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Dielectric Continuum Model (dielectric + continuum_model)

Distribution by Scientific Domains

Chemistry	50%

Selected Abstracts

Size-dependent Raman and infrared studies of PbSe nanoparticles

JOURNAL OF RAMAN SPECTROSCOPY, Issue 9 2008
F. S. Manciu
Abstract Micro-probe Raman and far-infrared absorption spectroscopies were used to prove the existence of optical phonon modes of PbSe nanoparticles prepared by colloidal chemistry and preliminarily characterized by transmission electron microscopy. To the best of our knowledge, this is the first time that evidence of the surface phonon (SP) mode by Raman spectroscopy has been experimentally observed. The wavenumber of the SP mode is consistent with its prediction by a dielectric continuum model. While for different PbSe nanoparticle sizes the observed SP mode does not show any obvious change in its position, there is a clear shift by approximately 4 cm,1 toward higher wavenumber in the appearance of the LO(,) in the Raman spectra from the 3 nm to the 7 nm PbSe nanoparticles. Far-infrared measurements demonstrate the presence of the transverse optical TO(,) and of the coupled phonon modes. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Excited Singlet (S1)-state Interactions of Nile Red with Aromatic Amines,

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2003
J. Mohanty
ABSTRACT Both steady-state (SS) and time-resolved (TR) studies show that the fluorescence of the dye Nile red (NR) is quenched by various aromatic amines (ArA). Bimolecular quenching constants (kq) from both SS and TR measurements are observed to match well, indicating that the interaction is dynamic in nature. The quenching interaction in the present systems has been attributed to electron transfer (ET) from ArA to excited NR, based on the variations in the kq values with the oxidation potentials of the amines. The kq values calculated within the framework of Marcus' outer-sphere ET theory at different free-energy changes (,G0) of the ET reactions match well with the experimental ones, supporting the ET mechanism in the systems studied. The reorganization energy (,) estimated from the correlation of the experimental and the calculated kq values is quite similar to the solvent reorganization energy (,s), calculated on the basis of the solvent dielectric continuum model along with the assumption that the reactants are the effective spheres. Although a modest error is involved in this ,s calculation, the similarity in , and ,s values suggests that the solvent reorganization plays a dominant role in governing the ET dynamics in the present systems. [source]

Phonons in InAs quantum dot structures

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2009
Alexander Milekhin
Abstract We present a Raman study of the phonon spectra of periodical structures with (In,Ga)As QDs in (Al,Ga)As matrix as well as AlAs QDs embedded in InAs grown by molecular beam epitaxy. Raman scattering by optical, interface and acoustic phonons was observed in the QD structures. TO and LO phonons in the QDs are strongly affected by both strain and confinement. The Raman study reveals a two-mode behavior of optical phonons in the whole composition range for both InGaAs QDs and the AlGaAs matrix. Raman scattering by InAs- and GaAs-like LO phonons in InGaAs QDs shows a size-selective resonant behaviour. Interface phonons were investigated in InGaAs QDs and the AlGaAs matrix. Their frequency positions were analyzed as a function of the alloy content within the dielectric continuum model. The positions of IF phonons in the QD structures observed in the experiment agree well with calculated ones assuming that the QDs have the shape of oblate ellipsoids. Multiple phonon Raman scattering involving both pure overtones of the first-order InAs, GaAs and AlAs optical and interface phonons and combination of phonons from the materials is observed in the vicinity with E0 resonance in QDs. Possible mechanisms of these processes are discussed. Low frequency resonant Raman scattering by acoustic phonons was observed in the QD structures. The periodic oscillations seen in the Raman spectra are well described by the elastic continuum model. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Behaviour of [PdH(dppe)2]X (X=CF3SO3,, SbF6,, BF4,) as Proton or Hydride Donor: Relevance to Catalysis

CHEMISTRY - A EUROPEAN JOURNAL, Issue 15 2004
Michele Aresta Prof.
Abstract The synthesis, characterization and properties of [PdH(dppe)2]+CF3SO3,,0.125,THF (1; dppe=1,2-bis(diphenylphosphanyl)ethane) and its SbF6, (1,) and BF4, (1,,) analogues, the missing members of the [MH(dppe)2]+X, (M=Ni, Pd, Pt) family, are described. The Pd hydrides are not stable in solution and can react as proton or hydride donors with formation of dihydrogen, [Pd(dppe)2]2+ and [Pd(dppe)2]. Complexes 1,1,, react with carbocations and carbanions by transferring a hydride and a proton, respectively. Such H, or H+ transfer occurs also towards unsaturated compounds, for example, hydrogenation of a CC double bond. Accordingly, 1 can hydrogenate methyl acrylate to methyl propionate. Complex 1,, is an effective (hourly turnover frequency=16) and very selective (100,%) catalyst for the hydrogenation of cyclohexen-2-one to cyclohexanone with dihydrogen under mild conditions. Density functional calculations coupled with a dielectric continuum model were carried out to compute the energetics of the hydride/proton transfer reactions, which were used to rationalize some of the experimental findings. Theory provides strong support for the thermodynamic and kinetic viability of a tetracoordinate Pd complex as an intermediate in the reactions. [source]