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1.
Jack G. Calvert 《Chemical physics letters》1973,20(5):484-488
New and previously published SO2 fluorescence emission data related to non-radiative decay processes are considered in light of the recent observations of Brus and McDonald. All the present data are consistent with the previous conclusion of Mettee that non-radiative processes in SO2 singlet photochemistry are unimportant. It appears that any small inefficiency in the emission of light quanta for SO2 excited at short wavelengths (2650 A) is largely due to the population of a second very short-lived state which is quenched effectively even at pressures down to 1 μ. The very low efficiency of quanta production which we observed at long wavelengths (3020 Å) appears to have its origin not only in the second easily quenched state, but more importantly, in the diffusional loss of the long-lived singlet which for these conditions has a 20-fold greated lifetime than was expected previously. 相似文献
2.
The biosynthesis of several sesquiterpenes has been proposed to proceed via germacrene A. However, to date, the production of germacrene A has not been proven directly for any of the sesquiterpene synthases for which it was postulated as an intermediate. We demonstrate here for the first time that significant amounts of germacrene A (7.5% of the total amount of products) are indeed released from wild-type aristolochene synthase (AS) from Penicillium roqueforti. Germacrene A was identified through direct GC-MS comparison to an authentic sample and through production of beta-elemene in a thermal Cope rearrangement. AS also produced a small amount of valencene through deprotonation of C6 rather than C8 in the final step of the reaction. On the basis of the X-ray structure of AS, Tyr 92 was postulated to be the active-site acid responsible for protonation of germacrene A (Caruthers, J. M.; Kang, I.; Rynkiewicz, M. J.; Cane, D. E.; Christianson, D. W. J. Biol. Chem. 2000, 275, 25533-25539). The CD spectra of a mutant protein, ASY92F, in which Tyr 92 was replaced by Phe, and of AS were very similar. ASY92F was approximately 0.1% as active as nonmutated recombinant AS. The steady-state kinetic parameters were measured as 0.138 min(-1) and 0.189 mM for k(cat) and K(M), respectively. Similar to a mutant protein of 5-epi-aristolochene (Rising, K. A.; Starks, C. M.; Noel, J. P.; Chappell, J. J. Am. Chem. Soc. 2000, 122, 1861-1866), the mutant released significant amounts of germacrene A (approximately 29%). ASY92F also produced various amounts of a further five hydrocarbons of molecular weight 204, valencene, beta-(E)-farnesene, alpha- and beta-selinene, and selina-4,11-diene. 相似文献
3.
J. M. Calvert D. G. Lees D. J. Derry D. Barnes 《Journal of Radioanalytical and Nuclear Chemistry》1972,12(2):271-275
The various nuclear techniques which have been used to study oxygen self-diffusion in oxides are discussed. Results are given
for measurements using resonance capture in the18O(p, α)15N and18O(p, γ)19F reactions and the different techniques are compared. 相似文献
4.
Thompson TS Noot DK Calvert J Pernal SF 《Rapid communications in mass spectrometry : RCM》2005,19(3):309-316
A simple and rapid analytical method was developed for the determination of lincomycin and tylosin residues in honey as part of field studies examining the efficacy and target animal safety of these antibiotics to control American foulbrood disease in honey bees. Residues of the antibiotics were determined using liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS). Honey samples were diluted and injected directly into the LC/MS/MS system without additional cleanup by solid-phase extraction or liquid-liquid partitioning. A six-port valve system was utilized to selectively route eluant from the LC column into the mass spectrometer only during a relatively short portion of the chromatographic run corresponding to the elution of the analytes of interest. Minimal contamination of the MS source chamber was observed despite the analysis of large numbers of samples. Using internal standard quantitation, excellent accuracy and precision were obtained with no apparent matrix-to-matrix variation. Based on the analysis of fortified replicates, the mean percent deviation from the theoretical concentration and the percent relative standard deviation were both less than 10% for tylosin over an analytical range of 10-1000 microg/kg. Slightly higher mean percent deviations and relative standard deviations were observed for the analysis of lincomycin in fortified replicate samples. The method detection limits were determined to be 5 and 2 microg/kg for lincomycin and tylosin, respectively. 相似文献
5.
Nitrous acid (HONO) has been observed to build in the atmosphere of cities during the nighttime hours and it is suspected that photolysis of HONO may be a significant source of HO radicals early in the day. The sources of HONO are poorly understood, making it difficult to account for nighttime HONO formation in photochemical modeling studies of urban atmospheres, such as modeling of urban O3 formation. This paper reviews the available information on measurements of HONO in the atmosphere and suggest mechanisms of HONO formation. The most extensive atmospheric measurement databases are used to investigate the relations between HONO and potential precursors. Based on these analyses, the nighttime HONO concentrations are found to correlate best with the product of NO, NO2 and H2O concentrations, or possibly the NO, NO2, H2O, and aerosol concentrations. A new mechanism for nighttime HONO formation is proposed that is consistent with this precursor relationship, namely, reaction of N2O3 with moist aerosols (or other surfaces) to form two HONO molecules. Theoretical considerations of the equilibrium constant for N2O3 formation and the theory of gas-particle reactions show that the proposed reaction is a plausible candidate for HONO formation in urban atmospheres. For photochemical modeling purposes, a relation is derived in terms of gas phase species only (i.e., excluding the aerosol concentration): NO + NO2 + H2O → 2 HONO with a rate constant of 1.68 x 10-17 e6348/T (ppm-2 min-1). This rate constant is based on an analysis of ambient measurements of HONO, NO, NO2 and H2O, with a temperature dependence from the equilibrium constant for formation of N2O3. Photochemical grid modeling is used to investigate the effects of this relation on simulated HONO and O3 concentrations in Los Angeles, and the results are compared to two alternative sources of nighttime HONO that have been used by modelers. Modeling results show that the proposed relation results in HONO concentrations consistent with ambient measurements. Furthermore, the relation represents a conservative modeling approach because HONO production is effectively confined to the model surface layers in the nighttime hours, the time and place for which ambient data exist to show that HONO formation occurs. The empirical relation derived here should provide a useful tool for modelers until such time as knowledge of the HONO forming mechanisms has improved and more quantitative relations can be derived. 相似文献
6.
Greenwald R. A. Geissler K. H. Calvert W. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1973,2(2):40-41
The low frequency cross spectrum of electron density fluctuations in a weakly ionized magnetoplasma is measured with two Langmuir probes aligned transverse to the static magnetic field. The shapes of the observed cross spectra are exponentionally damped cosines as predicted by theory. However, the frequency of the first zero crossing of the cross spectra is strongly dependent on probe bias. 相似文献
7.
The quantum yields of SO3 formation have been determined in pure SO2 and in SO2 mixtures with NO, CO2, and O2 using both flow and static systems. In separate series of experiments excitation of SO2 was effected within the forbidden band, SO2(3B1) ← \documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm SO}_2 (\tilde X,^1 A_1 ) $$\end{document}, and within the first allowed singlet band at 3130 Å. The values of Φ were found to be sensitive to the flow rate of the reactants. These results and the apparently divergent quantum yield results of Cox [10], Allen and coworkers [24, 26, 29], and Okuda and coworkers [11] were rationalized quantitatively in terms of the significant occurrence of the reactions SO + SO3 → 2SO2 (2), and 2SO → SO2 + S [or (SO)2] (3), in experiments of long residence time. From the present rate data, values of the rate constants were estimated, k2=(1.2±0.7) × 106; k3=(5±4) × 105 l˙/mole · sec. Φ values from triplet excitation experiments at high flow rates of NO? SO2 and CO2? SO2 mixtures showed the sole reactant with SO2 leading to SO3 formation in this system to be SO2(3B1); SO2(3B1) + SO2 → SO3 + SO(3Σ?) (la); k=(4.2±0.4) × 107 l./mole · sec. With excitation of SO2 at 3130 Å both singlet and triplet excited states play a role in SO3 formation. If the reactive singlet state is 1B1, the long-lived fluorescent state, SO2(1B1) + SO2 → SO3 + SO (1 Δ or 3Σ?) (lb), then k=(2.2±0.5) × 109 l./mole · sec. From the observed inhibition of SO formation by added nitric oxide, it was found that the SO3-forming triplet state, generated in this singlet excited SO2 system, had a relative reactivity toward SO2 and NO which was equal within the experimental error to that observed here for the SO2(3B1) species. Either SO2(3B1) molecules were created with an unexpectedly high efficiency in 3130 Å excited SO2(1B1) quenching collisions, or another reactive triplet (presumably 3A2 or 3B2) of almost identical reactivity to SO2(3B1) was important here. 相似文献
8.
Background
Growth hormone (GH) plays an incompletely understood role in the development of the central nervous system (CNS). In this study, we use transgenic mice expressing a growth hormone antagonist (GHA) to explore the role of GH in regulating postnatal brain, spinal cord and body growth into adulthood. The GHA transgene encodes a protein that inhibits the binding of GH to its receptor, specifically antagonizing the trophic effects of endogenous GH. 相似文献9.
Conventional use of magnetic bearings relies on a zero reference to keep the rotor centered in the radial and axial axes. This paper compares different control methods developed for the alternate control task of tracking an axial dynamic target. Controllers based on fuzzy logic, sliding mode, and direct linearization were designed to meet this task. Performance criteria, such as maximum axial displacement, minimum phase lag and I2R power losses were compared for each controller. The large motion, tracking problem for a rotor has utility in applications where dynamic seal clearances are required. This has a variety of potential applications in turbo-machinery, such as active stall control. 相似文献
10.
Bruce D. Calvert Armen H. Zemanian 《Transactions of the American Mathematical Society》2000,352(2):753-780
Given a nonlinear infinite resistive network, an operating point can be determined by approximating the network by finite networks obtained by shorting together various infinite sets of nodes, and then taking a limit of the nodal potential functions of the finite networks. Initially, by taking a completion of the node set of the infinite network under a metric given by the resistances, limit points are obtained that represent generalized ends, which we call ``terminals,' of the infinite network. These terminals can be shorted together to obtain a generalized kind of node, a special case of a 1-node. An operating point will involve Kirchhoff's current law holding at 1-nodes, and so the flow of current into these terminals is studied. We give existence and bounds for an operating point that also has a nodal potential function, which is continuous at the 1-nodes. The existence is derived from the said approximations.