
物理学 | 325篇 |
出版年
2020年 | 16篇 |
2019年 | 19篇 |
2018年 | 14篇 |
2017年 | 16篇 |
2016年 | 32篇 |
2015年 | 32篇 |
2014年 | 35篇 |
2013年 | 95篇 |
2012年 | 82篇 |
2011年 | 97篇 |
2010年 | 54篇 |
2009年 | 45篇 |
2008年 | 90篇 |
2007年 | 82篇 |
2006年 | 100篇 |
2005年 | 94篇 |
2004年 | 67篇 |
2003年 | 67篇 |
2002年 | 85篇 |
2001年 | 21篇 |
2000年 | 34篇 |
1999年 | 28篇 |
1998年 | 28篇 |
1997年 | 33篇 |
1996年 | 27篇 |
1995年 | 15篇 |
1994年 | 32篇 |
1993年 | 31篇 |
1992年 | 22篇 |
1991年 | 24篇 |
1990年 | 19篇 |
1989年 | 16篇 |
1988年 | 12篇 |
1987年 | 18篇 |
1986年 | 18篇 |
1985年 | 20篇 |
1984年 | 31篇 |
1983年 | 22篇 |
1982年 | 31篇 |
1981年 | 35篇 |
1980年 | 28篇 |
1979年 | 23篇 |
1978年 | 32篇 |
1977年 | 23篇 |
1976年 | 20篇 |
1975年 | 17篇 |
1974年 | 20篇 |
1973年 | 19篇 |
1972年 | 14篇 |
1971年 | 13篇 |
排序方式: 共有1934条查询结果,搜索用时 15 毫秒
101.
102.
Atom-probe field-ion microscopy together with X-ray photoelectron spectroscopy and secondary ion mass spectrometry have been applied to the microanalysis of fully ordered NiAl single crystals subjected to 3 keV inert gas ion bombardment. As with the studies of Ni3Al (the companion paper) aluminium was found to be preferentially sputtered by both argon and xenon bombardment. Comparisons between depth profiles through Ni3Al and NiAl targets have provided information about the role of binding energies in the selective sputtering process. These data have also permitted conclusions to be drawn about the correct choice of bombarding species for sample cleaning and depth-profiling applications in surface analysis. Examination of field-ion images from specimens after bombardment suggests that the surface is microroughened and this has been confirmed using transmission electron microscopy. 相似文献
103.
J.D. Petke Gerald M. Maggiora Lester L. Shipman Ralph E. Christoffersen 《Journal of Molecular Spectroscopy》1978,73(2):311-331
Ab initio configuration interaction wavefunctions and energies are reported for the ground state and many low-lying singlet and triplet states of magnesium chlorin and chlorin, and are employed in an analysis of the electronic absorption spectra of these systems.In chlorin, the calculated visible spectrum consists of two states, the lower energy, y-polarized state exhibiting moderate absorption intensity in contrast to the very weak absorption of the higher energy x-polarized state. The configurational composition of both states is well described by the four-orbital model. Five states are responsible for the Soret band envelope. A moderately intense y-state lies under the low energy edge of the band envelope, while two x-polarized states of moderate and strong intensity, respectively, are responsible for the band maximum. The final two states lie at the high energy edge of the Soret band and introduce a measure of asymmetry into the band envelope. Two states of very weak oscillator strength are also found in this region of the spectrum. All the Soret states are of complex configurational composition, and several of the higher lying states contain contributions from doubly excited configurations.The calculated visible spectrum of magnesium chlorin also consists of two states, with the weakly absorbing x-polarized state lying approximately 200 cm?1 lower in energy than the moderately intense y-polarized state. The configurational composition of both states is well described by the four-orbital model. Four states constitute the bulk of the intensity in the Soret band envelope. In distinction to chlorin, the moderately intense state at the low energy edge of the band envelope is x-polarized. Two intense states of y- and x-polarization, respectively, constitute the band maximum region, and a single x-polarized state of moderately strong intensity can be assigned to the high energy shoulder of the band envelope. Two other weakly absorbing states are also found in this region, along with another weakly absorbing state of mixed in-plane and out-of-plane polarization. No clearly defined states are observed. As was the case for chlorin, all the Soret states are of complex configurational composition, and some of the higher energy states contain significant contributions from doubly excited configurations.Chlorin and magnesium chlorin both possess three states which lie below S1 and a single which lies slightly above S2. All four of the low-lying states in each molecule are well described by the four-orbital model, with T1 being essentially a single configuration in each case. The remainder of the states are clustered in the same energetic region as the comparable Soret states, with comparably complex configurational compositions.Dipole moments and charge distributions for low-lying singlet and triplet states are also reported, and are used to rationalize chemical reactivity characteristics. 相似文献
104.
A relatively weak ESR spectrum is observed in single crystals of NaN3 after X-ray irradiation at 77 K. This spectrum, which has an anisotropic g value and exhibits a resolved 5-line hyperfine structure with components in the ratio 1:2:3:2:1, corresponds to a single unpaired electron interacting symmetrically with two spin-one nuclei, in three inequivalent sites. The spin-Hamiltonian parameters are: gx = 2.0054 ± 0.0005, gy = 2.0045 ± 0.0005, gz = 1.9688 ± 0.0005, |Ax| = 4.0 ± 0.2 G, |Ay| = 20.0 ± 0.2 G, and |Az| = 4.9 ± 0.2 G, c-axis, and y is perpendicular to the c-axis. This spectrum, which is clearly different from that of substitutional N2?reported by Gelerinter and Silsbee, is attributed to interstitial N2?. 相似文献
105.
Ralph P. Cooney Martin W. Howard Merrick R. Mahoney Terrence P. Mernagh 《Chemical physics letters》1981,79(3):459-464
A mechanism is proposed for surface-enhanced Raman scattering for pyridine which explains the enhancement, the anodizalion “activation”, the significance of silver, the photographitization of coordinated pyridine and formate, the participation of surface roughness, and the irreversibly held yet liquid-like nature of surface pyridine. 相似文献
106.
Let Mn be the algebra of n×n matrices over an algebraically closed field of characteristic zero. Let f(x) be a polynomial over with at least two distinct roots. Then all nonsingular linear maps L:Mn→Mn that map matrix roots of F(x)=0 into matrix roots of f(x}=0 are found. 相似文献
107.
Gary D. Bent Ralph H. Bartram Angelo R. Rossi Peter J. Kemmey 《Journal of Physics and Chemistry of Solids》1979,40(7):493-502
The energies of the ground state and low-lying excited states of the N4? defect in KN3 have been calculated using ab initio techniques. A rectangular equilibrium geometry with dimensions X = 2.76 and Z = 2.47 a.u. and ground state symmetry of Γ4+ was determined by calculating N4? as a free radical. For this ground state the unpaired electron is in a π orbital which is consistent with the experimental hyperfine tensor only if one edge of the N4? radical is parallel to the c axis in KN3. These results were used to calculate the X2Γ4+ state of N4? in the crystal field of KN3, yielding an energy of ?217.899 Hartrees. The isotropic hyperfine constant was calculated to be a = 2.1 G and the components of the anisotropic hyperfine tensor as Bxx = ?3.4 G, Byy = 7.0 G and Bzz = ?3.6 G, in good agreement with experimental and INDO results. Several excited states were calculated for the N4? defect in KN3. When an estimate was made of the correlation energy, the transition energy of the X2Γ4+→A2Γ3? transition agreed well with the peak energy of the 780 nm absorption band which has been attributed to N4?. 相似文献
108.
Ralph Hugh Francis Denniston 《manuscripta mathematica》1973,8(1):21-26
A search has shown that any inversive plane of order 7 must be isomorphic with the Miquelian plane. The method used was explicitly set out in a companion paper, which was concerned with a plane of order 5. 相似文献
109.
A 2(2J + 1)-component relativistic Hamiltonian H that describes free particles of mass m and spin J is said to be linear if it has the form , where , h is a numerical factor, and g commutes with x and p. All such Hamiltonians are found, provided that the metric is either the unit matrix or ?3 and provided that the theory is invariant under the discrete symmetries. If the operator Γ in the generator of Lorentz boosts is required to be local, there are only two possibilities; either Γ = 0, which generalizes the Dirac spin- theory, or , which generalizes the Sakata-Taketani spin-0 and spin-1 theories. The relationship to linear manifestly covariant equations and its significance is discussed. 相似文献
110.