Showing papers by "Johannes Kepler University of Linz published in 1990"

The relaxation of polymers with linear flexible chains of uniform length

Abstract: The analysis of dynamic mechanical data indicates that linear flexible polymer chains of uniform length follow a scaling relation during their relaxation, having a linear viscoelastic relaxation spectrum of the formH(λ) = n 1 G 0 × (λ/λ max) n1 forλ≤λ max. Data are well represented with a scaling exponent of about 0.22 for polystyrene and 0.42 for polybutadiene. The plateau modulusG 0 is a material-specific constant and the longest relaxation time depends on the molecular weight in the expected way. At high frequencies, the scaling behavior is masked by the transition to the glassy response. Surprisingly, this transition seems to follow a Chambon-Winter spectrumH(λ) = Cλ−n2, which was previously adopted for describing other liquid/solid transitions. The analysis shows that the Rouse spectrum is most suitable for low molecular-weight polymersM ≈ M c , and that the de Gennes-Doi-Edwards spectrum clearly predicts terminal relaxation, but deviates from the observed behavior in the plateau region.

Conduction electrons in GaAs: Five-level k⋅p theory and polaron effects

Abstract: Properties of conduction electrons in GaAs are described theoretically using a five-level k\ensuremath{\cdot}p model, which consistently accounts for inversion asymmetry of the material. The dispersion relation E(k) is computed and it is shown that the conduction band is both nonparabolic and nonspherical. The energy dependence of the electron effective mass, the energy-momentum relation in the forbidden gap, and the spin splitting of the band are calculated. Analytical expressions for the band-edge effective mass, the spin splitting, and the Land\'e factor ${g}^{\mathrm{*}}$ are presented, taking explicitly into account an interband matrix element of the spin-orbit interaction. A five-level P\ensuremath{\cdot}p theory for the conduction band in the presence of an external magnetic field is developed. Resonant and nonresonant effects due to polar electron--optic-phonon interaction are included in the theory. The spin g value of conduction electrons is calculated as a function of energy and magnetic field. Spin-doublet splitting of the cyclotron resonance and the cyclotron-resonance-mass anisotropy are described. A comparison of the theory with experimental data of various authors is used to determine important band parameters for GaAs. It is shown that away from the band edge the polaron effects in GaAs are comparable to the band-structure effects.

Finite-dimensional approximation of tikhonov regularized solutions of non-linear ill-posed problems

Abstract: In this paper we consider non-linear ill-posed problems F(x)=y0 in a Hilbert space setting. We solve these problems with Tikhonov regularization combined with finite-dimensional approximation where the data y0 and the non-linear operator F are assumed to be known only approximately. Conditions are given that guarantee optimal convergence rates with respect to both, the data noise and the finite-dimensional approximation. Finally, we present some numerical results for parameter estimation problems that verify the theoretical results.

Monte Carlo simulation of RBS spectra: Comparison to experimental and empirical results

Abstract: For low projectile energies, Rutherford backscattering (RBS) spectra from thin targets show a low energy background which can be explained by plural and multiple scattering of the projectiles. Knowledge about the influence of plural and multiple scattering on the shape of RBS spectra is important in many fields (e.g. RBS depth profiling, determination of projectile energy loss). Analytic calculation of RBS spectra with full inclusion of plural and multiple scattering is impossible. Therefore we used Monte Carlo computer simulations to calculate RBS spectra for 100–200 keV protons backscattered by thin gold films (1000 A). In order to limit the number of scattering events, a nonzero minimum scattering angle (cutoff angle) was used in the simulations. The cutoff angle must be chosen so small that a further reduction does not change the shape of the total spectrum to a significant extent. Therefore, for various cutoff angles, the contribution of single, double and plural scattering to the total RBS spectrum is discussed. Furthermore, simulated spectra are compared to experimental data and to the empirical formula presented by Weber and Mommsen [2].

Critical fluctuations in superconductors.

Abstract: The field-theoretical gauge model for a superconductor, generalized to a {ital n}/2-component complex order parameter, is renormalized in two-loop order. The question of a fluctuation-induced first-order transition is discussed, and the crossover functions for the effective exponents and the amplitude ratio of the specific heats above and below {ital T}{sub {ital c}} are given.

Stopping power of light ions near the maximum

Abstract: Some of the most interesting recent results are reviewed: in the regime of high ion velocities, there is large progress in the understanding of the stopping process beyond the first Born approximation (Bethe theory) due to theoretical and experimental investigations of the Barkas effect. Both investigations show that close collisions contribute to the Z31 correction, the importance of which becomes larger at lower ion velocities. For stopping measurements in metals and in semiconductors, the cooperation of two labs using different techniques has increased the attainable precision. The data may be compared to BEA calculations. The physical and chemical state of the material is of importance for ion stopping near the maximum on targets whose valence electrons dominate the stopping process. Ziegler has recently given a systematic treatment of the stopping of He and Li ions in hydrocarbons, using a core-and-bonds model. We have shown that, also for heavier compounds (Al2O3 and SiO2), large chemical effects are found. The stopping properties of these compounds can be well described by BEA calculations. Sabin et al. (1985) have predicted large phase effects for alkalis: for lithium and sodium targets and protons as projectiles they find a stopping ratio (vapor to solid) which exceeds a factor of 2 near the stopping maximum. The concept of an “effective charge” is useful to predict stopping powers for heavier ions from proton stopping data. For low ion velocities (near the velocity proportional region) and for some substances, the value of the He effective charge may exceed the value suggested by Ziegler et al. considerably.

Implicitization of Rational Parametric Curves and Surfaces

Abstract: In this paper we use Grobner bases for the implicitization of rational parametric curves and surfaces in 3D-space. We prove that the implicit form of a curve or surface given by the rational parametrization $$x_1 : = \frac{{p_1 }}{{q_1 }}x_2 : = \frac{{p_1 }}{{q_2 }}x_3 : = \frac{{p_1 }}{{q_3 }}$$ where the p's and q's are univariate polynomials in y 1 or bivariate polynomials in y 1 , y 2 over a field K, can always be found by computing $$GB(\{ q_{^{_1 } } \cdot x_1 - p_{1,} q_2 \cdot x_2 - p_{2,} q_3 \cdot x_3 - p_{3,} \} ) \cap K[x_1 ,x_2 ,x_3 ],$$ where GB is the Grobner basis with respect to the lexical ordering with x1≺x2≺x3≺y1≺y2, if for every i, j∈{1,2,3} with i≠j the polynomials p i , q i ,p j ,q j have no common zeros. This result leads immediately to an implicitization algorithm for arbitrary rational parametric curves.

Response of Si detectors to electrons, deuterons and alpha particles

Abstract: The response of silicon detectors to conversion electrons in the decay of 109 Cd, 233 Pa and 137 Cs, to deuterons in the energy range 35–440 keV from a Van de Graaff accelerator and to alpha particles from a mixed 239 Pu, 241 Am, 244 Cm source has been measured. The results were found to be in agreement with the model proposed by Lennard et al. (1986) which describes the nonlinear detector response in particular for the case of light ions. For electrons, the nonlinearity was found to be negligible within experimental uncertainties. For deuterons the nonlinearity observed in Rutherford-backscattering measurements at low energies was used to accurately measure the window thickness of the detectors. For alpha particles in Si it has been confirmed that the energy required to create an electron-hole pair depends on the stopping power. Our value for the nonlinearity parameter k = 2.8 ± 0.3 nm/electron-hole pair is larger than the one which was originally proposed.

On the Reaction Kinetics in Laser-Induced Pyrolytic Chemical Processing

Abstract: Reaction rates, particle densities, and temperature distribution in pyrolytic (photothermal) laser-induced microchemical processing are investigated with respect to temperature and concentration-dependent transport coefficients, and with respect to the effect of thermal diffusion. While the model employed is particularly suitable for laser-induced chemical vapor deposition (LCVD), it can also be applied to many cases of laser-induced surface modification and dry-etching.

Some good point systems for derivatives of lagrange interpolatory operators

Abstract: 1.1. Let us consider an arbitrary projection operator L,: C ~ , _ 1 (i.e. L, is a linear bounded operator, L,(f , x)E~,_ l if fEC and L,( f , x)=f(x) iff f E ~ l ; here C is the set of continuous functions on [ 1 , 1], ~ ,_ ~ denotes the set of algebraic polynomials of degree =cn ~" fEc (cf. D. L. Berman [10]). However, using a new norm introduced and investigated for projection operators by J. Szabados [3] we can have much better results. Namely, let (1.1) ~lts~ := sup l[L~)(f, x)ll, s = 0, 1, 2, p => 0 $EC

Beiträge zur Chemie der Pyrrolpigmente, 84. Mitt.: Darstellung und Lumineszenz bichromophorer 5-Aryl-dipyrrinderivate

Abstract: 1,3- and 1,4-Bis-dipyrrin-benzenes and their various zinc and difluoroboryl chelates are prepared. Their absorption and fluorescence spectra are compared with those of the corresponding mono-dipyrrin derivatives. The conformations of these compounds are analyzed using the ring current model and absorption spectra. Dipole-dipole type transfer of excitation energy is discussed to represent a main photophysical channel in these compounds.

Comparative optical investigations of ZnSe/GaAs epilayers grown by molecular beam and hot‐wall epitaxy

Abstract: Comparative photoluminescence and excitation spectra of ZnSe/GaAs epilayers grown by molecular beam epitaxy (MBE) and hot‐wall epitaxy (HWE) show likewise features in the exciton energy regime. Two strain‐split components of the free exciton are observed, as well as characteristic sets of transitions from or into ground and excited states of acceptor‐ and donor‐bound excitons. However, all respective lines are shifted to lower energies in the HWE samples, due to the increased thermally induced strain as a consequence of the substrate temperatures being enhanced compared to the MBE growth. Whereas the dominant donors are of the same nature in both kinds of samples, specific acceptor centers are incorporated in the HWE films. Although the MBE‐grown layers are of superior quality, it is shown that HWE under optimized growth conditions is a cheap and useful alternative to obtain ZnSe epilayers of reliable properties.

Density of bound states in a vortex core.

Abstract: The local density of states as derived from Kramer and Pesch's theory of bound states in a vortex core is compared with recent scanning-tunneling experiments. Effects of impurities and finite flux-line distance are approximately taken into account. In the isolated vortex regime one finds qualitative, but not quantitative, agreement with all experimental data reported so far. The results depend sensitively on impurity content. A nonmonotonic behavior of the density of states as a function of the distance from the flux-line center is predicted. The unexpected properties of the density of states are discussed in terms of the direction-dependent single-particle excitations bound to the core.

Weight functions admitting repeated positive Kronrod quadrature

Abstract: In this note it is shown that for weight functions of the formw(t)=(1 −t 2)1/2/s m (t), wheres m is a polynomial of degreem which is positive on [−1, +1], successive Kronrod extension of a certain class ofN-point interpolation quadrature formulas, including theN-point Gauss-formula, is always possible and that each Kronrod extension has the positivity and interlacing property.

Stopping cross section of water vapor for hydrogen ions

Abstract: In this contribution we describe a vapor cell with windows (Al), which we built to measure ϵ of deuterons in water vapor in the range 25–350 keV/u. The energy of the transmitted beam was measured by a surface-barrier detector after scattering by a thin Pt film. Operating the vapor cell, special care had to be taken in order to achieve thermal equilibrium. The resulting data confirm the results of Reynolds (1953) within 1%. The statistical uncertainty of our data is ∼1%, an additional (systematic) uncertainty of the same magnitude arises from the calibration of the temperature measurement. The deviation from Bragg's rule and the phase effect are discussed.

K-shell ionization cross sections for Si, P, K, Ca, Zn, and Ga by protons and carbon ions in the energy range 1-6.4 MeV.

Abstract: Absolute {ital K}-shell ionization cross sections have been measured for thin targets of Si, P, S, K, Ca, Zn, and Ga using carbon ions between 1.0 and 6.4 MeV and protons of 1 and 2 MeV. The dependence of x-ray production cross sections on target thickness was determined. The experimental results are compared to the semiclassical approximation (Laegsgaard, Andersen, and Lund in 3 Proceedings of the Tenth International Conference on the Physics of Electron and Atomic Collisions, Paris, 1977, edited by G. Watel (North-Holland, Amsterdam 1977)), to the theory for direct Coulomb ionization of the 1{ital s}{sigma} molecular orbital (Montenegro and Sigaud, J. Phys. B. 18, 299 (1985)), to the perturbed stationary-state approximation with energy-loss, Coulomb, and relativistic corrections (ECPSSR) (Brandt and Lapicki, Phys. Rev. A 23, 1717 (1981)), and to the modification of the ECPSSR approximation (MECPSSR) (Benka, Geretschlaeger, and Paul, J. Phys. (Paris) Suppl. 12, C9-251 (1987)). The results for carbon ions are also compared to the statistical molecular orbital theory of inner-shell ionization for symmetric or nearly symmetric atomic collisions (Mittelman and Wilets, Phys. Rev. 154, 12 (1967)).

Reconstitution of apomyoglobin with bile pigments

Abstract: Apomyoglobin was reconstituted with bile pigments of the verdinoid, 2,3-dihydroverdinoid, pterobilinoid, and violinoid type. Absorption and circular dichroism data as well as formation constants of the complexes were measured. From these results it was concluded that chromophore binding and induced chirality of these pigments are mainly governed by a lipophilic region opposite to the propionic side chain(s) and the asymmetric position of the hydrogen bonding acceptors of the propionic acid side chain(s) at the entrance of the protein pocket.

On the kinetics of non-equimolecular reactions in laser chemical processing

Abstract: The influence of counterdiffusion on the reaction rate in non-equimolecular laser-induced gas-phase processing is investigated.

The stopping power of Ag and Au, with regard to higher-order Z1-effects

Abstract: We have measured the absolute stopping cross section ϵ of Ag and Au with a precision of 3%, using backscattering geometry. The velocity of the projectiles (p, [d], He, Li, and C) ranges from about 1.5υ0 to 4υ0. These measurements are compared to calculations of ϵ including the Barkas and Bloch terms. We use a local density approximation employing the harmonic-oscillator calculations by Sigmund and Haagerup, by Mikkelsen and Sigmund, and the calculations by Bloch, introducing a simple shell correction. Within the energy range where our approximations apply, good agreement with the measured proton stopping cross section is found. Furthermore, we find from our calculations that the ratio of the stopping cross sections of Ag and Au does not depend on the type of projectile (in all orders of Z1), in agreement with our measurements.

Solvent donor and acceptor properties of pyridine

Abstract: Polarographic and cyclovoltammetric measurements on the perchlorates of Li+, Na+, K+, Rb+, Cs+, Tl+, Ba2+ and Ni2+ as well as on the trifluoromethane sulfonates of Zn2+, Cd2+, Pb2+, Cu2+, Cu+, Mn2+ and Co2+ were carried out. The data allowed the evaluation of the different donor behavior of pyridine towards hard, border line and soft cations. The conclusions drawn from electrochemical investigations were supported by Gibbs energies of transfer for cations, which were derived from both electrochemical measurements based on the bis(biphenyl)chromium assumption and from solubility studies based on the tetraphenylarsonium tetraphenylborate assumption. The acceptor properties of pyridine were obtained from the solvatochromic dyes bis(cyano)bis(1,10-phenanthroline)iron(II) and bis(cyano)bis(3,4,7,8-tetramethyl-1,10-phenanthroline)iron(II) and the results were compared with the acceptor number and the E T -value.