全文获取类型
收费全文 | 2735篇 |
免费 | 450篇 |
国内免费 | 258篇 |
专业分类
化学 | 515篇 |
晶体学 | 13篇 |
力学 | 473篇 |
综合类 | 31篇 |
数学 | 603篇 |
物理学 | 1808篇 |
出版年
2024年 | 7篇 |
2023年 | 22篇 |
2022年 | 36篇 |
2021年 | 55篇 |
2020年 | 64篇 |
2019年 | 54篇 |
2018年 | 60篇 |
2017年 | 73篇 |
2016年 | 68篇 |
2015年 | 85篇 |
2014年 | 78篇 |
2013年 | 196篇 |
2012年 | 97篇 |
2011年 | 165篇 |
2010年 | 121篇 |
2009年 | 165篇 |
2008年 | 182篇 |
2007年 | 190篇 |
2006年 | 200篇 |
2005年 | 167篇 |
2004年 | 148篇 |
2003年 | 131篇 |
2002年 | 175篇 |
2001年 | 122篇 |
2000年 | 131篇 |
1999年 | 96篇 |
1998年 | 115篇 |
1997年 | 80篇 |
1996年 | 54篇 |
1995年 | 54篇 |
1994年 | 46篇 |
1993年 | 38篇 |
1992年 | 36篇 |
1991年 | 25篇 |
1990年 | 18篇 |
1989年 | 8篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 14篇 |
1983年 | 3篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1974年 | 2篇 |
1936年 | 1篇 |
排序方式: 共有3443条查询结果,搜索用时 234 毫秒
61.
A spectro-streak photometer, an instrument for simultaneously measuring fluorescence intensity, time, and wavelength,I(t, ), with a single picosecond excitation pulse, has been constructed. Two typical and currently highly topical examples of mesurements are discussed. (1) the temporal development of the fluorescence form the intramolecular charge-transfer (ICT) state of the rigid aromatic compound 4,5-(1-methylindolino)3,4-naphthanthracene is studied in the protic solvent hexanol. (2) Propyl chain-linked pyrene/N,N-dimethylaniline is used as the model compound to study conformational changes associated with the transition from a contact ion pair to a sandwich exciplex. 相似文献
62.
Algebraic surfaces with log canonical singularities and the fundamental groups of their smooth parts
D.-Q. Zhang 《Transactions of the American Mathematical Society》1996,348(10):4175-4184
Let be a log surface with at worst log canonical singularities and reduced boundary such that is nef and big. We shall prove that either has finite fundamental group or is affine-ruled. Moreover, and the structure of are determined in some sense when .
63.
A stretch vibration Hamiltonian of H_2O has been derived by using the second quantization, unitary transformations and the optimization of coefficients. The energy spectrum obtained from this Hamiltonian is in good agreement with that of experiments. The multiphoton excitation and the energy absorption of H_2O in intense laser fields have been calculated. 相似文献
64.
Yuji Naruse Atsushi Takamori Itsuki Nagasawa 《Phosphorus, sulfur, and silicon and the related elements》2020,195(7):523-525
AbstractWe previously designed and prepared the first molecules to exhibit observable CD spectra by n-σ* excitation, 2,6-dithiaspiro[3.3]heptane 2,6-dioxide. Spiro[3.3]heptane frameworks possess axial asymmetry due to puckering of 2 four-membered rings; the ring bonds are rich in p-character due to acute bond angles, which lowers the σ* energy levels. In contrast, the lone pairs are rich in s-character, which results in a good donor with conformational fixation. We expected that, instead of lone pairs as donating orbitals, the use of σ-electron-donating Si-Si bonds should result in UV absorption in the observable range (>180?nm), so that the Cotton effect could appear, at least partially, in that range. We designed 2,6-disilyl-2,6-disilaspiro[3.3]heptanes as models, and performed theoretical calculations to confirm our idea. 相似文献
65.
Chaikovskaya O. N. Levin P. P. Sul"timova N. B. Sokolova I. V. Kuz"min A. V. 《Russian Chemical Bulletin》2004,53(2):313-317
The spectra and kinetics of short-lived intermediates formed from aqueous (0.1 N NaOH) solutions of the natural mixture of humic and fulvic acids (HFA) were studied by laser flash photolysis using excitation wavelengths of 337, 390, 470, and 520 nm. Laser photolysis of HFA with light of 520 and 470 nm results in the formation of triplet excited states (THFA) characterized by the broad absorption spectrum with a maximum near 630 nm and lifetimes of 0.15 ms in deoxygenated solutions. The formation of two types of THFA with lifetimes of 0.1 and 2 ms and absorption spectra with maxima at 570 nm is observed under photolysis with light of 337 and 390 nm. The estimation of quantum yields of THFA gives 1 and 0.3% under photolysis with excitation wavelengths of 337 and 520 nm, respectively. The rate constants of THFA quenching by molecular oxygen are equal to (7—8)·108 L mol–1 s–1. 相似文献
66.
Ema I García De La Vega JM Ramírez G López R Fernández Rico J Meissner H Paldus J 《Journal of computational chemistry》2003,24(7):859-868
We present three Slater-type atomic orbital (STO) valence basis (VB) sets for the first and second row atoms, referred to as the VB1, VB2, and VB3 bases. The smallest VB1 basis has the following structure: [3, 1] for the H and He atoms, [5, 1] for Li and Be, and [5, 3, 1] for the B to Ne series. For the VB2 and VB3 bases, both the number of shells and the number of functions per shell are successively increased by one with respect to VB1. With the exception of the H and Li atoms, the exponents for the VB1 bases were obtained by minimizing the sum of the Hartree-Fock (HF) and frozen-core singles and doubles configuration interaction (CISD FC) energies of the respective atoms in their ground state. For H and Li, we minimized the sum of the HF and CISD FC energies of the corresponding diatoms (i.e., of H(2) or Li(2)) plus the ground-state energy of the atom. In the case of the VB2 basis sets, the sum that was minimized also included the energies of the positive and negative ions, and for the VB3 bases, the energies of a few lowest lying excited states of the atom. To account for the core correlations, the VBx (x = 1, 2, and 3) basis sets for the Li to Ne series were enlarged by one function per shell. The exponents of these extended (core-valence, CV) basis sets, referred to, respectively, as the CVBx (x = 1, 2, and 3) bases, were optimized by relying on the same criteria as in the case of the VBx (x = 1, 2, and 3) bases, except that the full CISD rather than CISD FC energies were employed. We show that these polarized STO basis sets provide good HF and CI energies for the ground and excited states of the atoms considered, as well as for the corresponding ions. 相似文献
67.
68.
Quantum equations of motion describing the energy transfer dynamics via intramolecular anharmonic coupling are presented and solved numerically. Dynamical features of the average excitations of a homogeneous system (SF6) and a heterogeneous system (H/W) are quantitatively discussed in terms of the intramolecular vibrational relaxation (IVR) rates. Possible mechanisms of surface-enhanced desorption via low-power laser radiation are proposed.This work was supported in part by the Office of Naval Research, the Air Force Office of Scientific Research (AFSC), United States Air Force, under Grant AFOSR-82-0046 and the National Science Foundation under Grant No. CHE-8022874Camille and Henry Dreyfus Teacher-Scholar (1975–82) 相似文献
69.
在X射线荧光分析中,基本参数法是校正元素间吸收增强效应的数学方法之一。我们使用带有不锈钢窗的~(241)Am59.6keVγ射线作为激发源,用Si(Li)半导体探测器和多道脉冲幅度分析器作为实验手段。采用无标样法,即绝对测量法,因而具有更大的广泛性和实用性。实验中,我们除了考虑元素间的二次、三次荧光吸收增强作用外,对样品中元素 相似文献