A Class of Polynomials over Finite Fields

Generalizing the norm and trace mappings for _q^r/ _q, we introduce an interesting class of polynomials over finite fields and study their properties. These polynomials are then used to construct curves over finite fields with many rational points.     

MO study of the benzylperoxyl radical, an intermediate in atmospheric ozone formation

Unrestricted Hartree-Fock calculations have been carried out to study the benzylperoxyl radical, an intermediate atmospheric contaminant. For this purpose, quantum mechanical MP2//UHF ab initio calculations were performed by using 6-31G^** and 6-31 + G^** basis sets. Because no previous experimental nor theoretical data were available for benzylperoxyl, the results were compared with those of the smaller unsaturated peroxyl radicals, such as allylperoxyl. It is found that the calculated absolute minimal energy corresponds to a trans conformation in which the O---O bond is sticking away from the benzene ring. Benzylperoxyl is found also to exhibit an unusually O---O small bond order, which reflects the weakness of the bond, i.e. its ability to react with NO forming NO₂ and subsequently ozone. A peculiar MO diagram is obtained, with the unpaired MO deeply located in the “doubly” occupied MO space. This effect is also observed in allylperoxyl. Structural parameters, charges, spin densities and dipole moments are also reported.     

The surfactant-sensitized analytical reaction of niobium with 8-hydroxyquinoline-5-sulphonic acid

Cationic surfactants, such as cetylpyridinium bromide (CPB), sensitize the colour reaction of Nb(V) with 8-hydroxyquinoline-5-sulphonic acid (H(2)L). The formation of a ternary complex of stoichiometry 1:3:3 (Nb-L-CPB) is responsible for the observed enhancement in absorptivity and the quenching in the fluorescence of the Nb-L chelate, when a surfactant is present. The ternary complex exhibits maximum absorption at 383 nm ( = 1.46 +/- 0.01 x 10(4) l.mole(-1).cm(-1)) at pH 5.7, and Beer's law is obeyed up to 6-mug ml Nb concentration. Conditional formation constants of the niobium chelate in the presence and absence of CPB have been determined. On the basis of a detailed spectrophotometric and fluorimetric study the nature of the chromophoric reagent-surfactant interaction and the peculiar features of the sensitization by CPB are discussed.     

Determination of the degree of acetylation of chitin and chitosan by thermal analysis

Several methods for the rapid determination of the degree of acetylation of chitin and related polymers have been evaluated, including the use of the infrared and the mass spectra. Chitin and chitosan have characteristic degradation temperatures and it is possible to determine the acetylation degree by the use of empirical correlations based on the weight losses associated with the main decomposition peaks.     

Numerical Analysis of the Behaviour of Tooth Intrarradial Posts by the Finite Element Method

The aim of this work is to study the behaviour of a carbon/epoxi post housed in a canine tooth after endodontic treatment in order to support the typical loads present during mastication. The three-D basic design of the dental piece consisting of tooth + post was carried out with a three-dimensional parametric design program. We study the stresses and displacements of the different elements of the dental piece under normal load conditions, and present the results and conclusions.     

Pump-induced nonlinear refractive-index change in erbium- and ytterbium-doped fibers: theory and experiment

A 980-nm pump-induced nonlinear refractive-index (n2) change in erbium-doped (20-m) and ytterbium-doped (20-m) fibers has been measured at 1064 nm by time-delayed photorefractive beam coupling in a co-propagating and counterpropagating geometry. It was found that n2 decreases at the wavelength of the probe beam when the pump beam is present. We present a semiclassical theory based on a four-state system that accounts for the pump-induced change of n2. Both theoretical and experimental results show that a significant index change, of the order of 10%, can be obtained for cw pump powers as small as 10 mW.     

Superconductivity of bulk CaC6

We have obtained bulk samples of the graphite intercalation compound, CaC6, by a novel method of synthesis from highly oriented pyrolytic graphite. The crystal structure has been completely determined showing that it is the only member of the MC6, metal-graphite compounds that has rhombohedral symmetry. We have clearly shown the occurrence of superconductivity in the bulk sample at 11.5 K, using magnetization measurements.