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71.
The X-ray single crystal structure of 3-R Nb1.06S2 has been determined. The material crystallizes in the space group R3m with a = 3.3285(4) and c = 17.910(4) Å when indexed on a hexagonal lattice. The structure was refined by full matrix least squares procedures to a final residual of R = 0.026 based on 79 observed (I > 3σI) reflections. The sulfurs form closest-packed layers with the majority of the metal in sheets of trigonal prismatic sites. A small portion of niobium was found to occupy octahedral sites, between the van der Waals gaps of the sulfur lattice. Niobium in the van der Waals region is trigonally distorted from octahedral symmetry, with niobium-sulfur distances of 2.234(8) and 2.577(11) Å, because of repulsion from niobium in adjacent trigonal prismatic layers. 相似文献
72.
Host sensitized energy transfer in CaWO4 doped with Eu3+ and Sm3+ is investigated using pulsed laser time-resolved spectroscopy techniques. The excitation is into the long wavelength tail of the absorption edge and the results indicate that this selectively excites tungstate ions which are located next to activator impurity ions. The subsequent energy transfer is found to be consistent with a single step, nearest neighbor process. Both electric dipole-dipole and exchange interactions can predict strong enough transfer to account for the observed rates. These results are compared to those obtained previously for broad band excitation into the highest energy absorption bands of CaWO4 crystals. 相似文献
73.
1H NMR studies of an equilibrium mixture of 1,1,1,5,5,5-hexafluoropentane-2,4-dionato-π-allylplatinum(II) [IA] and bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato)bis(μ-allyl)diplatinum(II) [IB] show that the μ-allylic ligands of (IB) exhibit temperature dependent tautomeric behaviour, giving rise to a site-exchange between protons on these ligands. 相似文献
74.
Time resolved spectroscopy and fluorescence decay time measurements have been made on thin films of amorphous polyvinyl carbazole (PVK). The value obtained for the excimer fluorescence decay time in the undoped sample is less than that reported previously and a wavelength dependent double exponential decay is observed, which has been seen before only at low temperatures. The observed quenching of the excimer fluorescence decay times in the doped samples is evidence of direct energy transfer from excimers to perylene which is inconsistent with the model of energy transfer by exciton diffusion proposed to explain intensity quenching data. Several models for energy transfer in PVK are discussed and it is shown that the only model which predicts the observed time resolved spectroscopy results requires a long range resonant interaction strength much greater than that obtained theoretically. Several possible reasons for this are discussed. 相似文献
75.
Richard C. Powell 《Journal of luminescence》1973,6(4):285-295
Time resolved fluorescence spectroscopy measurements were used to investigate energy transfer in anthracene and tetracene doped fluorene crystals. The results for energy transfer from the host to anthracene molecules are consistent with both the recently proposed generalized random walk model for energy transfer and the combined exciton diffusion—long range resonance theory. The results for impurity sensitized energy transfer between anthracene and tetracene molecules in fluorene are in agreement with the predictions of long range resonance interaction. Comparisons are made with results of similar investigations of other systems. 相似文献
76.
Data for the resolution of americium from europium and terbium using 2,2′-diaminodiethylether-N,N,N′,N′-tetraacetate and 1,5-diaminopentane-N,N,N′,N′-tetraacetate are reported along with values of the formation constants of the La-Lu and Y chelates of the latter. It is shown that the minimum single-stage separation factor for Am3+ from Ln3+ cations, using 2,2′-diaminodiethylether-N,N,N′,-N′-tetraacetate with Dowex 50 resin, exceeds 1.7 for all Am-Ln pairs, and runs as high as 350 in the case of Am3+, La3+. The minimum of 1.7 occurs at Eu3+ in the lanthanon sequence. A novel separation of Am3+, Cm3+ and heavier actinons from each other and from all the lanthanons and yttrium appears to be feasible. 相似文献
77.
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79.
Bertone PF Champagne AE Powell DC Iliadis C Hale SE Hansper VY 《Physical review letters》2001,87(15):152501
The energy derived from the CN cycle at low stellar temperatures is regulated by the 14N(p,gamma)15O reaction. A previous direct measurement of this reaction has been interpreted as showing evidence for a subthreshold resonance which makes a major contribution to the reaction rate at low temperatures. This resonance, at E(c.m.) = -504 keV would correspond to the known Ex = 6793-keV state in 15O. We have measured a mean lifetime of 1.60(+0.75)(-0.72) fs (90% C.L.) for this state using the Doppler-shift attenuation method. This lifetime is a factor of 15 longer than that inferred from the (p,gamma) data and implies that the contribution of the subthreshold resonance is negligible. 相似文献
80.
Joosten R Powell J Guo FQ Haustein PE Larimer R McMahan MA Norman EB O'Neil JP Rowe MW VanBrocklin HF Wutte D Xu XJ Cerny J 《Physical review letters》2000,84(22):5066-5069
A light-element radioactive ion-beam capability has been developed at the LBNL 88-Inch Cyclotron. The system is based on the coupled-cyclotrons method and utilizes short-lived species, e.g., 11C, 14O, 13N produced by (p,n) and (p,alpha) reactions at the LBNL Biomedical Isotope Facility Cyclotron. In a first experiment, 197Au(11C,xn)208-xAt excitation functions have been measured for energies ranging from the Coulomb barrier up to 110 MeV using a beam of 11C with intensities up to (1-2)x10(8) ions/sec on target. The results of this experiment are compared to measurements of 197Au(12C, xn)209-xAt excitation functions. 相似文献