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41.
42.
We established that acetylacetone and acetone photolytically sensitize norbornene to undergo an efficient radical addition
of solvent (ranging from hexane, cyclic ethers, haloalkanes, acetone, alcohols and acetonitrile) across the double bond. In
view of its synthetic applicability, sensitized photoreactions of norbornene were reviewed and their mechanisms were compared.
Photolysis of acetylacetone in the presence of norbornene in hexane induced i) acetylacetone to cycloadd to norbornene giving
the expected 1,5-diketone, and ii) sensitization by triplet excited acetylacetone to generate reactive norbornene, which underwent
dimerization as well as the addition of a solvent molecule by radical chain processes. In other solvents, the radical chain
addition of solvent dominated the photoreaction, and superseded the cycloaddition, to give excellent to good yields of adducts
to norbornene. While the excited species of acetylacetone for the sensitization was deduced to be its spectroscopic triplet
excited state, that for the cycloaddition should involve a different one which may be a twisted triplet acetylacetone; sensitization
experiments showed that the cycloaddition did not occur from the spectroscopic triplet state. Triplet excited acetone sensitized
norbornene to undergo the same solvent addition more efficiently and cleanly than acetylacetone did. In view of various conflicts
existing in the proposed energy transfer mechanism, the sensitized norbornene reactions were rationalized with electron transfer
and a cation radical chain mechanism. 相似文献
43.
Zhai X. Garate E. Prohaska R. Benford G. Fisher A. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1993,21(1):142-150
Experimental studies of a plasma-filled X -band backward-wave oscillator (BWO) are presented. Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement by a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from ⩽0.5 GHz to 2 GHz when the BWO was working at the RF power enhancement pressure region. The RF power enhancement appeared over a much wider pressure range in a high beam current case (10-100 mT for 3 kA) than in a lower beam case (80-115 mT for 1.6 kA). The plasma-filled BWO has higher power output than the vacuum BWO over a broader region of magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) are observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the T-G modes and the X -band Tm01 mode prevailed when the background plasma density was below 6×1011 cm-3 . At a critical background plasma density of ≃8×1011 cm-3 power enhancement appeared in both X -band and the T-G modes. Power enhancement of the S -band in this mode collaboration region reached up to 8 dB. Electric fields measured by the Stark-effect method were as high as 34 kV/cm while the BWO power level was 80 MW. These electric fields lasted throughout the high-power microwave pulse 相似文献
44.
The standard model on non-commutative space-time 总被引:1,自引:0,他引:1
X. Calmet B. Jurčo P. Schupp J. Wess M. Wohlgenannt 《The European Physical Journal C - Particles and Fields》2002,23(2):363-376
We consider the standard model on a non-commutative space and expand the action in the non-commutativity parameter . No new particles are introduced; the structure group is . We derive the leading order action. At zeroth order the action coincides with the ordinary standard model. At leading order
in we find new vertices which are absent in the standard model on commutative space-time. The most striking features are couplings
between quarks, gluons and electroweak bosons and many new vertices in the charged and neutral currents. We find that parity
is violated in non-commutative QCD. The Higgs mechanism can be applied. QED is not deformed in the minimal version of the
NCSM to the order considered.
Received: 29 November 2001 / Published online: 25 January 2002 相似文献
45.
This paper presents an analysis of a portfolio model which can be used to assist a property-liability insurance company in determining the optimal composition of the insurance and investment portfolios. By introducing insurer's threshold risk and relaxing some non-realistic assumptions made in traditional chance constraint insurance and investment portfolio models, we propose a method for an insurer to maximize his return threshold for a given threshold risk level. This proposed model can be used to optimize the composition of underwriting and investment portfolios regarding the insurer's threshold risk level, as well as to generate the efficient frontier by adjusting insurer's threshold risk levels. A numerical example is given based on the industry's aggregated data for a sixteen year period. 相似文献
46.
47.
Observation of spontaneous pattern with six-fold symmetry in disk-shaped ZnO complex microstructures
L.W. Yang Y.J. Gao X.L. Wu Y.M. Yang G.S. Huang Z.Y. Zhang P.K. Chu 《Applied Physics A: Materials Science & Processing》2007,89(1):173-176
Using a simple vapor phase transport technique, we have fabricated unique complex disk-shaped ZnO microstructures comprising
a small disk coaxially grown on a large one and observed spatially perfect six-fold symmetric patterns. The observed results
can be explained based on the spontaneous nanoindentation (NI) effect under the geometric constraints and the explanation
can be extended to fathom the growth mechanism of other highly symmetrical ZnO nanostructures. Our results indicate that NI
not only can elucidate the mechanical properties of surfaces and thin films but also is an effective approach to fabricate
ordered nanostructures with high precision on the location of the building blocks.
PACS 81.16.Rf; 81.07.-b; 81.05.Dz 相似文献
48.
J. Xu J. Mei X.H. Huang X. Li Z. Li W. Li K. Chen 《Applied Physics A: Materials Science & Processing》2005,80(1):123-126
The effect of nitrogen surface doping on the electron field emission characteristics of hydrogenated amorphous carbon films was studied. It was found that the emission threshold electric field decreased after nitrogen surface doping which indicated the reduction of the emission barrier at the film surface. It was demonstrated that this improvement arose from the decrease of the effective work function due to the formation of a dipole layer induced by chemical termination and chemical structural change on the film surface. PACS 52.75.Xx; 73.30 +y; 73.61.Jc 相似文献
49.
The microstructure evolution of decagonal quasicrystals in Al72Ni12Co16 alloy was investigated by the electromagnetic melting and cyclic superheating method. Single-phase decagonal quasicrystals have been obtained when the undercoolings were larger than 60 K. The decagonal quasicrystals formed at various undercoolings show different microstructural morphologies. Furthermore, grain refinement was found near the undercooling of 120 K. Based on current thermodynamic and dendrite growth theories, a dimensionless superheating parameter was adopted to explain the effect of processing conditions on the microstructure of Al72Ni12Co16 alloy. The result indicate that the fine equiaxied microstructure of decagonal quasicrystal (D-phase) formed near on undercooling of 120 K originates from the break-up of dendrites. 相似文献
50.
应用七级目测法对浙江中部地区27个夏熟作物田样点的杂草进行了优势等级调查,将所得数据转换成重要值,以杂草在27个样点中的重要值为运算指标,应用主成分分析法和图论聚类中的最小生成树法,对24种杂草的生态学相似性进行了比较,指出了长期使用单一的除草剂导致农田杂草种群迅速更迭的原因:杂草对除草剂的敏感性差异及不同杂草间的生态学特性的差异。 相似文献