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排序方式: 共有109条查询结果,搜索用时 15 毫秒
1.
Brown coal from two areas in New Zealand was treated with acids (HF and HCl) to remove the clays. Iron was then introduced into the “cleaned” coal by ion exchange. Mössbauer spectra of the untreated coal, the low temperature ash of the coal, “cleaned” coal and ion-exchanged coal were analysed. From these spectra we conclude that a pair of lines with QS~1 mm s?1 and IS~0.45 mm s?1 is probably due to organically bonded iron. 相似文献
2.
The recently developed Gauge-Invariant (Including) Atomic Orbital (GIAO) based Time-Dependent Density Functional Theory (TDDFT) methodology for the calculation of transparent spectral region optical rotations of chiral molecules provides a new approach to the determination of absolute configurations. Here, we discuss the application of the TDDFT/GIAO methodology to chiral alkanes. We report B3LYP/aug-cc-pVDZ calculations of the specific rotations of the 22 chiral alkanes, 2-23, of well-established Absolute Configuration. The average absolute deviation of calculated and experimental [alpha](D) values for molecules 2-22 is 24.8. In two of the molecules 2-23, trans-pinane, 10, and endo-isocamphane, 13, the sign of [alpha](D) is incorrectly predicted. Our results demonstrate that absolute configurations of alkanes can be reliably assigned by using B3LYP/aug-cc-pVDZ TDDFT/GIAO calculations if, but only if, [alpha](D) is significantly greater than 25. In the case of (-)-anti-trans-anti-trans-anti-trans-perhydrotriphenylene, 1, [alpha](D) is -93 and TDDFT/GIAO calculations reliably lead to the absolute configuration R(-). 相似文献
3.
J. T. Glass J. F. McCann D. S. F. Crothers 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1996,36(2):119-124
Starting with the impulse approximation, we analyse second-order effects in relativistic electron capture. The relation of this model with relativistic distorted-wave approximations is clarified. In particular it is shown that the second-order spin-coupling terms in the RCDW theory are consistent with the correct form given by perturbation theory. In the semirelativistic limit, the RCDW results are shown to accord with the formulae of Moiseiwitsch for flip and nonflip transitions in the ultra-relativistic limit. This confirms that the continuum-distorted-wave model generalises to relativistic spinors, and highlights the defects of scalar models. We also present a new symmetric eikonal theory which gives reliable results for capture without change of spin, but leads to a divergent total cross section for spin-flip transitions in the second-order term. This effect, which is quite distinct from the spurious spin-flip amplitudes of the scalar symmetric eikonal theory, is taken as further evidence that the eikonal approximation is not valid for magnetic transitions. 相似文献
4.
5.
Shiga Tânia M. Yang Haibing Penning Bryan W. Olek Anna T. McCann Maureen C. Carpita Nicholas C. 《Cellulose (London, England)》2021,28(9):5305-5319
Cellulose - A modified TEMPO-catalyzed oxidation of the solvent-exposed glucosyl units of cellulose to uronic acids, followed by carboxyl reduction with NaBD4 to 6-deutero- and... 相似文献
6.
In many diffusive settings, initial disturbances will gradually disappear and all but their crudest features — such as size and location — will eventually be forgotten. Quantifying the rate at which this information is lost is sometimes a question of central interest. Here this rate is addressed for the fastest conservative nonlinearities in the singular diffusion equation which governs the decay of any integrable, compactly supported initial density towards a characteristically spreading self-similar profile. A potential theoretic comparison technique is outlined below which establishes the sharp conjectured power law rate of decay uniformly in relative error, and in weaker norms such as . To cite this article: Y.J. Kim, R.J. McCann, C. R. Acad. Sci. Paris, Ser. I 341 (2005). 相似文献
7.
Peng LY McCann JF Dundas D Taylor KT Williams ID 《The Journal of chemical physics》2004,120(21):10046-10055
The full-dimensional time-dependent Schr?dinger equation for the electronic dynamics of single-electron systems in intense external fields is solved directly using a discrete method. Our approach combines the finite-difference and Lagrange mesh methods. The method is applied to calculate the quasienergies and ionization probabilities of atomic and molecular systems in intense static and dynamic electric fields. The gauge invariance and accuracy of the method is established. Applications to multiphoton ionization of positronium, the hydrogen atom and the hydrogen molecular ion are presented. At very high laser intensity, above the saturation threshold, we extend the method using a scaling technique to estimate the quasienergies of metastable states of the hydrogen molecular ion. The results are in good agreement with recent experiments. 相似文献
8.
We set out a model, algorithms and strategies for the train pathing and timetabling problem for rail lines of the type normally found in Britain and Europe; that is, lines having separate tracks for trains in each direction. As the pathing problem is combinatorially difficult we propose solution heuristics and strategies analogous to those which have enabled ‘expert’ train pathers to plan large-scale complex rail systems by traditional manual graphical methods. This approach also has the potential advantage of facilitating user interaction, understanding and acceptance. We report computational results for several examples. Without the above strategies we found even very small problems intractable. With these strategies we could solve realistic size problems in acceptable times, while exploring more options and trade-offs than do traditional methods. More work is needed and we suggest natural extensions, and directions for further research and development. 相似文献
9.
Jochen?DenzlerEmail author Robert J.?McCann 《Archive for Rational Mechanics and Analysis》2005,175(3):301-342
The complete spectrum is determined for the operator on the Sobolev space W1,2(Rn) formed by closing the smooth functions of compact support with respect to the norm Here the Barenblatt profile is the stationary attractor of the rescaled diffusion equation in the fast, supercritical regime m the same diffusion dynamics represent the steepest descent down an entropy E(u) on probability measures with respect to the Wasserstein distance d2. Formally, the operator H=HessE is the Hessian of this entropy at its minimum , so the spectral gap H:=2–n(1–m) found below suggests the sharp rate of asymptotic convergence: from any centered initial data 0u(0,x)L1(Rn) with second moments. This bound improves various results in the literature, and suggests the conjecture that the self-similar solution u(t,x)=R(t)–n(x/R(t)) is always slowest to converge. The higher eigenfunctions – which are polynomials with hypergeometric radial parts – and the presence of continuous spectrum yield additional insight into the relations between symmetries of Rn and the flow. Thus the rate of convergence can be improved if we are willing to replace the distance to with the distance to its nearest mass-preserving dilation (or still better, affine image). The strange numerology of the spectrum is explained in terms of the number of moments of .Dedicated to Elliott H. Lieb on the occasion of his 70th birthday. 相似文献
10.
Mössbauer spectra of 57Fe-doped CoS2, NiS2, CoSe2 and NiSe2 have been taken at various temperatures between 300 and 16 K, and the hyperfine parameters H and θ measured for those substances showing magnetic hyperfine splitting. Quadrupole splitting was also measured and for NiS2 and CoS2 the sign of QS was determined.The room temperature value of QS showed a similar behaviour in the series FeS2CoS2NiS2 to that in the series FeSe2CoSe2NiSe2. The absolute value of QS decreased slightly below the magnetic transition temperature in those compounds which showed magnetic ordering. 相似文献