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71.
M Garcia-Viloca J M González-Lafont ALluch 《Journal of the American Chemical Society》2001,123(4):709-721
The experimentally postulated mechanism for the interconversion between (S)-vinylglycolate and (R)-vinylglycolate catalyzed by mandelate racemase enzyme consists of a two-step quite symmetric process through a dianionic enolic intermediate that is formed after the abstraction of the alpha-proton of vinylglycolate by a basic enzymatic residue and is then reprotonated by another residue. The challenging problem behind this reaction is how the enzyme manages to stabilize such an intermediate, that is, how it lowers enough the high pK(a) of the alpha-proton for the reaction to take place. The QM/MM simulations performed in this paper indicate that catalysis is based on the stabilization of the negative charge developed on the substrate along the reaction. We have identified three different reaction mechanisms starting from different quasi-degenerate structures of the substrate-enzyme complex. In two of them the stabilizing role is done by means of a catalytic proton transfer that avoids the formation of a dianionic intermediate, and they involve six steps instead of the two experimentally proposed. On the contrary, the third mechanism passes through a dianionic species stabilized by the concerted approach of a protonated enzymatic residue during the proton abstraction. The potential energy barriers theoretically found along these mechanisms are qualitatively in good agreement with the experimental free energy barriers determined for racemization of vinylglycolate and mandelate. The theoretical study of the effect of the mutation of Glu317 by Gln317 in the kinetics of the reaction reveals the important role in the catalysis of the hydrogen bond formed by Glu317 in the native enzyme, as only one of the mechanisms, the slower one, is able to produce the racemization in the active site of the mutant. However, we have found that this hydrogen bond is not an LBHB within our model. 相似文献
72.
Zofia Stepien/snm> 《Geometriae Dedicata》2002,93(1):37-45
We prove the formality property of any homogeneous space G/G
generated by an automorphism of finite order of a compact simple Lie group G. 相似文献
73.
Thomas J. Tucker with an Appendix by Olivier Debarre 《Transactions of the American Mathematical Society》2002,354(8):3011-3029
This paper deals with generalizations of Hilbert's irreducibility theorem. The classical Hilbert irreducibility theorem states that for any cover of the projective line defined over a number field , there exist infinitely many -rational points on the projective line such that the fiber of over is irreducible over . In this paper, we consider similar statements about algebraic points of higher degree on curves of any genus. We prove that Hilbert's irreducibility theorem admits a natural generalization to rational points on an elliptic curve and thus, via a theorem of Abramovich and Harris, to points of degree 3 or less on any curve. We also present examples that show that this generalization does not hold for points of degree 4 or more. These examples come from an earlier geometric construction of Debarre and Fahlaoui; some additional necessary facts about this construction can be found in the appendix provided by Debarre. We exhibit a connection between these irreducibility questions and the sharpness of Vojta's inequality for algebraic points on curves. In particular, we show that Vojta's inequality is not sharp for the algebraic points arising in our examples.
74.
D. T. Khoa W. von Oertzen Amand Faessler
M. Ermer
H. Clement 《Physics letters. [Part B]》1991,260(3-4):278-284The real optical potential for 16O+16O system is calculated within a generalized version of the double-folding model with the Pauli knock-on exchange effects between the projectile nucleons and the target nucleons taken into account from first principle. The elastic scattering data at Elab=350 MeV supplemented by the new measurement at larger angles seem to be the first case in heavy-ion scattering where one can test the reliability of different theoretical heavy-ion optical potentials. Predictions are made for the elastic scattering of 16O+16O at laboratory energies of 240–480 MeV to illustrare the energy dependence of the rainbow structure which has been clearly observed in experiment at 350 MeV. 相似文献
75.
W. Korneta
Z. Pytel
《Surface science》1988,200(2-3):374-380The random walk of a particle on a three-dimensional semi-infinite lattice is considered. In order to study the effect of the surface on the random walk, it is assumed that the velocity of the particle depends on the distance to the surface. Moreover it is assumed that at any point the particle may be absorbed with a certain probability. The probability of the return of the particle to the starting point and the average time of eventual return are calculated. The dependence of these quantities on the distance to the surface, the probability of absorption and the properties of the surface is discussed. The method of generating functions is used. 相似文献
76.
A novel method is described for preparing reactive mixed crystals by vapor deposition in argon and subsequent evaporation of the argon. The kinetics of photochlorination chain reactions of ethylene-chlorine mixtures prepared by this method have been studied by dynamic UV and IR spectroscopy. Activationless chain growth occurs with mean length 260±70 in the temperature range 17–45 K. Chain lengths greater than 100 are observed up to 60–70% conversion to products. Comparison of the experimental data with computer simulations shows that the tunneling reaction is due to formation of a close-packed reaction complex with high amplitude zero-point vibrations which lower the vibrationally adiabatic barrier to reaction. The commensurability of reactant and product crystal lattices results in linear propagation of long chains without accumulation of deformation strain. Spatial correlations caused by nonlinear propagation are considered. 相似文献
77.
M. Metcalf G. Neuhofer F. Niebergall M. Regler H. Stier K. Winter
J. J. Aubert
X. De Bouard V. Lepeltier L. Massonnet H. Pessard M. Vivargent T. R. Willitts M. YvertW. Bartl
M. Steuer 《Physics letters. [Part B]》1972,40(6):703-707The time-dependence of the decay rate of initially pure K0 into the final state (π+π−π0) has been studied in search for the decay kS0→π+π−π0. No evidence is found in a sample of 384 observed events. The ratio of the CP -violating KS0 amplitude and the KL0 amplitude is η+−0 = (0.13−0.20+0.17) + i(0.17−0.26+0.27); the ratio of the CP-conserving KS0 amplitude and the KL0 amplitude is < 0.4. The energy dependence of the K0→π+π−π0 matrix element is found to be a+−0 = −0.31 ± 0.03. 相似文献
78.
79.
The ground-state energy per particle of symmetrical nuclear matter has been calculated as a function of Fermi wave number for two representative central potentials containing hard cores, by means of both reaction-matrix and Jastrow methods. The agreement of results from the two methods is remarkably good in the density region near equilibrium. 相似文献
80.
The decay of 6.1 sec 203mPb has been examined with a high-resolution Ge(Li) spectrometer, and a 5.1 keV M2 transition was found to compete with the 825.2 keV M4 isometric transition. The M4 transition probability, thus corrected, is now found to be consistent with those of the other M4 transitions in the lead region. 相似文献