Paramagnetic defects in BaTiO3 and their role in light-induced charge transport — Optical studies

On the basis of a previous identification of paramagnetic defects in nominally undoped as grown BaTiO₃ single crystals, we have investigated the changes of the concentrations of these centers and their optical absorptions under illumination with light of varying wavelengths. The most pronounced charge transfers occur by hole ionization of Fe⁴⁺ and — to a lesser extent — of Cr⁵⁺ and Cr⁴⁺. At low temperatures the created holes are trapped in the form of O^–-ions next to Al³⁺ or unknown acceptor defects. Corresponding Fe⁴⁺ and O^– absorptions have been identified.     

Characterization of carbide phases in high speed tool steels

A study is reported on carbides electrolytically extracted from two different types of high speed tool steels as quenched and after tempering. According to X-ray diffraction analysis and EDS microanalysis, Mössbauer spectra show the presence of MC and M₆C in every analysed sample. Only a few substitutional Fe atoms should be present in MC and they give a minor contribution to the Mössbauer spectrum. A model for evaluating the Mössbauer spectrum of M₆C is proposed which is based on the symmetry of lattice sites occupied by Fe atoms and partially substituted by other metallic atoms. The proposed model also allows to get information on the amount of this substitution.     

Charm production in two-photon collisions: Calculation of the leading nonperturbative contribution in QCD

The leading nonperturbative contribution to the cross section of charm production in two-photon collisions is calculated in QCD. Operator expansion is used and the c-quark interaction with vacuum gluon condensate is taken into account. As a result, the moments of the charm contribution to the photon structure function are obtained. The moments with sufficiently high numbers turn out to be sensitive to the gluon condensate.     

Validation of a solid-phase extraction high-performance liquid chromatographic assay for doxazosin.

The determination of doxazosin by high-performance liquid chromatography with fluorescence detection is described. Propanolol was used as the internal standard. Plasma samples were treated with methanol to precipitate the proteins. Doxazosin was isolated with C18 reversed-phase extraction columns. The determination limit is 1 ng/ml of plasma, while the extraction columns can be reused frequently. The method is applied to clinical trial samples.     

Low energy positron scattering from atoms in and on solids observed with low energy positron diffraction and reemission microscopy

The present status of low energy positron diffraction (LEPD) and positron reemission microscopy (PRM) is reviewed in the context of unanswered questions regarding the elastic scattering of positrons in the 1–300 eV energy range from atoms in solids and at solid surfaces. Recent LEPD studies yield an agreement between theoretical and experimental diffraction intensities for semiconductors that has never been equalled in electron diffraction studies. This situation is discussed in terms of the repulsive interaction between the positron and the embedded atomic potential and the lack of exchange with the nonspherically distributed valence electrons. The scattering of re-emitted positrons in PRM from atoms chemisorbed or physisorbed on the re-emitting surface has not yet received the same theoretical attention as scattering from embedded atoms in LEPD. Possible ways in which positron scattering from overlying atomic structures manifests itself in PRM as well as positron re-emission holography are discussed, both from the practical viewpoint of observing these structures and in the context of fundamental questions regarding the positron re-emission process itself.     

Electron Energy Distribution Functions in CO2 Laser Mixture: The Effects of Second Kind Collisions from Metastable Electronic States

Electron energy distribution functions (eedf) in CO₂ laser discharge (He—CO₂—N₂—CO mixture) have been calculated by solving the Boltzmann equation in the presence of given concentrations of excited (vibrational and electronic) states. The results show a well structured eedf as a result of second kind collisions coming from metastable electronic states of N₂ and He as well as a strong dependence of rate coefficients for CO₂ dissociation and for the ionization of the different species.     

Extended transverse-momentum analysis for intermediate-energy heavy-ion collisions

An extension of the transverse-momentum analysis of Danielewicz and Odyniec is proposed. This extension involves a consideration of baryonic p_T-distribution widths in and out of the reaction plane. Calculations within one- and two-fluid mean-field models show that such an analysis allows us to extract more information on the nuclear equation of state and stopping power than the conventional one does. Different criteria for selecting collision events (i.e. sorting them out according to impact parameters) are also examined. Selections according to the baryonic multiplicity in the midrapidity region and the transversal-to-longitudinal energy ratio are found to be suitable for this purpose.     

Approximation of a bounded solution of a difference equation by solutions of corresponding boundary problems in Banach space

Translated from Matematicheskie Zametki, Vol. 51, No. 4, pp. 17–22, April, 1992.