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排序方式: 共有59条查询结果,搜索用时 265 毫秒
1.
Leo Valenta Petr Kovaí
ek Vclav Vale Zdenk Bastl Karolina A. Drogowska Timotheus A. Verhagen Radek Cibulka Martin Kalb
《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(5):1338-1342
Spatially resolved functionalization of 2D materials is highly demanded but very challenging to achieve. The chemical patterning is typically tackled by preventing contact between the reagent and material, which brings various accompanying challenges. Photochemical transformation on the other hand inherently provides remote high spatiotemporal resolution using the cleanest reagent—a photon. Herein, we combine two competing reactions on a graphene substrate to create functionalization patterns on a micrometer scale via the Mitsunobu reaction. The mild reaction conditions allow introduction of covalently dynamic linkages, which can serve as reversible labels for surface‐ or graphene‐enhanced Raman spectroscopy characterization of the patterns prepared. The proposed methodology thus provides a pathway for local introduction of arbitrary functional groups on graphene. 相似文献
2.
Einstein's field equations with variablegravitational coupling G = G(t) and decaying vacuumenergy density = (t) are considered asdescribing matter creation in a cosmological framework.The particle creation rate is determined by thevariation rate of both G and . By consideringsimple phenomenological evolution laws for G and Lambda,an exact solution of the gravitational field equationsfor a flat Friedmann-Robertson-Walker (FRW)space-time is obtained leading to a self-consistentcosmological model describing matter and entropygeneration in the very early Universe. 相似文献
3.
A. M. W. Verhagen 《Journal of statistical physics》1976,15(3):219-231
An anisotropic triangular Ising model in which the first- and second-order parameters and the field parameters are functionally related is solved exactly by representing the distribution of the atom patterns in terms of a suitably constructed Markov process. The probabilities of patterns, defined as the probabilities generated by this process, are a mathematically tractable alternative to the classical representation of these probabilities in terms of the partition function. The interaction and field parameters of this Ising model, its magnetization, free energy, and its nearest neighbor correlation functions, are expressed in terms of the parameters of this Markov process. Special cases are worked out in detail and numerical examples are given. 相似文献
4.
B. Th. Verhagen 《Journal of Radioanalytical and Nuclear Chemistry》2003,257(1):17-26
Ground water has increasingly taken its place in the provision of safe, potable supply in the developing world. Large investments have been made in infrastructural development for rural ground water supply schemes, but far too little attention has been given to assess the sustainability of these supplies. Overexploitation of aquifers, evident in failing boreholes and deteriorating water quality, has become a world-wide concern. Developments in physics half a century ago established the basis of isotope hydrology. Radioactive isotopes give information on ground water dynamics and recharge rates whilst non-radioactive - or stable - isotopes indicate origins of ground water and delineate ground water bodies. Environmental isotope hydrology is increasingly seen as a powerful discipline in assessing ground water systems. This is particularly important in developing environments, where historical data is rarely available. Brief examples are presented of isotope applications to collaborative ground water studies conducted at the University of the Witwatersrand. Recharge estimates based on isotope snapshot data conform well with results from subsequent long-term water level observations in the Kalahari of Botswana. The importance is demonstrated of irrigation return flow and pollution hazard to the Lomagundi dolomite of Zimbabwe. Isotopes suggest the source of high nitrate concentrations to an important ground water supply in Tanzania. Mechanisms of the release of arsenic into millions of tube wells in Bangladesh are put into perspective. Isotope hydrology as appropriate technology is highlighted in terms of its cost-effectiveness and the investigative empowerment of local investigators. 相似文献
5.
Iuga D Schäfer H Verhagen R Kentgens AP 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2000,147(2):192-209
We have recently shown that the sensitivity of single- and multiple-quantum NMR experiments of half-integer (N/2) quadrupolar nuclei can be increased significantly by introducing so-called double frequency sweeps (DFS) in various pulse schemes. These sweeps consist of two sidebands generated by an amplitude modulation of the RF carrier. Using a time-dependent amplitude modulation the sidebands can be swept through a certain frequency range. Inspired by the work of Vega and Naor (J. Chem. Phys. 75, 75 (1981)), this is used to manipulate +/-(m - 1) <--> +/-m (3/2 < or = m < or = N/2) satellite transitions in half-integer spin systems simultaneously. For (23)Na (I = 3/2) and (27)Al (I = 5/2) spins in single crystals it proved possible to transfer the populations of the outer +/-m spin levels to the inner +/-1/2 spin levels. A detailed analysis shows that the efficiency of this process is a function of the adiabaticity with which the various spin transitions are passed during the sweep. In powders these sweep parameters have to be optimized to satisfy the appropriate conditions for a maximum of spins in the powder distribution. The effects of sweep rate, sweep range, and RF field strength are investigated both numerically and experimentally. Using a DFS as a preparation period leads to significantly enhanced central transition powder spectra under both static and MAS conditions, compared to single pulse excitation. DFSs prove to be very efficient tools not only for population transfer, but also for coherence transfer. This can be exploited for the multiple- to single-quantum transfer in MQMAS experiments. It is demonstrated, theoretically and experimentally, that DFSs are capable of transferring both quintuple-quantum and triple-quantum coherence into single-quantum coherence in I = 5/2 spin systems. This leads to a significant enhancement in signal-to-noise ratio and strongly reduces the RF power requirement compared to pulsed MQMAS experiments, thus extending their applicability. This is demonstrated by (27)Al 3QMAS experiments on 9Al(2)O(3). 2B(2)O(3) and the mineral andalusite. In the latter compound, Al experiences a quadrupolar-coupling constant of 15.3 MHz in one of the sites. Finally a 5QMAS spectrum on 9Al(2)O(3). 2B(2)O(3) demonstrates the sensitivity enhancement of this experiment using a double frequency sweep. 相似文献
6.
We propose that the Universe is filled with a massive vector field non-minimally coupled to gravitation. The field equations
of the model are consistently derived, and their application to cosmology is considered. The Friedmann equations acquire an
extra dark-energy component, which is proportional to the mass of the vector particle. This leads to a late-time accelerated
de Sitter type expansion. The free parameters of the model (gravitational coupling constants and initial value of the cosmological
vector field) can be estimated by using the PPN solar system constraints. The mass of the cosmological massive vector particle,
which may represent the main component of the Universe, is of the order of 10-63 g.
PACS 03.70.+k; 11.90.+t; 11.10.Kk 相似文献
7.
8.
9.
Acta Mathematicae Applicatae Sinica, English Series - We present some exact integrability cases of the extended Liénard equation y″ + f(y)(y′)n + k(y)(y′)m + g(y)y′ +... 相似文献
10.
G. de Jongh MIEKE BERGERS J. B. M. Boezeman A. R. Verhagen P. D. Mier 《Photochemistry and photobiology》1984,39(5):625-629
Abstract— The lethal effect of UVA and PUVA radiation was studied in cultures of fresh and mature monocytes. UVA radiation alone was shown to possess a lethal effect at doses which are attained in the dermis in vivo. The synergistic action of 8-methoxypsoralen and UVA radiation predominated in PUVA radiation, but again a residual effect of UVA alone was demonstrated mathematically. Mature cells were less sensitive than fresh monocytes.
Our results indicate that a monolayer culture of non-dividing, mesenchymal cells offers considerable advantages over in vivo systems as a model for the study of phototoxicity. 相似文献
Our results indicate that a monolayer culture of non-dividing, mesenchymal cells offers considerable advantages over in vivo systems as a model for the study of phototoxicity. 相似文献