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621.
622.
Michael P. Barnett 《Theoretical chemistry accounts》2002,107(4):241-245
In the course of checking our work on the symbolic calculation of molecular integrals over Slater orbitals, we obtained some
results in substantial disagreement with two recent articles that describe numerical schemes. We believe that these schemes
suffer from digital erosion, possibly because recurrence formulas were used outside their regions of stability. Our results
were obtained using the ζ-function method, which expands the orbital on one atom onto the other, and integrates in polar coordinates.
They were checked using elliptic coordinates. Both sets of calculations were performed symbolically. We summarize these calculations
and discuss the impact of symbolic calculation on the accuracy of molecular computations.
Received: 31 August 2001 / Accepted: 29 October 2001 / Published online: 8 April 2002 相似文献
623.
A model describing the incorporation of thermal dopants into single crystal films grown by molecular beam epitaxy (MBE) is presented. The model is general, accounts for dopant surface segregation during deposition, and allows dopant incorporation probabilities and depth profiles to be calculated as a function of film growth conditions (e.g. deposition rate, dopant beam flux, and growth temperature Ts). Input data to the model include thermodynamic parameters such as the free energy of segregation and dopant-surface binding energies together with kinetic parameters such as incident fluxes and dopant diffusivities. The model is applied here to Si MBE in which common dopants are typically characterized by strong surface segregation and temperature-dependent incorporation probabilities σ. Calculated values of σ(Ts) and calculated depth profiles were found to agree very well with available experimental data for both group-III acceptors and group-V donors in Si. In addition, the model predicts, in agreement with limited experimental data, that a growth parameter range exists in which abrupt doping profiles can be obtained, even for dopants which exhibit strong surface segregation. Finally, transition temperatures from equilibrium to kinetically-limited segregation are determined for several dopants. 相似文献
624.
Hai-Ping Cheng R. N. Barnett Uzi Landman 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,26(1):296-300
Energetics and structures of small aluminum-lithium clusters were investigated using structure-minimization and dynamic simulated annealing on the Born-Oppenheimer surface, calculated via local-spin-density functional method. A transition from atomic-based characteristics forn≤5 to a perturbed delocalized electronic cluster-shell behavior forn≥6, containing an AiLi5 “core”, is suggested. 相似文献
625.
Ronald G. Rehm P.Darcy Barnett Howard R. Baum Daniel M. Corley 《Applied Numerical Mathematics》1985,1(6):515-529
Solutions are presented to nonlinear finite difference equations used to represent fire-driven buoyant convection in enclosures. The solutions depend upon the fact that these difference equations permit the decomposition of the discretized velocity field into solenoidal and irrotational components. The irrotational field is shown to satisfy a finite difference analog of Bernoulli's equation when the density is constant. This leads to a three-dimensional time-dependent solution to the difference equations. The solenoidal field is shown to possess steady-state two-dimensional solutions corresponding to a constant non-zero value of the discretized vorticity. The two solutions, together with results presented elsewhere describing finite difference approximations to linear internal waves in enclosures, have been used in the development and testing of the computer-based algorithms used to solve these equations. They have proved particularly useful in assessing the accuracy of finite difference approximations to the equations of inviscid fluid mechanics, as well as in debugging the computer codes implementing these algorithms. 相似文献
626.
Continued-fraction expansions of Stieltjes and second Cauer from are considered for a ratio of polynomials expressed relative to an arbitrary real orthogonal polynomials basis. Simple tabular arrays are developed for obtaining the coefficients in the expansions. Inverse procedures are also obtained. An application to a diophantine equation, and corresponding results for matrix continued fractions, are also given. 相似文献
627.
D. K. Olsen A. R. Barnett S. F. Biagi N. H. Merrill W. R. Phillips 《Nuclear Physics A》1974,220(3):541-568
The static quadrupole moments of the first excited Jπ = 2+ states in 20Ne and 22Ne and the reduced electric quadrupole transition probabilities of these states to the ground states were measured via projectile Coulomb excitation. The quadrupole moments were deduced from the shapes of γ-ray angular distributions. The results are: Q(20Ne, 2+) = −0.20±0.05 b and Q(22Ne, 2+) = −0.11±0.05 b. The transition strengths were deduced from yield measurements and by comparison with the yields of target γ-rays. The results are: B(E2; 0+ → 2+, 20Ne) = 0.037±0.003 e2 · b2 and B(E2; 0+ → 2+, 22Ne) = 0.025±0.002 e2· b2. The results for the transition strengths are consistent with the results of accurate timing methods and resolve discrepancies between previous experiments. The results for the quadrupole moments are consistent with earlier measurements, although the mean values we obtain are slightly lower. The experimental measurements are compared with theoretical predictions and a detailed discussion is given of corrections to this type of reorientation experiment. 相似文献
628.
629.
630.
The four coproporphyrin “type-isomers” have been synthesized as their tetramethyl esters (1) through modifications of existing procedures. When treated with thallium(III) trifluoroacetate, these porphyrins furnish the corresponding aquo porphinatothallium(III) hydroxides (2) after ligand exchange induced by chromatography on deactivated alumina.The proton NMR spectra of the chloroform solutions of thallium(III) coproporphyrins show a pronounced concentration dependence, all resonances moving to low field upon dilution; the spectra of the type-III and -IV isomers show additional fine structure in solution. Both the meso- and β-Me protons show thallium-proton spin couplings. These results are interpreted in terms of a monomer-dimer equilibrium. In the dimers of the type-I and -II chelates, the rings lie directly one above another, whereas with the type-III and -IV complexes, steric repulsions of the propionate side-chains cause lateral displacement of one molecule in the dimer relative to the other, resulting in the observed fine structure in the spectra. The inter-porphyrin distances and lateral displacements are calculated on this basis and are compared with the corresponding dimers of the parent coproporphyrins, with which there are considerable similarities. 相似文献