Spectral heterogeneity of oxygen-deficient centers in Ge-doped silica

We report an experimental investigation of the emission spectra of a 1000 mol ppm sol–gel Ge-doped silica by fine tuning the excitation energy in the ultraviolet (UV) range, around 5 eV, and in the vacuum-UV range, around 7.3 eV, at room temperature and at 10 K. The sample is characterized by a blue (centered at 3.2 eV) and an UV (centered at 4.3 eV) bands. We have found that the ratio between the area of the blue and the UV bands depends on the temperature and on the excitation energy in both the vacuum-UV and the UV range. At both temperatures the spectral features of the blue and the UV bands are weakly affected when the excitation is varied in the vacuum-UV. At variance, under UV excitation the peaks of the bands are shifted and also their widths are changed. These results are interpreted in terms of distinct excitation channels of the luminescence that are influenced in a different way by the structural inhomogeneity of point defects.     

Optical properties of Ti1−x Hf x Se2 layered compounds from 1 to 9 eV

Summary The main structures of interband spectra of Ti_1−x Hf _x Se₂ crystals (0≤x≤0.3 andx=1) have been investigated by means of thermoreflectance from 1 to 9 eV. New indications about the correspondence of the interband spectra of TiSe₂ and HfSe₂ are given. Detailed data at intermediate stoichiometries, which give also evidence of possible bowing effects, are reported and discussed. No relevant effect of the 200 K phase transition in TiSe₂ is observable above 2 eV.

---

Riassunto Sono state studiate, mediante la tecnica della termoriflettanza, le principali strutture dello spettro interbanda dei cristalli Ti_1−x Hf _x Se₂ (0≤x≤0.3 ex=1), fra 1 e 9 eV. Si sono così ottenute nuove indicazioni circa la corrispondenza fra gli spettri interbanda del TiSe₂ e dell'HfSe₂. Si riportano in dettaglio e discutono i dati di termoriflettanza a valori intermedi dix con evidenza di possibili effetti di ?bowing?. Non sono stati osservati apprezzabili effetti della transizione di fase a 200 K ad energie superiori a 2 eV.

---

Резюме Исследуются главные особенности междузонных спектров кристаллов Ti_1−xHf _x Se₂ (0≤x≤0.3 иx=1) с помощью техники термоотражения в области от 1 до 9 эВ. Приводятся новые данные о соответствии между междузонными спектрами TiSe₂ и HfSe₂. Подробно обсуждаются данные при промежуточных значенияхx. Не наблюдаются заметные эффекты фазового перехода при 200 К в TiSe₂ при энергиях выше 2 эВ.

     

Interval linear systems: the state of the art

Summary  Linear systems represent the computational kernel of many models that describe problems arising in the field of social, economic as well as technical and scientific disciplines. Therefore, much effort has been devoted to the development of methods, algorithms and software for the solution of linear systems. Finite precision computer arithmetics makes rounding error analysis and perturbation theory a fundamental issue in this framework (Higham 1996). Indeed, Interval Arithmetics was firstly introduced to deal with the solution of problems with computers (Moore 1979, Rump 1983), since a floating point number actually corresponds to an interval of real numbers. On the other hand, in many applications data are affected by uncertainty (Jerrell 1995, Marino & Palumbo 2002), that is, they are only known to lie within certain intervals. Thus, bounding the solution set of interval linear systems plays a crucial role in many problems. In this work, we focus on the state of the art of theory and methods for bounding the solution set of interval linear systems. We start from basic properties and main results obtained in the last years, then we give an overview on existing methods.     

A structural variability of copper(I) chloride-tetrahydrothiophene adducts crystallizing in polymeric forms and exhibiting polymorphism: The role of the solvent

The function of the solvent in the self-assembling mode of [CuCl] with tetrahydrothiophene is reported. Copper(l) chloride has been used in the form of [CuCOCl] _n , which is slightly soluble in the most common solvents, and to allow an homogeneous phase reaction. The reaction of [CuCOCl] _n with THT gave [(CuCl)₂(THT)₃] _x ,1, in CH₃OH, [(CuCl)(THT)₂] _x ,2 in THF [(CuCl)(THT)] _x ,3, in CH₂Cl₂, and [(CuCl)₃(THT)₂] _x ,4, in DME. Compound1 consists of polymeric chains of centrosymmetric Cu₂Cl₂ dinuclear units bridged by THIT molecules running parallel to the [101] axis. In the structure of2 we found polymeric layers generated from the [(CuCl)(THT)] asymmetric unit by the center of symmetry and by the twofold axis. In compound3 the structure consists of layers generated through the center of symmetry by the [(Cu₂Cl₂)(THT)₂] asymmetric unit, while4 consists of layers generated through the center of symmetry by the [(Cu₃Cl₃)(THT)₂] asymmetric unit. Crystallographic details are as [ollows:2 is monoclinic, space group P2₁/c.a=9.657(3) A,b=6.441(2) A,c=11.459(3) A,\=111.96(2)°,V=661.0(4) A³. andR=0.075;3 is monoclinic, space group P2₁/n,a=19.327(7) A,b=6.703(2) A,c=10.116(3) A,ß=103.04(3)°,V=1276.7(7) A³ andR=0.043:4 is triclinic, space group Pl,a=12.513(2) A,b=6.698(1) A,c=9.651(1) A,=91.98(1)°,\=107.86(1)°,=74.59(1)°,V=741.2(2) A³, andR=0.044.     

Chiral trans-carboxylic trifluoromethyl 2-imidazolines by a Ag2O-catalyzed Mannich-type reaction

Trifluoromethyl aldimines derived from α-amino esters have proven to be very good starting materials to obtain the title compounds. A Ag₂O-catalyzed Mannich-type/cyclization cascade reaction starting from suitable α-isocyano acetates leads to enantiopure valuable trans-carboxylic trifluoromethyl substituted 2-imidazolines by a highly stereoselective addition without the need to add organocatalysts.     

Unitary colligations of operators in Krein spaces

A model for non-contractive functions is given, according to which every bounded analytic function coincides with the characteristic function of a suitable unitary colligation. In our construction, the function is expressed in terms of a fundamentally reducible unitary dilation of the basic operator of the colligation.The results in this paper first appeared as part of the author's doctoral thesis [13] at the Universidad Central de Venezuela, under the supervision of Mischa Cotlar. The work has been supported by the Consejo Nacional de Investigaciones Cientificas y Techológicas (CONICIT-Venezuela). I am grateful to Professor Aad Dijksma, who read an earlier version of this note and made several helpful criticisms. I also thank Professor Mischa Cotlar for his kind encouragement and Professor Alejandra Cabana for correcting the english text.     

Hybrid Inorganic-Organic Sol–Gel Coatings in the SiO2-TiO2 System

In the present work, the influence of substituted Si-alkoxides on the structural and optical properties of films obtained in the SiO₂-TiO₂ system was studied. Methyltriethoxysilane (MTEOS) and 3-(tri-methoxysilyl)propyl methacrylate (TSPM) were used as SiO₂ sources and Ti(OBu)₄ was used as TiO₂ source. Acetylacetone was added to the Ti(OBu)₄ as chelating agent and the synthesis was carried out in acid medium. The films were deposited on oxidized Si-wafers by spin-coating. The films were characterized by XRD, spectro-ellipsometry (SE), and atomic force microscopy (AFM). The results obtained have shown that in the case of hybrid films the desired thickness could be obtained in a single deposition step. The thickness of the films and the optical properties are controlled by the bulkiness of the organic substitute bounded to Si. Among other optical applications, the potential use of such films as optical waveguides is proposed.     

Selective inhibition of 6-phosphogluconate dehydrogenase from Trypanosoma brucei

A number of triphenylmethane derivatives have been screened against 6-phosphogluconate dehydrogenase from Trypanosoma brucei and sheep liver. Some of these compounds show good inhibition of the enzymes and also selectivity towards the parasite enzyme. Modelling was undertaken to dock the compounds into the active sites of both enzymes. Using a combination of DOCK 3.5 and FLEXIDOCK a correlation was obtained between docking score and both activity for the enzymes and selectivity. Visualisation of the docked structures of the inhibitors in the active sites of the enzymes yielded a possible explanation of the selectivity for the parasite enzyme.     

Synthesis, X-ray diffraction characterization, and radiative properties of Er(2)O(3)-ZrO(2) nanocrystals embedded in LAS glass ceramic

Low thermal expansion Li(2)O-Al(2)O(3)-SiO(2) (LAS) glass ceramic was examined as a host matrix for erbium ions. ZrO(2) was added to the glass since it serves as a nucleating agent and as a good environment for the luminescent ions. The study was carried out on amorphous powders of the Li(2)O-Al(2)O(3)-SiO(2)/ZrO(2)/Er(2)O(3) system prepared by the sol-gel method and successively crystallized at different temperatures. X-ray diffraction (XRD), transmission electron microscopy (TEM), and infrared (IR) spectroscopy were employed to study the evolution of the crystalline phases and the distribution of the erbium ions. The TEM micrographs confirmed that, after thermal treatment at 1000 degrees C, the crystallization of nanoparticles constituted by an Er(2)O(3)-ZrO(2) solid solution with narrow size distribution could be achieved. On the contrary, erbium silicate was detected in the samples without ZrO(2). The repartition constant of Er(2)O(3) between ZrO(2) and LAS matrix has been also evaluated.     

Determination of trace amounts of highly hydrophilic compounds in water by direct derivatization and gas chromatography — mass spectrometry

A new procedure has been developed to derivatize a large set of highly hydrophilic substances ((poly) hydroxy and/or (poly)carboxylic acids, glycols and dihydroxybenzenes) directly in water. The key element of the method is the derivatizing agent used, n-hexyl chloroformate, which proved to be much more effective than other alkyl and aryl chloroformates. Detection limits in the low μg/L range were found for most of the compounds studied, using positive ion chemical ionization mass spectrometry as the detection technique. Calibration curves exhibited a good linearity over 2–3 orders of magnitude, so that quantitative determinations were possible. Among the experimental parameters tested, it was found crucial to introduce the chloroformate slowly and under sonication. The whole derivatization procedure takes only 2–3 min from sample collection to injection into the gas chromatograph.