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311.
Photoinduced electron-transfer processes along molecular wires based on phenylenevinylene oligomers: a quantum-chemical insight 总被引:1,自引:0,他引:1
Pourtois G Beljonne D Cornil J Ratner MA Brédas JL 《Journal of the American Chemical Society》2002,124(16):4436-4447
Quantum-chemical techniques are applied to model the mechanisms of photoinduced charge transfer from a pi-electron donating group (tetracene, D) to a pi-electron-acceptor moiety (pyromellitimide, A) separated by a bridge of increasing size (p-phenylenevinylene oligomers, B). Correlated Hartree-Fock semiempirical approaches are exploited to calculate the four main parameters controlling the transfer rate (k(RP)) in the framework of Marcus-Jortner-Levich's formalism: (i) the electronic coupling between the initial and final states; (ii) and (iii) the internal and external reorganization energy terms; and (iv) the variation of the free Gibbs energy. The charge transfer is shown to proceed in these compounds through two competing mechanisms, coherent (superexchange) versus incoherent (bridge-mediated) pathways. While superexchange is the dominant mechanism for short bridges, incoherent transfer through hopping along the phenylene vinylene segment takes over in longer chains (for ca. three phenylenevinylene repeat units). The influence of the chemical structure of the pi-conjugated phenylenevinylene bridge on the electronic properties and the rate of charge transfer is also investigated. 相似文献
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Significant quantum effects in chemistry range from static structure (electronic and geometrical) through dynamical behavior, including optical properties, conductance, relaxation, decoherence, and thermalization. We outline seven situations in which molecular systems exhibit ineluctably quantum behavior. These range from situations in which the community can understand the problem quantitatively and conceptually (for example for dilute sets of spins in NMR) to femtosecond/attosecond situations, which the community understands only primitively. In condensed phase, the dynamics will always evolve in a system/bath environment, and we discuss here how to pose, and to start understanding, problems of that sort. 相似文献
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E. Ratner 《Mathematische Annalen》1888,32(4):566-582
Ohne Zusammenfassung 相似文献
317.
Analysis of the phenomenon of self-heating as a result of competition between hysteresis heating and heat losses to the ambient medium shows that for polymers two zones of steady-state heating are possible: a low-temperature zone, corresponding to a high endaurace limit, and a high-temperature zone, corresponding to low endurance. Between these zones is a temperature region in which steady-state heating is impossible. The high-temperature steady-state zone is frequently not realized as a consequence of the sharp drop in strength at high temperatures. The transition from one steady-state zone to the other is discontinuous, the occurrence of one or the other zone being determined by the deformation conditions (stress, cycling speed, size of specimen, heat conductivity, etc.). However, the self-heating temperature at which this transition takes place does not depend on the deformation conditions, but is determined only by the properties of the material. These conclusions have been confirmed experimentally.Mekhanika Polimerov, Vol. 1, No. 3, pp. 93–100, 1965 相似文献
318.
Upon cooling and compression, both the thermodynamic and the kinetic properties of glassformers change. In fragile glassformers, these changes suggest changes in the local structure of the liquid. Thermodynamic data may then provide a measure of structure changes. Special criteria for structure changes in terms of energy and pressure fluctuations in the isochoric system are proposed; recent theoretical results allow one to rewrite the criteria in terms of long- and short-time thermodynamic characteristics of the liquid. Based on the proposed criteria, we discuss the changes in the local structure due to temperature and pressure variations. 相似文献
319.
Monte Carlo simulation has been used to characterize the capillary force due to the condensation of a liquid meniscus between a tip with a nanoscale asperity and a flat surface. To consider both hydrophobic and hydrophilic molecules coating the tip as a model of dip-pen nanolithography, tips with various wettabilities are studied. The capillary force due to the meniscus is calculated for various saturations (humidities). We have implemented a thermodynamic integration technique that can project the force into energetic and entropic contributions. In most cases, the force is mainly energetic in origin. At the snap-off separation where the meniscus disappears, the tip feels a significant entropic force at high saturation. Our calculation shows nonmonotonic behavior of the pull-off force as a function of saturation, which is in qualitative accord with experiments. 相似文献
320.
M. Ratner 《Israel Journal of Mathematics》1973,15(1):92-114
In this work we construct a Markov partition for transitive Anosov flows, such that the measure of the boundary of the partition
is zero. Symbolic dynamics for these flows is also developed. 相似文献