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111.
We show that classical chaotic scattering has experimentally measurable consequences for the quantum conductance of semiconductor microstructures. These include the existence of conductance fluctuations-a sensitivity of the conductance to either Fermi energy or magnetic field-and weak-localization-a change in the average conductance upon applying a magnetic field. We develop a semiclassical theory and present numerical results for these two effects in which we model the microstructures by billiards attached to leads. We find that the difference between chaotic and regular classical scattering produces a qualitative difference in the fluctuation spectrum and weak-localization lineshape of chaotic and nonchaotic structures. While the semiclassical theory within the diagonal approximation accounts well for the weak-localization lineshape and for the spectrum of the fluctuations, we uncover a surprising failure of the semiclassical diagonal-approximation theory in describing the magnitude of these quantum transport effects. 相似文献
112.
S. Recknagel P. Brätter A. Tomiak U. Rösick 《Fresenius' Journal of Analytical Chemistry》1993,346(6-9):833-836
Summary An analytical procedure for the determination of Se, as well as of other trace elements of clinical interest, in undiluted blood serum samples is presented. This procedure allows the on-line wet ashing of the sample, the hydride generation and the determination of the SeH2 formed in one step. For this purpose, carrier solution and the injected sample (300 l) were merged with an acid stream (80% H2SO4, 12% HClO4, 8% HNO3) and passed through a reaction coil heated up to 240°C. To increase the dispersion, the wet digestion was carried out in an ultrasonic field. After trapping of the gas bubbles, 5 mol/l HCl and 0.053 mol/l NaBH4 solutions were added via T-junctions for Se-hydride generation. The nebulizing system of the ICP-OES was used as gas/liquid separator. The hydride was swept from the spray chamber into the plasma by an argon gas flow with droplets containing other sample constituents. This allows the simultaneous determination of other trace elements of clinical interest, for example Fe, Cu and Zn. The relative detection limit for Se in blood serum was 5 g/l. The quality of the developed procedure was verified in two ways: by measuring SRM Seronorm 116 and by comparing the results for different serum samples with the results obtained with ETAAS. Our results were in good agreement in both cases.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday 相似文献
113.
114.
115.
U. Groß P. Dietrich G. Engler D. Prescher J. Schulze K. Lunkwitz A. Ferse 《Journal of fluorine chemistry》1982,20(1):33-52
The present work studies the changes in polymer structure and the mechanism of the decomposition of polytetrafluoroethylene resin (PTFE) exposed to high energy radiation (electron beam). Spectroscopic and kinetic observations are used to interpret the degradation process. For the first time the decomposition of PTFE has been carried out on a preparative scale and new results obtained by analysing the degradation products. The radiation-induced degradation of PTFE is accompanied by thermal degradation under certain irradiation conditions. This is due to an increase in temperature of the polymer caused by retardation of highly accelerated electrons (heat accumulation effect).The kinetics are discussed in terms of the reactions and recombination of radicals produced by high-energy radiation both in the polymer melt and the polymer surface. These are related to the overall rate of decomposition.The primary radicals formed by decomposition of PTFE in an inert atmosphere (N2, Ar) react to produce perfluorinated alkanes and alkenes. In the presence of reactive gases the decomposition fragments originated will react rapidly; e.g. if oxygen is present in the reactive area the radicals form perflourinated peroxyl and oxyl radicals which finally stabilize themselves by CC-scission to perfluorocarbon acid fluorides and carbonyldifluorides. 相似文献
116.
Fuchslueger U Rissler K Grether HJ Grasserbauer M 《Analytical and bioanalytical chemistry》1996,356(8):495-499
High-performance liquid chromatography (HPLC) coupled to mass spectrometry (MS) is used to detect 2,4-dinitrophenyl (DNP) derivatives of amines by means of negative chemical ionisation at atmospheric pressure. The high sensitivity and good comparability of UV- and MS-detection of DNP-derivatives of amines is shown by several examples. The high selectivity of the derivatisation and the detection method (UV and MS) is used for the analysis of unknown amines in aqueous phases after hydrolytic degradation of polyamide-amine- or polyamine-epoxide-adducts as well as for the characterisation of commercial products. 相似文献
117.
Lundström U 《Talanta》1982,29(4):291-296
The optimum conditions for the oxidation of bromide to bromate by peroxodisulphate at 120 degrees as well as for the decomposition of the excess of oxidant have been determined. The predicted advantages of this oxidizing agent, viz. minimal blanks and destruction of small amounts of interfering organic matter and reducing substances, were confirmed. The bromate was determined iodometrically either by titration with thiosulphate or by spectrophotometry in absence of oxygen at 355 nm. The titrimetric finish applied to 0.8-8 micromole of bromide gave a mean yield of 100.0%, s = 6 nmole. The spectrophotometric finish applied to 0.05-0.25 micromole of bromide gave a mean yield of 98.9%, s = 1.1 nmole. Interfering amounts of iodide present in the sample and oxidized to iodate can be corrected for by making use of the pH-dependence of the reaction of iodide with bromate and iodate. 相似文献
118.
U.T. Mueller-Westerhoff A. Nazzal W. Prössdorf 《Journal of organometallic chemistry》1981,205(3):C21-C23
[1,1]Ferrocenophane can be prepared by reduction of 1,1′-bis(6-fulvenyl)ferrocene with complex borohydrides to give the dianion of 1,1′-bis(cyclopentadienylmethyl)ferrocene. Reaction of this dianion with ferrous chloride produces the ferrocenophane in good yield. 相似文献
119.
Graham R. Underwood Brajeswar Paul M. Alice Becker 《Journal of heterocyclic chemistry》1976,13(6):1229-1232
Pyridoxol, protected by acetylation of the hydroxyl groups, has been converted to its N-oxide which upon reaction with perfluoroacetie anhydride yields a 2-nor-2-hydroxymethylpyridoxol derivative as an intermediate. This compound undergoes acyl migration from the 3-position. Protection of the pyridoxol hydroxyls by benzylalion followed by the same treatment yields the unrearranged α2-hydroxy derivative. This compound has been converted to a series of α2-substituted pyridoxols (X = -Cl, -Br, -OCOCH3, -OCH3, -OC2H5). 相似文献
120.
A new route has been developed for the micro-determination of227Ac in geological materials by neutron activation. The method is based on intense neutron irradiation of the analysed samples followed by separation and α-spectrometric determination of228Th, the β-decay product of the 6.1 hrs228Ac isotope formed. Two alternatives are considered for analysis related to the origin of the analysed matrix. The high sensitivity of the method is documented by the determination of 10?17 g227Ac/g sample. The method is successfully applied for age determination of five uranium containing materials and old uranium glass from Bohemia, CSSR. 相似文献