全文获取类型
收费全文 | 70篇 |
免费 | 2篇 |
专业分类
化学 | 57篇 |
数学 | 4篇 |
物理学 | 11篇 |
出版年
2022年 | 4篇 |
2021年 | 5篇 |
2020年 | 2篇 |
2018年 | 3篇 |
2015年 | 1篇 |
2014年 | 4篇 |
2013年 | 1篇 |
2012年 | 3篇 |
2011年 | 5篇 |
2010年 | 1篇 |
2009年 | 2篇 |
2008年 | 1篇 |
2007年 | 4篇 |
2006年 | 2篇 |
2005年 | 7篇 |
2004年 | 6篇 |
2003年 | 2篇 |
2002年 | 5篇 |
2001年 | 2篇 |
2000年 | 1篇 |
1998年 | 1篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1989年 | 1篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
排序方式: 共有72条查询结果,搜索用时 15 毫秒
21.
The gas-phase ion chemistry of silane-allene-ammonia, germane-allene (or propyne)-ammonia (or phosphine) systems was studied by ion trap mass spectrometry. Reaction sequences were determined and rate constants were measured for the main processes observed. The mixture containing silane displays higher reactivity with respect to that with germane. Comparison with analogous systems provides useful information about the reactivity of different hydrocarbon molecules and the different affinities of silicon and germanium towards nitrogen and phosphorus. The most interesting product ions observed are those containing Si (or Ge), C and N (or P) elements together, as these ion species may be considered precursors of doped amorphous carbides, which are widely used in semiconductor devices. 相似文献
22.
We investigate the capability of dynamical decoupling techniques to reduce decoherence from a realistic environment generating 1/f noise. The predominance of low frequency modes in the noise profile allows for decoherence scenarios where relatively slow control rates suffice for a drastic improvement. However, the actual figure of merit is very sensitive to the details of the dynamics, with decoupling performance which may deteriorate for non-Gaussian noise and/or high frequency working points. Our results are promising for robust solid-state qubits and beyond. 相似文献
23.
We present a general control-theoretic framework for constructing and analyzing random decoupling schemes, applicable to quantum dynamical control of arbitrary finite-dimensional composite systems. The basic idea is to design the control propagator according to a random rather than deterministic path on a group. We characterize the performance of random decoupling protocols, and identify control scenarios where they can significantly weaken time scale requirements as compared to cyclic counterparts. Implications for reliable quantum computation are discussed. 相似文献
24.
We show that the solutions of a population equation with age structure and delayed birth process have asynchronous exponential growth (AEG). We use operator matrices and Hille-Yosida operators as well as Perron-Frobenius techniques. 相似文献
25.
Mancinelli V Rinaldi M Finessi E Emblico L Mircea M Fuzzi S Facchini MC Decesari S 《Journal of chromatography. A》2007,1149(2):385-389
The chemical composition of water-soluble organic carbon (WSOC) in atmospheric aerosol particles is largely unexplored, due to the myriad of individual compounds, which has hampered attempts to attain a full characterization at the molecular level. An alternative approach, focusing on the analysis of a few main chemical classes, allowed the quantitative fractionation of WSOC into neutral compounds (NC), mono- and di-acids (MDA) and polyacids (PA) through an anion-exchange liquid chromatographic method. Previous attempts to quantify NC, MDA and PA relied on a low-pressure chromatographic technique using a volatile buffer, followed by total organic carbon (TOC) analysis of the fractions, or alternatively on a faster HPLC-UV method which provided a quantification of the fractions based on empirical relationships between UV signal and TOC concentration. Here, we report an upgraded anion-exchange HPLC technique, allowing direct TOC analysis of the eluted fractions, without any pre-treatment, thus permitting a great simplification of quantitative analysis and preventing sample losses. The new HPLC-TOC methodology, employing completely inorganic eluents shows the same efficiency of the former HPLC-UV method employing organic additives, with the exception of phenolic compounds, which are retained on the column by secondary interactions, and low-molecular weight tricarboxylic acids, which are recovered among MDA. The new anion-exchange liquid chromatographic method can recover a substantial amount (86-100%) of water-soluble organic carbon from atmospheric aerosol extracts, thus allowing a higher retention and fractionation capacity with respect to alternative techniques, like solid phase extraction. 相似文献
26.
Operti L Rabezzana R Turco F Borocci S Giordani M Grandinetti F 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(38):10682-10689
The xenon–difluoronitrenium ion F2N? Xe+, a novel xenon–nitrogen species, was obtained in the gas phase by the nucleophilic displacement of HF from protonated NF3 by Xe. According to Møller–Plesset (MP2) and CCSD(T) theoretical calculations, the enthalpy and Gibbs energy changes (ΔH and ΔG) of this process are predicted to be ?3 kcal mol?1. The conceivable alternative formation of the inserted isomers FN? XeF+ is instead endothermic by approximately 40–60 kcal mol?1 and is not attainable under the employed ion‐trap mass spectrometric conditions. F2N? Xe+ is theoretically characterized as a weak electrostatic complex between NF2+ and Xe, with a Xe? N bond length of 2.4–2.5 Å, and a dissociation enthalpy and free energy into its constituting fragments of 15 and 8 kcal mol?1, respectively. F2N? Xe+ is more fragile than the xenon–nitrenium ions (FO2S)2NXe+, F5SN(H)Xe+, and F5TeN(H)Xe+ observed in the condensed phase, but it is still stable enough to be observed in the gas phase. Other otherwise elusive xenon–nitrogen species could be obtained under these experimental conditions. 相似文献
27.
Lorenza Gardella Dario Cavallo Samuele Colonna Alberto Fina Orietta Monticelli 《Journal of polymer science. Part A, Polymer chemistry》2014,52(22):3269-3282
Novel poly(l ‐lactide) (PLLA)/poly(d ‐lactide) (PDLA)/poly(tetrahydrofuran) (PTHF) multiblock copolymers with designed molecular structure were synthesized by a two‐stage procedure. Well‐defined PDLA‐PLLA‐PTHF‐PLLA‐PDLA pentablock copolymers were prepared by sequential ring opening polymerization of l ‐ and d ‐lactides starting from PTHF glycol, with the length of the (equimolar) PLLA and PDLA blocks being varied. Then, these dihydroxyl‐terminated pentamers were transformed into multiblock copolymers by melt chain‐extension with hexamethylene diisocyanate–being the first time that the coupling of pentablock units is reported. The successful formation of macromolecular chains with a multiblock and well‐defined architecture was demonstrated by 1H NMR spectroscopy. The thermal properties and structuring of the resulting materials were investigated by means of DSC and WAXD measurements and DMA analysis. Stereocomplexation was found to be promoted during solution and melt crystallization. This approach affords materials combining the high rigidity and strength (other than improved thermal resistance) of the hard stereocomplex crystallites with the flexibility imparted by the soft block, whereby their properties can be finely tailored through the composition of the basic pentablock units without limitations on the final molecular weight. The adopted reaction conditions make this process highly appealing in view of the possibility to perform it in extruder. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3269–3282 相似文献
28.
29.
Calderan S Carbone P Operti L Rabezzana R Vaglio GA 《Journal of mass spectrometry : JMS》2002,37(12):1205-1212
The gas-phase ion chemistry of propene-ammonia and silane-propene-ammonia mixtures was studied by ion trap mass spectrometry. As far as the binary mixture is concerned, the effect of different molar ratios of the reactants on the trend of ion species formed was evaluated, the ion-molecule reaction processes were identified and the rate constants for the main processes were measured. The results were compared with the collisional rate constants to determine the reaction efficiencies. In the ternary silane-propene-ammonia mixture the mechanisms of formation of Si(m)C(n)N(p)H(q)(+) clusters were elucidated and the rate constants of the most important steps were measured. For some species, selected by double isolation (MS/MS), the low abundance of the ions allowed us to determine the reaction paths but not the rate constants. Ternary ions are mainly formed by reactions of Si(m)C(n)H(q)(+) ions with ammonia, whereas a minor contribution comes from reactions of Si(m)N(p)H(q)(+) ions with propene. On the other hand, the C(n)N(p)H(q)(+) ions showed a very low reactivity and no step leading to ternary ion species was identified. The formation of hydrogenated ternary ions with Si, C and N has a basic importance in relation to their possible role as precursors of amorphous silicon carbides doped with nitrogen obtained by deposition from silane-propene-ammonia mixtures properly activated. 相似文献
30.
Olimpia Gambino Paola Michelin-Lausarot Gian Angelo Vaglio Mario Valle Paolo Volpe Lorenza Operti 《Transition Metal Chemistry》1982,7(6):330-332
Summary The positive chemical ionization mass spectra, with ammonia and isobutane at 0.5 torr of (6-arene)Cr(CO)3, where arene is C6H5COMe, C6H5COEt, C6H5COC3H7, C6H5COCMe3, 2-MeC6H4COC3H7, C6H5COOMe, C6H5CH2 COEt, C6H5Me and 1,3,5-Me3C6H3 are reported. All these compounds exhibit [M + H]+ as base peak, with isobutane. In the presence of ammonia, [M + NH4]+ is the most abundant ion, when the arene is a ketone or an ester and the addition site is suggested to be the ligand. Conversely [M + H]+ is the base peak when arene is toluene and mesitylene and the protonation site is likely to be the metal atom. In the presence of ammonia, ions such as [M — 3 CO + NH3]+ and [M — 3 CO + 2 NH3]+ are also observed and their abundances strongly depend on the pressure of the reagent gas. 相似文献