Showing papers by "Johannes Kepler University of Linz published in 1992"

Excitation-power dependence of the near-band-edge photoluminescence of semiconductors.

Abstract: We present a model calculation for the dependence of the near-band-edge photoluminescence (NBEPL) on the power of the exciting laser light. Our model explains all features of the NBEPL power dependence that were previously observed in experiment: (i) the variation of the excitonic photoluminescence intensity I with ${\mathit{L}}^{\mathit{k}}$, where L is the excitation power and k is an exponent between 1 and 2, (ii) deviations from the I\ensuremath{\sim}${\mathit{L}}^{\mathit{k}}$ law as L is varied by more than two orders of magnitude, and (iii) the variation of k for exciton emission lines when the wavelength of the exciting laser radiation is varied. Furthermore, our model relates the k values of the free exciton, bound exciton, and the free-to-bound transitions. The results are in excellent agreement with experimental data.

Indirect UV detection of carbohydrates in capillary zone electrophoresis

Abstract: A new system for the rapid and sensitive analysis of underivatized carbohydrates has been established using capillary zone electrophoresis with indirect UV detection. At an applied potential of 28 kV, sugars and sugar acids could be separated by the combined effects of electroendosmosis and electrophoresis within 20 minutes in a fused silica capillary of 50 μm internal diameter and an effective length of 100 cm using 6mM sorbic acid, pH 12.1, as both carrier electrolytie and chromophore. The alkaline pH ensured ionization of the sugars and, hence, their detection by means of charge displacement. Furthermore, the chosen concentration of sorbic acid allowed the smallest fractional change in the background signal to be measured. While the electrophoretic mobilities of the sugars were found to increase within a pH range of 11.9 to 12.3, those of the sugar acids were not affected. Due to the increasing competition of hydroxide ions in the displacement of the chromophore with rising pH, a significant loss of sensitivity is observed at pH values higher than 12.1 and this pH was found to provide sufficient resolution, optimum sensitivity, and a acceptably short analysis time. Under these conditions, a lower detection limit of 2 pmol was obtained for glucose.

Capillary zone electrophoretic analysis of carbohydrates by direct and indirect UV detection

Abstract: The analysis of carbohydrates has been always hampered by their lack of UV absorbance above 200 nm, which is an especially challenging problem in capillary electrophoresis due to the very small (nl) sample volumes injected. The introduction of 2-aminopyridine as derivatizing agent allows sensitive direct UV detection of saccharides in the fmol range. However, due to the requirement of the presence of a free aldehyde group only aldoses and uronic acids can be determined. This limitation was recently overcome by means of precolumn derivatization withp-aminobenzoic acid or ethylp-aminobenzoate, which permits the analysis of fructose with a lower mass detection limit of 0.3 and 0.14 pmol, respectively. The detection limits for aldoses were even as low as 15 and 7 fmol. A more universal approach is the use of indirect UV detection, which permits the analysis of carbohydrates, including (1–2)-linked disaccharides and aldonic acids, at the lower pmol level without the need for derivatization.

Tikhonov regularization of nonlinear III-posed problems in hilbert scales

Abstract: In this paper we consider nonlinear ill-posed problems F(x) = y 0, where x and y 0 are elements of Hilbert spaces X and Y, respectively. We solve these problems by Tikhonov regularization in a Hilbert scale. This means that the regularizing norm is stronger than the norm in X. Smoothness conditions are given that guarantee convergence rates with respect to the data noise in the original norm in X. We also propose a variant of Tikhonov regularization that yields these rates without needing the knowledge of the smoothness conditions. In this variant F is allowed to be known only approximately and X can be approximated by a finite-dimensional subspace. Finally, we illustrate the required conditions for a simple parameter estimation problem for regularization in Sobolev spaces.

Crystal structure and superconductivity in REBaSrCu3Ox

Abstract: The preparation, crystal structure and physical properties of REBaSrCu 3 O x compounds have been studied (Re≡La, Pr, Nd, Sm, Eu, Gd, Dy, Y, Ho, Er, Tm, or Lu). These compounds are found to be superconducting with a transition temperature within the range 54 K ≤ T c ≤86 K, except for RE = Pr, which is semiconducting. The maximum transition temperature is observed with RE=Gd and Dy where the lattice structure changes from tetragonal to orthorhombic.

Capillary zone electrophoresis of aldoses, ketoses and uronic acids derivatized with ethylp-aminobenzoate

Abstract: A number of mono- and oligosaccharides derivatized with ethylp-aminobenzoate have been separated as their borate complexes by capillary zone electrophoresis, using a fused silica capillary containing 175 mM borate buffer, pH 10.5, as carrier. The derivatized carbohydrates were sufficiently separated within 17 min at an applied voltage of 25 kV. On-column UV monitoring allowed detection of these derivatives at the 10 fmol level, and quantification by the relative peak area method allowed reproducible determination of the saccharides at least in the concentration range of 0.5–20 mM in reaction solutions. This method has been applied to the determination of the monosaccharide compositions of polysaccharides extracted from Flos matricariae and Radix althaeae.

On the nature of “soluble” hypericin inHypericum species

Abstract: The two red and two violet “soluble” pigments ofHypericum species were isolated by means of extraction, chromatography, and counter-current droplet chromatography. In contrast to authentic hypericin, they are soluble in common organic solvents and even in water. Using NMR experiments it was deduced that hypericin, pseudohypericin, protohypericin, and protopseudohypericin are present in the plant as their rapidly interconverting 3- and 4-phenolate ions. From AAS the main counter-ion of these phenolates was derived to be potassium. The potassium and N-ethyl-N,N-diisopropylammonium salts of hypericin were synthesizcd for comparison. A preparative procedure to isolate hypericin and pseudohypericin from plant material was developed.

Wigner-Seitz approach to spin splitting.

Abstract: A method is proposed that allows for a more precise evaluation of the spin splitting of electronic levels induced by the exchange interaction between effective-mass electrons and localized magnetic moments in dilute magnetic semiconductors. The method is nonperturbative, surpasses the molecular-field approximation (MFA), and is developed in the spirit of the muffin-tin model. Surprisingly, the corrections to the MFA exhibit a strong variation with the concentration of magnetic ions, accounting for the hitherto unexplained dependence of the apparent p-d exchange integral on x in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$S.

The width of an RBS spectrum: influence of plural and multiple scattering

Abstract: The width of RBS spectra from thin films depends on film thickness and stopping power, hence it is possible to determine one of these quantities when the other is known. For low ion energies or large film thicknesses, however, the spectrum shape significantly differs from that obtained by the single scattering calculation normally used. In this contribution, we determine quantitatively the influence of plural and multiple scattering on the width of RBS spectra: we calculate RBS spectra for protons and He ions by Monte Carlo simulation and by an analytical single scattering model using the same stopping power and interaction potential. As a result, quantitative agreement of the Monte Carlo spectra with measurements is found, while the single scattering model fails, especially near the low energy edge and below. Not only the spectrum shape but also the position of the low energy edge is affected by plural and multiple scattering. We determine this shift by varying projectile and target parameters and we find it is mostly governed by the angular width α 1 2 of the multiple scattering distribution. In bad cases, the shift may be as large as 3–4% for α 1 2 ≅ 20°.

Enhanced adhesion of metal films on PET after UV-laser treatment

Abstract: The adhesion-force of thin metal films on PET foils can be significantly improved by UV excimer-laser irradiation of the polymer surface prior to metal deposition. The laser fluences required are well below the ablation threshold.

On the synthesis of hypericin by oxidative trimethylemodin anthrone and emodin anthrone dimerization: Isohypericin

Abstract: The base catalyzed oxidative dimerization of emodin anthrone exclusively yields hypericin. However, on oxidative dimerization of trimethylemodinanthrone a mixture of hexamethylhypericin and hexamethylisohypericin was obtained. Chromatographic separation of the hexabenzoyl derivatives was achieved, and by saponification about equal amounts of hypericin and isohypericrin were produced. Isohypericin could be characterized for the first time by its spectroscopic data and its protonation and deprotonationpK a andpK a * values.

The distribution function of the convolution square of a convex symmetric body in ℝ n

Abstract: By analyzing the distribution function of the convolution square of a convex and symmetric body we obtain some affine invariants related to the body. These invariants have a geometric interpretation.

Structure formation in UV-laser-ablated polyimide foils

Abstract: The formation of nap-type and wall-type structures on laser-irradiated polyimide (PI) films is reported for the first time. These investigations demonstrate that such structures can develop on non-melting polymer surfaces.

Deprotonation and protonation of hydroxyphenanthroperylenes

Abstract: Ground and excited state deprotonation and protonationpK a values of hydroxyanthraquinones, hydroxyanthrones, hydroxyphenanthroperylenes, and the natural pigments hypericin and pseudohypericin were determined by means of spectrophotometric titrations and Forster cycle calculations. It was concluded that there is a strong intramolecular excited state proton transfer in the hydroxyanthraquinones and hydroxyanthrones due to a reversion of acidity and basicity of the hydroxyl and carbonyl groups in the excited state. However, in the hydroxyphenanthroperylene and the natural pigment excited states the order of basicity and acidity of these two functional groups remain unchanged. The site of deprotonation in hypericin and pseudohypericin was deduced by comparison between thepK a values of suited model compounds and these pigments to be the hydroxyl group in position 3 or 4, respectively.

Persistence of critical multitype particle and measure branching processes

Abstract: We consider a class of systems of particles ofk types inR d undergoing spatial diffusion and critical multitype branching, where the diffusions, the particle lifetimes and the branching laws depend on the types. We prove persistence criteria for such systems and for their corresponding high density limits known as multitype Dawson-Watanabe processes. The main tool is a representation of the Palm distributions for a general class of inhomogeneous critical branching particle systems, constructed by means of a “backward tree”.

Oxygen ordering and superconductivity in DyBaSrCu3Oy

Abstract: A new superconducting compound, DyBaSrCu 3 O y , has been synthesized. By different thermal treatments, this compound was crystalized in either orthorhombic or quasitetragonal structure, with oxygen content of y ⋍6.93 to 6.94. Superconducting properties are similar for both phases. The magnetic moment of the Dy atoms is not influenced by the difference in structures and is close to the value for Dy 3+ . At low temperatures a tendency towards magnetic ordering was observed.

Labor Market Institutions and the Industry Wage Distribution: Evidence from Austria, Norway, and the U.S.

Abstract: The paper studies the industry wage structures of Austria, Norway, the union sector of the U. S. as well as the non-union sector of the U. S. We make comparable regressions for each country, and are thus able to compare the sectoral earnings patterns controlling for the usual individual characteristics. Our results confirm the hypothesis that the pattern of the inter-industry pay structure is largely independent of labour market institutions: High paying industries in a non-union environment tend to pay high wages also in regimes where bargaining is very centralised and coordinated.

Energy loss of hydrogen and helium ions in hydrogen and helium gas: looking for exceptions from velocity proportionality

Abstract: At projectile velocities ν 1 below the Bohr velocity the electronic stopping cross section e of hydrogen (H 2 , D 2 ) and helium gas for hydrogen isotopes (H, D) and of helium gas for helium projectiles has been measured by a time-of-flight technique We find the hydrogen stopping in He to show a significant departure from the ν 1 -proportionality commonly assumed (e α ν 1 33 at 4 keV per nucleon) [R Golser and D Semrad, Phys Rev Lett 66 (1991) 1831] Electronic stopping of H, D onto H 2 , D 2 (e α ν 1 09 at 4 keV per nucleon) and He onto He (e α ν 1 08 at 4 keV per nucleon,if nuclear stopping is not subtracted) may be considered velocity proportional We attribute this behavior to the probability of charge changing processes, which is small if projectile and target ground state levels are strongly different The helium to hydrogen stopping ratio of helium gas has the strange value 58 at 4 keV per nucleon; possible implications to the concept of “effective charge” are discussed

Stopping of hydrogen ions in oxides: influence of the chemical bond

Abstract: The stopping cross sections (SCS) of Al2O3 and SiO2 for hydrogen ions have been measured by Rutherford backscattering relative to that of Al and Si, respectively, the velocity range being 3.45 < v/v0 < 3.75 (equivalent to proton energies of 300 to 350 keV). Using these data we are able to re-evaluate the thicknesses of the targets used in our earlier measurements. Thus we get consistent stopping data in the velocity range 0.77 < v/v0 < 5.4 (i.e., 15 to 730 keV for protons) for all targets and measuring methods used. The resulting absolute values are more accurate than our earlier data and consistent with those within the experimental uncertainties. Comparing our results for these compounds to stopping cross sections calculated for the corresponding mixtures we find that due to the influence of the chemical bond, the stopping in the compounds is less effective than in the mixtures; the largest deviations (up to 27% for Al2O3 and up to 14% for SiO2) appear at our lowest ion velocities. Furthermore we use our data to check whether the stopping behaviour of the oxides can be described by standard theoretical models, especially at low velocities where our experiment yields a stopping cross section proportional to velocity despite the rather large band gap of the insulators. We find that neither the density functional approach, nor linear response theory nor a binary encounter calculation can fit our experiment at low energies. Above proton energies of ≈ 40 keV, the BEA calculation fits perfectly, while the LRT is in only qualitative agreement with our data. The reason for the failure of the theories at low velocities is still unknown.

Do they come back again? Job search, labour market segmentation and state dependence as explanations of repeat unemployment

Abstract: This study investigates the causes of recurrent unemployment. Using data from the Austrian unemployment register we test the explanatory power of three different approaches which appear in the literature: job search theory, labour market segmentation and state dependence. Whereas job search theory does not seem to be able to explain anything, labour market segmentation does. However, the most powerful determinant of the risk of unemployment repetition is past unemployment history. This micro finding is not inconsistent with theories explaining the persistent high level of unemployment rates.

Non-linear Real Constraints in Constraint Logic Programming

Abstract: Dealing with non-linear constraints over real numbers is one of the most important and non-trivial problems in constraint logic programming. We report our initial effort in tackling the problem with two methods developed in computer algebra during last three decades: Partial Cylindrical Algebraic Decomposition and Grobner basis. We have implemented a prototype called RISC-CLP(Real). Experience with the prototype suggests that it is desirable and in fact feasible to provide a full support of non-linear constraints.