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151.
It is frequently desirable to be able to delay audio frequencies by times ranging from several milliseconds to hundreds of milliseconds. Although several methods have been developed to achieve this, most of them employ electromechanical delay systems. The article describes an all-electronic technique of delaying acoustic signals; experimental work on the system has established its feasibility. 相似文献
152.
153.
We report the preparation of a new class of copolymer films that consist of polymethylene (PM) with a low percentage ( approximately 1-4%) of randomly distributed ethyl ester side groups, consistent with poly(methylene-co-ethyl acetate). The films are prepared through a surface-catalyzed polymerization on gold surfaces upon exposure to a dilute solution of diazomethane (DM) and ethyl diazoacetate (EDA) in ether at 0 degrees C. While EDA alone does not polymerize at gold surfaces but DM does decompose on gold to grow PM films, the combined presence of EDA and DM results in dramatic enhancements in film growth and promotes an alternative mechanism for propagation as compared with that for the PM homopolymerization. The rate of copolymer film growth is constant over a approximately 24 h period, consistent with a controlled polymerization in which chain terminations are minimized. Carefully controlled experiments indicate that chain propagation does not occur at the outer film-solvent interface, but more likely, at the film-metal interface, suggesting a catalyzed insertion mechanism that extends the chain and pushes the outer chain termini further away from the metal-polymer interface. The results also suggest that adsorbed intermediates of EDA function as co-catalysts to promote the propagation reaction. Of particular importance for materials modification is that the ester side chains of these copolymer films can be hydrolyzed to carboxylate groups that exhibit pH-dependent wettability. 相似文献
154.
P.J. Jennings 《Surface science》1978,75(4):L773-L776
155.
P.J. Jennings 《Surface science》1973,34(3):668-678
The structure of surface barrier resonances in LEED is shown to be strongly dependent upon the shape and position of the potential energy barrier at the crystal surface. This suggests that systematic high-resolution measurements of LEED intensities in the energy range of surface barrier resonances may be a useful tool for studies of surface barrier structure. Such measurements should be performed at several angles of incidence and at a variety of azimuths with primary beams of very low energy (<30 eV).Computations are presented for a model crystal surface of Cu(001) to illustrate these conclusions. The results are in agreement with McRae's model for the surface barrier resonance in LEED. 相似文献
156.
A recently proposed semiclassical method for extracting the smoothly varying part of the total energy of an independent particle system is applied to a rotating system. Expressions for the average density distribution, angular momentum, total energy and effective moment of inertia are given. 相似文献
157.
U. Bogner R. Seel F. Graf C. R. Pollock D. A. Jennings F. R. Petersen R. E. Drullinger E. C. Beaty J. S. Wells J. L. Hall H. P. Layer K. M. Evenson Ch. Salomon Ch. Bréant A. van Lerberghe G. Camy Ch. J. Bordé K. J. Siemsen H. D. Riccius A. S. Südbø Guiguang Xiong 《Applied physics. B, Lasers and optics》1982,29(3):152-156
158.
Analysis of the cathodoluminescence emission spectra of neodymium implanted cubic single crystal ZnS as a function of post-implantation annealing temperature has enabled groups of emission lines to be identified as originating from transitions within particular luminescence centres. For each group of lines the wavelength separations yield energy splittings characteristic of the crystal field at the luminescence centre. Comparison of these splittings with values calculated by crystal field theory on the basis of a point charge model, with due regard to possible sites of cubic symmetry in ZnS, has allowed two luminescence centres (A and B) to be identified.Centre A, which exists at low annealing temperatures, consists of a Nd3+ ion situated interstitially with 4 tetrahedrally disposed nearest neighbour S2? ions. At higher annealing temperatures the emissions from centre A were found to decline giving way to those of centre B, which analysis shows to be a Nd3+ ion situated on a zinc substitutional site. 相似文献
159.
Complete separation of the TMS derivatives of histamine, 1-methyl histamine and 3-methyl histamine was achieved in two minutes in a 7 m × 0.25 mm column coated with SP 2250. Using NPD, histamine was detectable at levels as low as 20 pg. This offers a rapid, sensitive procedure with potential for clinical diagnosis and food analysis. 相似文献
160.
C.G. Cassapakis H.C. Bryant B.D. Dieterle C.P. Leavitt D.M. Wolfe B.E. Bonner J.E. Simmons C.W. Bjork P.J. Riley M.L. Evans G. Glass J.C. Hiebert M. Jain R.A. Kenefick L.C. Northcliffe D.W. Werren 《Physics letters. [Part B]》1976,63(1):35-38
The measured cross sections show a narrow high-energy peak (20–30 MeV/cFWHM) and a broad bump at lower momenta associated with pion production. Comparison is made to an intranuclear cascade calculation. 相似文献