Fluorescence studies and semiempirical calculations on alkali ion indicators

Two newly synthesized cryptands act as sensitive Na⁺- and K⁺-selective indicators for cation concentrations above 20 M. The fluorescence properties change markedly upon cation binding. In addition, the free ligands exhibit a pronounced sensitivity to pH, which is considerably lower for the cation complexes. Time resolved fluorescence is characterized by a decay time of about 5 ns that is attributed to the diprotonated protolytic state of the uncomplexed ligands. Semiempirical calculations show the systematic influence of the nitrogen lone pairs or the N–H bond on the stability of the system. The cause of the strong fluorescence intensity increase observed upon protonation of the fluorescent cryptands may be attributed to an increase in the S₁–T _x energy gap as a consequence of bridgehead nitrogen protonation.This is a peer-reviewed conference proceeding article from the Third Conference on Methods and Applications of Fluorescence Spectroscopy, Prague, Czech Republic, October 18–21, 1993.     

Röntgenographische Untersuchungen zum Abbau von bleihaltigen Stabilisatoren im PVC

Investigation methods of X-ray diffraction cannot only be used for characterizing technical crystalline admixtures of plastic material. In the case of stabilizers „dibasic lead stearate”︁ and „three-basic lead sulphate”︁ in PVC, there has been shown that the degradation of these substances at thermal stress of the material can be illustrated by diffractograms proving basic lead oxichlorides in cristalline intermediate stages. As final product of the reaction of lead stabilizers with HCl liberated when PVC decomposes, lead chloride appears, and in this connection the conversion of the sulphate part of th three-basic lead sulfate in this compound must also be taken into account. The intensities of the interferences of the three-basic lead sulphate and of the lead chloride on the X-ray diffractograms show the degree of degradation of the material after a processing operation due to the thermal stress. There is also the possibility of comparing the thermal stability of various lead stabilized PVC materials.     

Development of a time-resolved white-light interference microscope for optical phase measurements during fs-laser material processing

A modified Mach–Zehnder interferometer set-up combined with microscope objectives has been developed for the measurement of phase changes in the processed material sample, like modification and melting of glass. The white light is generated by focusing ultrafast laser radiation (t _p=80 fs) in a sapphire crystal using a micro-lens array to minimize temporal and spatial fluctuations in the white-light continuum. Lateral and coaxial pump-probe measurements of the phase changes during material processing are performed using two coupled ultrafast laser sources at different repetition rates (f _rep=1 Hz–1 MHz). The optical phase shift and therefore the refractive index of the material are calculated from the interference images using two approaches. The knowledge of the refractive index during the laser processing with a temporal resolution in the ps-range and a spatial resolution of several microns leads to a better understanding of the initial processes for the permanent material modifications.     

Relationship between coffee husk caffeine degradation and respiration of Aspergillus sp. LPBx in solid-state fermentation

Studies were carried out in a packed-bed column fermentor using coffee husk as substrate in order to verify a relationship between caffeine degradation and the respiration of Aspergillus sp. LPBx. Fermentation conditions were optimized by using factorial design experiments. The kinetic study showed that the caffeine degradation was related to the development of mold and its respiration and also with the consumption of reducing sugars present in coffee husk. From the values obtained experimentally for oxygen uptake rate and CO₂ evolved, we determined a biomass yield of 3.811 g of biomass/g of consumed O₂ and a maintenance coefficient of 0.0031 g of consumed O₂/(g of biomass·h). The maximum caffeine degradation achieved was 90%.     

Synthesis of a giant 222 carbon graphite sheet

In this paper we present the synthesis and characterization of the so far largest polycyclic aromatic hydrocarbon (PAH), containing 222 carbon atoms or 37 separate benzene units. First a suitable three-dimensional oligophenylene precursor molecule is built up by a sequence of Diels-Alder and cyclotrimerization reactions and then planarized in the final step by oxidative cyclodehydrogenation to the corresponding hexagonal PAH. Structural proof is based on isotopically resolved MALDI-TOF mass spectra and electronic characteristics are studied by UV/Vis spectroscopy.     

A thermal X-RAY and resistivity study of the heusler alloy Cu2NiSn

The compound Cu₂NiSn was analyzed by DTA, X-ray and electrical resistivity measurements. No single-phase structure was found at room temperature. The alloy decomposes upon heating, starting at 160°, forming copper and two hcp structures. At 500° the Heusler structure is restored. Electrical resistivity results are discussed in terms of Markowitz's theory for disordered metals.

---

Zusammenfassung Die Verbindung Cu₂NiSn wurde durch DTA, Röntgen- und elektrische Widerstandsmessungen analysiert. Keine Einphasenstruktur wurde bei Zimmertemperatur gefunden. Die Legierung zersetzt sich beim Erhitzen ab 160°, wobei Kupfer und zwei hcp-Strukturen gebildet werden. Bei 500° wird die Heusler-Struktur wieder hergestellt. Die Ergebnisse der elektrischen Widerstandsmessungen werden aufgrund der Markowitzschen Theorie für ungeordnete Metalle diskutiert.

---

Résumé Le composé Cu₂NiSn a été examiné par ATD, rayons X et mesures de résistance électrique. A température ambiante on n'a pas trouvé de structure correspondant à une phase unique. L'alliage se décompose par chauffage à partir de 160° en formant du cuivre et deux structures hcp. A 500° la structure Heusler est rétablie. On discute les résultats des mesures de résistance électrique à partir de la théorie de Markowitz pour des métaux désordonnés.

---

Cu₂NiSn , . . , 160° . 500° . a .

---

---

Work supported in part by Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) and Financiadora de Estudos e Projetos (FINEP).     

A phenomenological theory of the isotropic to chiral smectic-C phase transition

In this paper we discuss the direct isotropic to chiral smectic-C phase transition on the basis of a phenomenological theory. The model free energy is written in terms of the coupled order parameters including the spontaneous polarization. We present a detailed analysis of the different phases that can occur and analyze the question under which conditions a direct isotropic to chiral smectic-C phase transition is possible when compared to other phase transitions. On the basis of this model the isotropic-smectic-C^* transition is always of first order. The theoretical predictions are compared with the available experimental results.-1     

A search for the exotic Ξ<Superscript>−−</Superscript> (1860) in the hyperon beam experiment WA89(1860) in the hyperon beam experiment WA89

We report on a high statistics search for the Ξ^??(1860) resonance in Σ^?-nucleus collisions at 340 GeV/c. No evidence for this resonance is found in our data sample which contains 676000 Ξ^? candidates above bakground. For the decay channel Ξ^??(1860)→Ξ^? π ^? and the kinematic range 0.15<xF<0.9 we find a 3σ upper limit for the production cross section of 3.1 and 3.5 μb per nucleon, for reactions with carbon and copper, respectively.