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201.
The electron states in Ge and Si under uniaxial stress in the (100) direction are computed with the pseudopotential method. The theoretical results are compared with optical measurements of the indirect absorption edge in Si and Ge under uniaxial stress and with the polarization dependence and the splittings of donor levels. 相似文献
202.
The conformations of diastereomers of Z-Phe-Ala-OCH3 were investigated by analyzing NMR and IR spectra in the presence and absence of Eu(DPM)3. NMR ΔEu values and ratios of integral intensities for NH(bonded)/NH(free) confirm the stereochemistry of the time-averaged conformation for the peptide-Eu(DPM)3 complex as that proposed by Bystrov for a dipeptide in solution. Total H-bonding by the peptide was increased by the presence of Eu(DPM)3. 相似文献
203.
Various finite volume mixing conditions in classical statistical mechanics are reviewed and critically analyzed. In particular somefinite size conditions are discussed, together with their implications for the Gibbs measures and for the approach to equilibrium of Glauber dynamics inarbitrarily large volumes. It is shown that Dobrushin-Shlosman's theory ofcomplete analyticity and its dynamical counterpart due to Stroock and Zegarlinski, cannot be applied, in general, to the whole one phase region since it requires mixing properties for regions ofarbitrary shape. An alternative approach, based on previous ideas of Oliveri, and Picco, is developed, which allows to establish results on rapid approach to equilibrium deeply inside the one phase region. In particular, in the ferromagnetic case, we considerably improve some previous results by Holley and Aizenman and Holley. Our results are optimal in the sene that, for example, they show for the first time fast convergence of the dynamicsfor any temperature above the critical one for thed-dimensional Ising model with or without an external field. In part II we extensively consider the general case (not necessarily attractive) and we develop a new method, based on renormalizations group ideas and on an assumption of strong mixing in a finite cube, to prove hypercontractivity of the Markov semigroup of the Glauber dynamics.Work partially supported by grant SCi-CT91-0695 of the Commission of European Communities 相似文献
204.
ESR and ENDOR experiments using a disc-shaped resonator working in the Whispering Gallery Mode (WGM)
Different instrumental apparatuses are presented employing disc-shaped dielectric resonators working in the Whispering Gallery Mode (WGM). The conditions suitable to take the best from the microwave power obtained from a trade klystron in the X-band have been verified for the ESR part of the experiments. It is well known that a good ESR saturation condition is an essential prerequisite to have satisfactory ENDOR results. The WGM dielectric resonators seem to be very suitable for multiple irradiation purposes and in particular for ENDOR experiments which is the aspect we wanted to exploit here. Various radiofrequency ENDOR irradiation circuits have been tested and the obtained spectra of a γ-irradiated single crystal are presented and criticized. Some results for very high frequency ESR applications are also reported. 相似文献
205.
M. Ciuchini E. Franco G. Martinelli L. Reina L. Silvestrini 《Zeitschrift fur Physik C Particles and Fields》1995,68(2):239-255
An upgraded analysis of ,x
d
and /, using the latest determinations of the relevant experimental and theoretical parameters, is presented. Using the recent determination of the top quark mass,m
t
=(174±17) GeV, our best estimate is /=(3.1±2.5)×10–, which lies in the range given by E731. We describe our detemination of / and make a comparison with other similar studies. A detailed discussion of the matching of the full theory to the effective Hamiltonian, written in terms of lattice operators, is also given. 相似文献
206.
A. A. Marchetti T. A. Brown C. C. Cox T. F. Hamilton R. E. Martinelli 《Journal of Radioanalytical and Nuclear Chemistry》2005,263(2):483-487
Summary Accelerator mass spectrometry (AMS) is a sensitive and robust technique typically applied to the quantification of long-lived radioisotopes in samples too small to be decay-counted. AMS is characterized by a high rejection of interferences and a low susceptibility to matrix components, which reduce the demands on sample preparation chemistry. At Lawrence Livermore National Laboratory (LLNL), Center for Accelerator Mass Spectrometry (CAMS), we have developed an AMS capability for the measurement of actinide concentrations and isotopic ratios. To date, this capability has been primarily devoted to the measurement of 239Pu and 240Pu in bioassay and environmental samples including soils, sediments, waters, and human urine. For these analyses, a known amount of 242Pu is added to the samples as a reference isotope for normalization. Measurements of standard and intercomparison samples have shown that quantification is accurate and precise from at least 106 to 1011 atoms/sample. Recently, the ratios of 240Pu, 241Pu, 242Pu, and +Pu to intrinsic 239Pu have been successfully measured in soil samples from nuclear test sites. In addition, initial measurements of U and Np isotopes have yielded results consistent with the Pu measurements with respect to sensitivity, accuracy, precision, and linear range. 相似文献
207.
Yonath A 《Current protein & peptide science》2002,3(1):67-78
Structural analysis of the recently determined high resolution structures of the small and the large ribosomal subunits from three bacterial sources, assisted by the medium resolution structure of a complex of the entire ribosome with three tRNAs, led to a quantum jump in our understanding of the process of the translation of the genetic code into proteins. Results of these studies highlighted dynamic aspects of protein biosynthesis; illuminated the modes of action of several antibiotics; indicated strategies adopted by ribosomes for maximizing their functional activity and revealed a wealth of architectural elements, including long tails of proteins penetrating the particle s cores and stabilizing the intricate folds of the RNA chains. Binding of substrate analogues showed that the decoding and the peptide-bond formation are accomplished mainly by RNA. However, several proteins may be functionally relevant in directing the mRNA and in mediating the proper orientation of the tRNA molecules within the ribosomal rRNA frame. Elements involved in intersubunit contacts or in substrate binding are inherently flexible, but maintain well-ordered characteristic conformations in unbound particles. The ribosomes utilize this conformational variability for optimizing their efficiency and minimizing non-productive interactions, hence disorder of functionally relevant features may be linked to less active conformations or to far from physiological conditions. Clinically relevant antibiotics bind almost exclusively to rRNA. In the small subunit they affect the decoding accuracy or limit conformational mobility and in the large subunit they either interfere with substrate binding, by interacting with components of the peptidyl transferase cavity, or hinder the progression of the growing peptide chain. 相似文献
208.
Fabio Martinelli Enzo Olivieri Elisabetta Scoppola 《Journal of statistical physics》1991,62(1-2):117-133
We present rigorous results on the exponential convergence to equilibrium for the Swendsen-Wang stochastic dynamics for thed-dimensional Ising ferromagnet with external magnetic fieldh in the thermodynamic limit. We consider various situations, mainly in the low-temperature regime, in which boundary conditions are homogeneous and parallel or opposite to the external field. In the latter case we relate directly the tunneling from the metastable phase to the stable one with the exponential convergence to equilibrium. 相似文献
209.
Summary Primary and secondary crystallographic constants can be utilized conveniently for identifications of crystalline organic and inorganic materials by microscopy and X-ray diffractometry. Several specific properties can be measured under the petrographical microscope, including optic axial angle, character, dispersion, and in addition, refractive index directions can be established. The d spacings and their intensities as determined with the X-ray diffractometer serve to identify crystalline materials. The two instruments should be considered as supplementary tools for identifying crystals.Both instruments supply a means for identification and semiquantitative analysis of mixed crystal systems. Data are presented for the systems: acetic-propionic p-bromoanilides and acet-propion-2.4-dinitro-phenylhydrazones.
Zusammenfassung Primäre und sekundäre kristallographische Konstanten können zur Identifizierung kristallisierter organischer und anorganischer Stoffe auf mikroskopischem Wege und durch Röntgendiffraktometrie gut herangezogen werden. Einzelne spezifische Eigenschaften können unter dem petrographischen Mikroskop gemessen werden, so der optische Achsenwinkel, der Charakter, die Dispersion. Außerdem kann man Angaben über den Brechungsindex erhalten. Die mit dem Röntgendiffraktometer bestimmten Netzebenenabstände d und deren Intensitäten dienen zur Identifizierung kristallisierter Stoffe. Die beiden Instrumente sind als Hilfsmittel zur Erkennung von Kristallen anzusehen.Sie bieten die Möglichkeit zur Identifizierung und halbquantitativen Analyse von Mischkristallsystemen. Daten für die Systeme Essigsäure-Propionsäure-p-Bromanilid und Acetaldehyd-Propionaldehyd-2,4-Dinitrophenylhydrazon werden mitgeteilt.
Résumé Les constantes cristallographiques primaires et secondaires sont utilisables pour l'identification de composés cristallins organiques et minéraux par microscopie et diffractométrie de rayons X. Il est possible de mesurer diverses propriétés spécifiques sous le microscope pétrographique, savoir: l'angle des axes optiques, le caractère et la dispersion optiques. Il est en outre possible de déterminer les directions des indices de réfraction primaires. Les espacements «d» et leurs intensités déterminés par le diffractomètre à rayons X permettent d'identifier les substances cristallines. Les deux instruments doivent être considérés comme complémentaires pour l'identification de cristaux. Chacun d'entre eux fournit un moyen d'identification et d'analyse semi quantitative des systèmes de cristaux mixtes. On rend compte de résultats obtenus pour les systèmes: p-bromoanilides acétique et propionique ainsi que 2, 4-dinitro phénylhydrazones de l'acétaldéhyde et du propionaldéhyde.相似文献
210.
Several examples of 1,7-dioxaspiro[5.5]undecane spiroketal systems have been synthesized from the common bicyclic intermediate 1 via acid-catalyzed rearrangement. Intermolecular ketalization of C(2) symmetric diene diol 3 with ketone 9 and then desymmetrization by ring-closing metathesis rapidly constructs bicyclic acetal 1. The locked conformation and steric bias of 1 allow stereoselective functionalization of one or both double bonds before spiroketalization. 相似文献