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1.
One of the open questions in the geometry of line arrangements is to what extent does the incidence lattice of an arrangement determine its fundamental group. Line arrangements of up to 6 lines were recently classified by K.M. Fan (Michigan Math. J. 44(2) (1997) 283), and it turns out that the incidence lattice of such arrangements determines the projective fundamental group. We use actions on the set of wiring diagrams, introduced in (Garber et al. (J. Knot Theory Ramf.), to classify real arrangements of up to 8 lines. In particular, we show that the incidence lattice of such arrangements determines both the affine and the projective fundamental groups. 相似文献
2.
Uzi Vishne 《Archiv der Mathematik》2003,80(2):119-122
We prove that assuming enough roots of unity in the base field, a central simple algebra of exponent 2 which is split by a dihedral group, is also split by certain abelian groups. 相似文献
3.
Separation of lithium and potassium isotopes has been achieved using the isotopic differences in the radiofrequency spectrum. The isotope separation was studied in a supersonic molecular beam magnetic resonance spectrometer, and enrichment factors up to 30 obtained. 相似文献
4.
Intercalation of trioxatriangulenium ion in DNA: binding,electron transfer,x-ray crystallography,and electronic structure 总被引:1,自引:0,他引:1
Reynisson J Schuster GB Howerton SB Williams LD Barnett RN Cleveland CL Landman U Harrit N Chaires JB 《Journal of the American Chemical Society》2003,125(8):2072-2083
Trioxatriangulenium ion (TOTA(+)) is a flat, somewhat hydrophobic compound that has a low-energy unoccupied molecular orbital. It binds to duplex DNA by intercalation with a preference for G-C base pairs. Irradiation of intercalated TOTA(+) causes charge (radical cation) injection that results in strand cleavage (after piperidine treatment) primarily at GG steps. The X-ray crystal structure of TOTA(+) intercalated in the hexameric duplex d[CGATCG](2) described here reveals that intercalation of TOTA(+) results in an unusually large extension of the helical rise of the DNA and that the orientation of TOTA(+) is sensitive to hydrogen-bonding interactions with backbone atoms of the DNA. Electronic structure calculations reveal no meaningful charge transfer from DNA to TOTA(+) because the lowest unoccupied molecular orbital of TOTA(+), (LUMO)(T), falls in the gap between the highest occupied molecular orbital, (HOMO)(D), and the (LUMO)(D) of the DNA bases. These calculations reveal the importance of backbone, water, and counterion interactions, which shift the energy levels of the bases and the intercalated TOTA(+) orbitals significantly. The calculations also show that the inserted TOTA(+) strongly polarizes the intercalation cavity where a sheet of excess electron density surrounds the TOTA(+). 相似文献
5.
The photoexcited triplet state of phenazine in toluene glasses at 35 K is investigated by light modulation-EPR spectroscopy. From the transient EPR spectra and the kinetics in the three canonical orientations (p = x, y, z) the rate parameters are determined. Thus, the depopulation rate constants kp, the anisotropic spin lattice relaxation rate constants Wp, and the ratios between the population constants Ap are calculated: kx = (2.2 ± 0.3) × 102 s?1, ky = (0.21 ± 0.04) × 102 s?1, kz = (0.06 ± 0.03) × 102 s?1, Wx = (8.6 ± 0.9) × 103 s?1, Wy = (11.0 ± 1.0) × 103 s?1, Wz = (14.0 ± 1.4) × 103 s?1, and Ax: Ay:Az ≈ 1:0.04:0.02. It is concluded therefore that the in-plane spin state |τx > is the active one. 相似文献
6.
7.
Electronic structure and bonding in anionic coinage metal clusters are investigated via density-functional calculations, focusing on an extensive set of isomers of Cu(-)(7), Ag(-)(7), and Au(-)(7). While the ground states of Cu(-)(7) and Ag(-)(7) are three dimensional (3D), that of Au(-)(7) is planar, separated from the optimal 3D isomer by 0.5 eV. The simulated thermally weighted photoabsorption spectrum of Au(-)(7) is dominated by planar structures, and it agrees well with the measured one. The propensity of Au(-)(N) clusters to favor planar structures (with N as large as 13) is correlated with strong hybridization of the atomic 5d and 6s orbitals due to relativistic effects. 相似文献
8.
Bréchignac C Cahuzac P Carlier F Colliex C Leroux J Masson A Yoon B Landman U 《Physical review letters》2002,88(19):196103
Formation and evolution of fragmentation instabilities in fractal islands, obtained by deposition of silver clusters on graphite, are studied. The fragmentation dynamics and subsequent relaxation to the equilibrium shapes are controlled by the deposition conditions and cluster composition. Sharing common features with other materials' breakup phenomena, the fragmentation instability is governed by the length-to-width ratio of the fractal arms. 相似文献
9.
Models of synchronized parallel computation in which all the processors have access to a common memory are considered. We focus on algorithms in models that allow simultaneous access to the same memory location, for both read and write instructions. Assume that such an algorithm uses p processors, d time units, and s memory space. We present a universal algorithm that implements this algorithm in models that forbid simultaneous access to the same memory location, using p processors, O(dlog2p) time units, and O (s + p) memory space his implementation algorithm is shown to compare favorably with its conventional naive counterpart, as the extra memory space it requires is independent of the implemented algorithm. 相似文献
10.
The enhancement by water molecules of the catalytic activity of gas-phase and supported gold nanoclusters toward CO oxidation is investigated with first-principles calculations. Coadsorption of H(2)O and O(2) leads to formation of a complex well bound to the gold cluster, even on a defect-free MgO(100) support. Formation of the complex involves partial proton sharing between the adsorbates, that in certain configurations results in proton transfer leading to the appearance of a hydroperoxyl-like complex. The O-O bond is activated, leading to a weakened peroxo or superoxolike state, and consequently the reaction with CO to form CO2 occurs with a small activation barrier of approximately 0.5 eV. A complete catalytic cycle of the water-enhanced CO oxidation is discussed. 相似文献