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  • 1
    ISSN: 0392-6737
    Keywords: Time-resolved optical phenomena (including electron-hole drops) ; Optical properties of thin films surfaces, and layer structures (superlattices, heterojunctions and multilayers) ; Electron states in low-dimensional structures (including quantum wells, superlattices, layer structures, and intercalation compounds) ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary We show that a detailed understanding of the influence of the exciton-exciton interaction on the non-linear excitonic response in a GaAs/AlGaAs superlattice can be obtained by changing the external applied field. In time-resolved four-wave mixing this implies a change from a third-order polarization signal dominated by phase space occupation to a Coulomb interaction-induced signal when the bias is increased at high excitation densities. The experimental data are compared with theoretical results based on the numerical solution of the semiconductor Bloch equations.
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  • 2
    ISSN: 1432-0630
    Keywords: 71.35 ; 78.20
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract We discuss the evolution of optical properties of semiconductor quantum wells, as the quasi-two-dimensional electronic states are further confined into quasi-zero dimensions by a perpendicular magnetic field. We show that confinement in all three directions strongly modifies both linear and nonlinear optical response. In particular, quasi-zero-dimensionality makes an ensemble of magneto-excitons a unique many-body system, distinct from higher-dimensional excitons and the one-component Coulomb system in the fractional quantum Hall regime or Wigner crystal.
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  • 3
    ISSN: 1434-4475
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Zusammenfassung Die Umsetzung von α-Mercaptoketonen mit Propylenimin führt zu 2,3-disubstituierten 5-Methyl-5,6-dihydro-4H-(bzw. 2H)-1,4-thiazinen. Sie reagieren im Gegensatz zu den in Stellung 5 nicht substituierten Äthylenimin-Reaktionsprodukten nicht mit einem weiteren Mol-α-Mercaptoketon unter Bildung von Thiazolo[2,3-c]-1,4-thiazinen. Die Dihydrothiazine lassen sich mit Ameisensäure oder NaBH4 zu sterisch identischen Thiomorpholinen hydrieren; in einigen Fällen gelingt diese Hydrierung auch mit H2S, wobei jedoch die stereoisomeren Thiomorpholine gebildet werden. Thiazolo [2,3-c]-1,4-thiazine ergeben bei der Behandlung mit Ameisensäure oder NaBH4 stereoisomere Perhydro-thiazolo[2,3-c]-1,4-thiazine.
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  • 4
    ISSN: 1434-4475
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Abstract Reaction of elementary sulfur with phenacylsulfenylmorpholide gives phenylglyoxylic acid thionmorpholide while hydrogen sulfide is eliminated. The rearrangement proceeds, as was shown by use of radioactive sulfur, via agem. dimercaptocompound as intermediate.
    Notes: Zusammenfassung Bei der Einwirkung von elementarem Schwefel auf Phenacyl-sulfenylmorpholid entsteht unter H2S-Abspaltung Phenylglyoxyl-säurethionmorpholid. Durch Einsatz von radioaktivem Schwefel wurde festgestellt, daß diese Umlagerungsreaktion über die Stufe einergem. Dimercaptoverbindung verläuft.
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  • 5
    ISSN: 1434-4475
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Abstract o-Methoxyphenols withp-side chains labelled with14C at the α-C-atoms were synthesized and mildly oxygenated in dilute aqueous NaOH (0.2n, 1–2 mole equivalents NaOH) at 70° C. Derivatives with α-C-atoms in different states of oxidation exhibit different reaction patterns. In contrast top-alkyl-substitutedo-methoxyphenols which dimerize too,o′-dihydroxybiphenyls, eliminations of side chains according to theDakin reaction could be observed in the case of α-carbonyl, and α-carbinol derivatives, respectively. The carbon dioxide formed in the course of oxidation of models labelled with14C in α-position proved to be inactive and does definitely not originate from α-C-atoms. Possible reaction mechanisms are discussed, and to the importance of these particular elimination reactions in lignin chemistry is referred to.
    Notes: Zusammenfassung o-Methoxyphenole mit para-ständigen Seitenketten (an den α-C-Atomen mit14C markiert) wurden dargestellt und einer milden Sauerstoffoxydation in verd. wäßr. NaOH (0,2n, 1–2 Moläquivalente NaOH) bei 70° unterworfen. Das Reaktionsverhalten hängt dabei von der Struktur (Oxydationsstufe) am α-C-Atom der Seitenkette ab. Im Gegensatz zup-alkylsubstituierteno-Methoxyphenolen, die zuo,o′-Dihydroxydiphenylverbindungen dimerisiert werden, konnten bei den α-Carbonyl- bzw. α-Carbinolderivaten Seitenketteneliminierungen im Sinne derDakin-Reaktion beobachtet werden. Das bei diesen Oxydationen abgespaltene CO2 erwies sich bei den Versuchen mit α-14C-markierten Modellen als inaktiv und entsteht daher mit Sicherheit nicht aus den α-C-Atomen. Mögliche Reaktionsmechanismen werden diskutiert; auf die Bedeutung dieser Eliminierungsreaktionen für die Ligninchemie wird hingewiesen.
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  • 6
    ISSN: 1434-4475
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Abstract Imidazolidine-thiones-(4) react with H2O2 in methyl alcohol or in basic solution to give the corresponding imidazolin-ones-(4) in very good yields. Some new chemical compounds of that type are described. On reduction of 2.2.5.5-tetramethylimidazolidin-thione-(4) and-one-(4), as examples, with LiAlH4 only 1-[N-isopropyl]-2.2-dimethylethylendiamine is formed, demonstrating that the heterocyclic ring is cleaved between the positions 1 and 2. The structur of the open chain diamine is proved. With anorganic, water-soluble salts of Ag, Cu and Cd the corresponding imidazoline-(3)-mercaptides-(4) are formed. By treating the imidazolidine-thiones-(4) with ketones or aldehydes the substituents on the C-(2)-position can be exchanged.
    Notes: Zusammenfassung Imidazolidin-thione-(4) setzen sich mit H2O2 in Methylalkohol oder in basischem Milieu mit sehr guten Ausbeuten zu Imidazolidin-onen-(4) um. Einige bisher unbekannte Verbindungen dieses Typs werden beschrieben. Am Beispiel des 2.2.5.5-Tetramethyl-imidazolidin-thion-(4) bzw.-on-(4) wird gezeigt, daß bei durchgreifender Reduktion mit LiAlH4 das einheitliche, durch Ringsprengung zwischen Position 1 und 2 bedingte 1-[N-Isopropyl]-2.2-dimethyläthylendiamin entsteht. Die Struktur dieses Amins wird bewiesen. 2.2.5.5-Tetramethylimidazolidin-thion-(4) bildet mit anorganischen Silber-, Kupfer- und Cadmium salzen die entsprechenden Imidazolin-(3)-mercaptide-(4). Durch Austauschreaktion mit Carbonylverbindungen lassen sich die Substituenten der Imidazolidin-thione-(4) in 2-Stellung gegen andere Substituenten austauschen.
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  • 7
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The paramagnetic resonance of Gd in YAl2 was measured in the temperature range from 5 °K to 70 °K. The experimental results are discussed with Hasegawa's theory. The dependence of linewidth on temperature and Gd-concentration indicates the existence of a “bottleneck” in the relaxation between the conduction electrons and the lattice. If nonmagnetic thorium impurities are added an additional relaxation becomes effective, caused by spin-flip scattering of the conduction electrons at these impurities. In this case the bottleneck is removed.
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  • 8
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Mass andZ-distributions of fusion products from the reaction 5.8 MeV/u46Ti+12C were measured in the angular range betweenθ lab=1.5° and 8° using a time-of-flightΔE−E telescope. The results are compared with an earlier measurement of the reaction32 S+26Mg, in which the same compound nucleus was produced at higher excitation energy. The residual nuclide distributions are interpreted with the aid of evaporation calculations and the influence of shell effects in the level densities is discussed. The data are consistent with the assumption that the shell effects vanish above 15 to 20 MeV excitation energy, that is, that they are essentially only important for the last evaporation step.
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  • 9
    ISSN: 1434-6036
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present an investigation of the optical properties of semiconductors, which are excited by a strong laser pulse. On the basis of a nonequilibrium Green's function technique (Keldysh formalism) we study the system of electrons and holes, interacting among themselves and with the exciting pulse. A set of coupled equations for the one-particle Green's function and the transverse and longitudinal polarizability is derived, which is valid for arbitrary excitation conditions. For an initially prepared state, which is not too far from a quasiequilibrium state of the excited system, the theory is elaborated in detail, using a screened Hartree-Fock approximation scheme. Within this approximation earlier results of various theoretical approaches are found to be special cases of the equilibrium limit of the present theory.
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  • 10
    ISSN: 1434-601X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Fast heavy ions penetrating through solids initiate density oscillations of the electron plasma, forming Mach cones if the projectile velocity exceeds a critical valuec s . These Mach waves should lead to the directed emission of electrons from the target. The preferential emission angleϑ as a function of the bombarding energy is calculated for various projectile-target combinations.
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