Study of the glass transition region in amorphous selenium by EXAFS

Summary EXAFS spectra of amorphous bulk selenium and Se₈₀Te₂₀ alloy have been analysed in the temperature range of the glass transition region (−15 to 37°C). α-Se shows an unexpected behaviour because the modulated part of the X absorption increases with the temperature, irrespectively of the action of the EXAFS Debye-Waller factor from which one expects an opposite behaviour. Analogous features do not appear in the Se₈₀Te₂₀ alloy, probably because of the role played by the intramolecular covalent Se−Te bond strength, weaker than the corresponding Se−Se. In the absence of suitable models for the temperature dependence of the microscopic processes occurring in glasses nearT _g, a tentative explanation is proposed in terms of the temperature dependence of the backscattering amplitude functionF(k).

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Riassunto Il selenio amorfo e lega Se₈₀Te₂₀ sono stati studiati con tecnica EXAFS nell'intervallo delle temperature noto come regione della transizione vetrosaT _g (da −15 a 37°C). Il selenio amorfo mostra un comportamento inaspettato per effetto dell'aumento, con la temperatura sottoT _g, della parte modulata dell'assorbimento dei raggi X, in contrasto con quanto ci si aspetterebbe per l'effetto di smorzamento del fattore di Debye-Waller. Un comportamento analogo non si riscontra invece nella lega Se₈₀Te₂₀ dove, probabilmente, il legame covalente intracatena Se−Te, piú debole del corrispondente Se−Se, rende piú efficace l'azione di smorzamento del fattore di Debye-Waller, smascherando altri effetti. In assenza di modelli adeguati che descrivono la dipendenza dalla temperatura dei processi microscopici nei materiali vetrosi nella regione della transizione vetrosa, si ipotizza che il comportamento ≪anomalo≫ del selenio sia attribuibile ad una dipendenza dalla temperatura della funzione ampiezza di backscatteringF(k).

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Резюме Анализируются спектры EXAFS аморфного объемного селена и сплава Se₈₀Te₂₀ в области температур переходной области стекла (от 15° до 37°C). а-Se обнаруживает неожиданное поведение, потому что модулированная часть Х поглощения увеличивается с температурой, независимо от действия фактора дебая-Уоллера EXAFS, который предполагает противоположное поведение. Аналогичные особенности не возникают в сплаве Se₈₀Te₂₀, так как сила межмолекулярной ковалентной связи Se−Te, по-видимому, слабее, чем соответствующвя связь Se−Se. В отсутствии соответствующих моделей для температурной зависимости микроскопических процессов, происходящих в стеклах вблизиT _g,a, предлагается предварительное объяснение в терминах температурной зависимости функцииF(k) амплитуды обратного рассеяния.

     

Bregman Forward-Backward Operator Splitting

Set-Valued and Variational Analysis - We establish the convergence of the forward-backward splitting algorithm based on Bregman distances for the sum of two monotone operators in reflexive Banach...     

Nature of depolarized quasi-elastic light scattering in associated and non-associated liquids

Summary An analysis of Rayleigh-wing spectra on two isomers of propanol (C₃H₇OH) and on two of their Br-based isomorphous systems (C₃H₇Br) is presented. The performed measurements, over a wide temperature range, allow us to identify two different processes characterizing the response of both the alcohols. A comparison between the rotational dynamics of the linear normal-propanol (1-propanol) and the more sterically hindered iso-propanol (2-propanol) is performed. At the same time, the role played by the H-bond in the associative properties of hydrogenous systems is investigated by comparing the dynamical response of the two alcohols with that of 1-bromopropane and 2-bromopropane, in which the O−H ?active? sites are replaced by ?inert? bromine atoms. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Two-electron reduction of alkyl(sulfanyl)porphyrazines: a route to free-base and peripherally metallated asymmetric porphyrazines

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Unusual optical properties of porphyrin fractal J-aggregates

A simple mix and shake procedure in aqueous solution from water-soluble tetrakis(4-sulfonatophenyl)porphyrin and spermine leads to fractal non-covalent supramolecular systems which exhibit unusual large enhancement of resonant light scattering, behaving as metal particles.     

Ricci Collineations for Non-degenerate, Diagonal and Spherically Symmetric Ricci Tensors

The expression of the vector field generator of a Ricci collineation for diagonal, spherically symmetric and non-degenerate Ricci tensors is obtained. The resulting expressions show that the time and radial first derivatives of the components of the Ricci tensor can be used to classify the collineation, leading to 64 families. Some examples illustrate how to obtain the collineation vector.     

Supramolecular binding of cationic porphyrins on a filamentous bacteriophage template: toward a noncovalent antenna system

Engineered viruses act as scaffolds to bind porphyrins on their surfaces, exploiting mainly electrostatic interactions. The close proximity between porphyrins and tryptophan residues, exposed on the solvent-accessible surface, leads to an efficient resonant energy transfer, which makes these systems suitable for developing noncovalent antenna systems.     

The shape of dilepton spectra in heavy ion collisions as a signature of quark-gluon plasma

The formation of quark-gluon plasma in heavy ion collisions should manifest itself by the increase of the ratio of medium to large mass dileptons, the former being produced predominantly in the QGP and the mixed phase whereas the latter are produced mostly by the Drell-Yan process. The increase of this ratio should be visible either when comparing collisions of lighter with heavier ions, e.g. O+O with Pb+Pb or when comparing low and large transverse energy events in heavy ion e.g. Pb+Pb collisions.