Photoluminescence of cadmium selenide quantum dots in polymer solutions

The effect of solvent on the photoluminescence of cadmium selenide quantum dots stabilized by oleic acid is examined with the example of two organic solvents: toluene and THF. It is found that THF favors desorption of ligands and formation of surface defects to a greater extent than toluene; as a result, the maximum of the photoluminescence band shifts to the red spectral region and its intensity decreases. The addition of polymers to the solution of quantum dots causes changes in the efficiency of photoluminescence and in the kinetics of its quenching. In the range of low concentrations (≤2 wt %) of quantum dots in polymer solutions, the intensity of luminescence first grows and then passes through a maximum and decreases. This effect may be explained both by the increasing number of surface defects and by quenching via energy transfer to polymers, especially in the case of polymers containing aromatic groups.     

Giant magnetoresistance oscillations induced by microwave radiation and a zero-resistance state in a 2D electron system with a moderate mobility

The effect of a microwave field in the frequency range from 54 to 140 GHz on the magnetotransport in a GaAs quantum well with AlAs/GaAs superlattice barriers and with an electron mobility no higher than 10⁶ cm²/V s is investigated. In the given two-dimensional system under the effect of microwave radiation, giant resistance oscillations are observed with their positions in the magnetic field being determined by the ratio of the radiation frequency to the cyclotron frequency. Earlier, such oscillations had only been observed in GaAs/AlGaAs heterostructures with much higher mobilities. When the samples under study are irradiated with a 140-GHz microwave field, the resistance corresponding to the main oscillation minimum, which occurs near the cyclotron resonance, appears to be close to zero. The results of the study suggest that a mobility value lower than 10⁶ cm²/V s does not prevent the formation of zero-resistance states in a magnetic field in a two-dimensional system under the effect of microwave radiation.     

Quadratic crystal field tensors and spin Hamiltonian tensors of Fe3+ in Li2Ge7O15 above and below the phase transition temperature

Dopant Fe³⁺ ions in tetrahedral and octahedral positions of Ge⁴⁺ in the crystal Li₂Ge₇O₁₅ were studied using EPR. Fe³⁺ substitutes for Ge⁴⁺ with a local charge compensation. The octahedral site and the tetrahedral sites significantly differ by the value of the invariant sumS(B ₄) of theB ₄ tensor of the spin Hamiltonian of Fe³⁺. The irreducible tensor products {V ₄ ? V ₄}₄ and {V ₄?V ₄}₂ theV ₄ tensor of the crystal field calculated using the point-charge model for octahedral and tetrahedral complexes provide the predominant contribution of the crystal field to theB ₄ andB ₂ tensors of the spin Hamiltonian of Fe³⁺, respectively. A comparison of the fourth-rank tensorsB ₄ of the spin Hamiltonian and {V ₄?V ₄}₄ of the crystal field determined at 300 K with those determined at 77 K supports the conclusion that the phase transition is accompanied by combined rotation of the [GeO₄] tetrahedra with the [Ge(1)O₆] octahedron almost unaltered. The spectrum lines are narrow and the variety of point defects in the vicinity of the paramagnetic impurity ions Fe³⁺, Cr³⁺ and Cu²⁺ is not detected. These facts are inconsistent with the statistically distributed model for the Li(2) atom. In specific cases at 300 K, when the wings of the two spectrum lines of theM→M+1 and theM+2 →M+3 transitions of Fe³⁺ ions belonging to one system of translationally equivalent positions overlap an extra line appears in the center between these lines. It is suggested that this effect is due to the soft phonon mode above the phase transition temperature.     

Nickel catalysts for the addition polymerization of norbornene and its derivatives and for their copolymerization with ethylene

The addition polymerization of norbornene and its derivatives has been carried out in the presence of a nickel complex or carboxylate and an electron acceptor to obtain amorphous polymers with bicyclic units. Norbornene copolymers with conjugated dienes or ethylene cannot be obtained with these catalysts because of rapid chain transfer reactions. Norbornene can be copolymerized with ethylene under mild conditions in the presence of nickel phosphorylide chelates without using any cocatalyst. In most cases, the backbone of the resulting copolymer consists of alternating comonomer units. The new catalysts allow ethylene to be copolymerized with norbornene derivatives containing ester substituents.     

Parametric analysis of kinetic models, XI. Influence of the number of active sites

The influence of active sites number on the stationary characteristics of kinetic models is analyzed for some nonlinear schemes of conversion on catalyst surface. Mechanisms involving one and two types of active sites are considered. A possibility of critical effects is shown.     

Peculiarities of the radiospectroscopy line shape in nanomaterials

The theory of the radiospectroscopy (nuclear magnetic resonance [NMR] and electron spin resonance [ESR]) line shape for nanomaterials is developed. The consideration was performed in the core and shell models which are, respectively, the nanoparticle regions unperturbed and perturbed by the surface influence. The shift of the resonance frequency by the surface tension was taken into account. The homogeneously broadened line shape was supposed to be Gaussian or Lorentzian. Inhomogeneous broadening of lines via the distribution of nanoparticle sizes was calculated for several forms of the size distribution function. The splitting of radiospectroscopy spectra into two lines decreases with particle sizes, which looks like that in the bulk and on the surface. It was shown to be the characteristic feature of nanomaterial spectra. The changing of these lines’ intensity and width with the change of the distribution function parameters and the particle size decrease was considered. The comparison of the theory with NMR spectra of¹⁷O and²⁵Mg observed recently in nanocrystalline MgO is performed. The calculations fit pretty good the observed size dependence of the line shape, intensity and width.     

Water-vapor absorption spectrum in the 0.59-μm region

The paper gives the results of investigation of the water-vapor absorption spectrum in the range of 586.9–596.6 nm. In the given range, 282 water-vapor absorption lines were recorded and were identified as belonging to the bands 4ν₁ + ν₃, 3ν₁ + 2ν₂ + ν₃, 3ν₁ + 2ν₃. Theoretical justification of the technique for interpreting the observed spectrum was made. The values of more vibration-rotation levels of energy were determined: for state (401) up to J = 10, for state (321) up to J = 10, and for state (302) up to J = 8. The rotational and centrifugal constants of the reduced Watson's Hamiltonian for the vibrational states (401), (321), (302), and (222) and constants of Fermi resonance (401)-(321), (302)-(222), and Coriolis resonance between the states (401), (321), and (302), (222) were determined.     

Marcelin-de Donder kinetics near equilibrium

A linear approximation for the Marcelin-de Donder kinetics (MC) near the point of detailed equilibrium (p.d.e./is studied. In particular it is shown that in the invariant plane the p.d.e. is stable knot.

---

- (MD) (...). , , ... — .

     

Analysis of the stationary kinetic characteristics of an n-stage unbranched catalytic reaction (linear mechanism)

Conclusions We shall discuss what differences the complexity (n-stage character) brings into the kinetic characteristics of the simplest case examined by us: an unbranched cycle, linear with respect to the intermediate substances. In the range in which the reaction proceeds in the forward direction the temperature dependence for an exothermal complex reaction is identical to that for a simple reversible exothermal stage [1]. For a complex endothermal reaction are possible characteristics which are analogous to an elementary endothermal stage: a monotonic dependence, but also a novel property may appear: an extreme (no more than one) due to the strong hidden endothermal character. As was proved in the present study, the action of the partial pressures of the reactants is analogous to their effect on the rate of an elementary reaction: monotonic functions. The difference consists in that here in most cases a limiting rate exists as a result of, for example, the circumstance that the detailed mechanism embraces stages in which no gas phase participates.Consequently, we classified the typical stationary kinetic characteristics of an n-stage unbranched catalytic reaction with a mechanism linear with respect to the intermediate products. The results obtained in our study can be easily extended to other cases, too, when the kinetic equations are of the shape (1), for example, to some empirical equations.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 14, No. 4, pp. 529–533, July–August, 1978.