Showing papers by "Chalmers University of Technology published in 1981"

Effective medium models for the optical properties of inhomogeneous materials

Abstract: The Maxwell Garnett and Bruggeman effective medium theories are derived for the average dielectric permeability of heterogeneous materials from a unified theoretical approach. It starts by specifying two random unit cells which represent different microstructures. Requiring that these cells should not be detectable by electromagnetic radiation when embedded in an effective medium, we show from an extended optical theorem that the forward scattering amplitude must vanish. Setting the leading term in the expansion series of this quantity equal to zero yields the effective medium theories pertaining to the two microstructures. The remaining terms provide estimates of the accuracy of the approximations. This approach is then used in numerical computations for Co-AI(2)O(3) cermets.

Vibrational interaction between molecules adsorbed on a metal surface: The dipole-dipole interaction

Abstract: We have developed a theory for the properties of vibrational excitations in molecules adsorbed on a metal surface. The coherent potential approximation (CPA) is used in the treatment of the vibrational interaction between the molecules. We show, by interpreting infrared spectra of substitutionally disordered systems consisting of isotopic mixtures of CO on Cu(100), that the molecules interact mainly through their dipole fields. We also show that in interpreting the integrated absorptance in infrared spectroscopy or the relative loss intensity in electron-energy-loss spectroscopy it is necessary to take into account the screening due to the electronic polarizability of the adsorbed molecules. A simplified version of the CPA result is used for a discussion of the absorption spectra of partial monolayers of one isotope. With the assumption that the CO molecules are randomly distributed, comparison between theory and experiment indicates that the dipole-dipole interaction alone is responsible for the coverage-dependent frequency shift for CO adsorbed on a transition metal [Ru(001)], whereas there is an almost equally large counteracting chemical shift on a noble metal [Cu(100)]. The meaning and origin of the dynamical dipole moment of adsorbed CO molecules are discussed. We find that the increase of the dynamical dipole moment (by a factor 2-3) upon adsorption probably is due to charge oscillations between CO $2{\ensuremath{\pi}}^{*}$ molecular orbitals and the metal. Finally, we outline how the theory developed here can be applied to a fundamental step in photosynthesis.

On the convergence of supercritical general (C-M-J) branching processes

Abstract: Convergence in probability of Malthus normed supercritical general branching processes (i.e. Crump-Mode-Jagers branching processes) counted with a general characteristic are established, provided the latter satisfies mild regularity conditions. If the Laplace transform of the reproduction point process evaluated in the Malthusian parameter has a finite ‘x log x-moment’ convergence in probability of the empirical age distribution and more generally of the ratio of two differently counted versions of the process also follow.

Radiative heating and cooling with spectrally selective surfaces.

Abstract: Matter continuously exchanges energy with its surroundings. This exchange can be dominated by radiation, conduction, or convection. In this brief review we discuss how proper design of radiative surface properties can be used for heating and cooling purposes. The desired properties can be understood once it is realized that solar and terrestrial radiation take place in different wavelength ranges and that only part of the solar spectrum is useful for vision and for photosynthesis in plants. These facts allow the possibility of tailoring the spectral absorptance, emittance, reflectance, and transmittance of a surface to meet different demands in different wavelength intervals, i.e., to take advantage of spectral selectivity. One example is the selective surface for efficient photothermal conversion of solar energy, which has high absorptance over the solar spectrum but low emittance for the longer wavelengths relevant to thermal reradiation. Below we discuss the pertinent spectral radiative properties of our ambience. These data are then used as background to the subsequent sections treating four examples of spectrally selective surfaces. The first example is the previously mentioned selective surface for converting solar radiation to useful heat. The second example considers surfaces capable of reaching low temperatures by benefiting from the spectral emittance of the clear night sky. The third example concerns two related types of transparent heat mirror. The fourth example, finally, treats radiative cooling of green leaves; this part is included since it gives a nice example of how nature solves a difficult problem in an elegant and efficient way. This example hence provides an interesting background to the other cruder types of artificial selective surfaces. Throughout our discussion we treat the ideal spectral properties, give an illustrative experimental example of how well this goal can be realized, and—where this is possible—show a corresponding theoretical curve indicating to what extent the measured results can be theoretically understood.

Numerical results from large deflection beam and frame problems analysed by means of elliptic integrals

Abstract: Numerical evaluations of elliptic integral solutions of some large deflection beam and frame problems are presented. The values are given in tabular form with up to six significant figures. The numerical technique used for evaluating the elliptic integrals is described.

Determination of micelle aggregation numbers and micelle fluidities from time-resolved fluorescence quenching studies

Abstract: A critical study of the quenching of the fluorescence of tris(2,2′-bipyridyl)ruthenium(II) by 9-methylantracene in sodium dodecyl sulfate micelles at various ionic strength, in microemulsions and in pentanol-swollen micelles is presented. The results are discussed in terms of aggregation numbers and quenching constants obtained from single-photon counting data and steady-state fluorescence measurements. In connection with the analysis of the time-resolved data, an approach to choose correct time zero is outlined. The choice of the time where the excitation profile has its maximum is suggested. Varying the ionic strength in the micelle solutions resulted in an increase of the mean aggregation number from 63 at zero concentration of sodium chloride to 373 at 0.75M of the electrolyte. The first-order rate constant for quenching decreased in these solutions from 2 × 107 to 0.2 × 107 sec−1 in an inverse proportion to the estimated aggregation numbers. The correct values of the aggregation numbers are probably higher than the estimated values, which is shown in computer simulations assuming a distribution of micelle sizes. The necessity of time-resolved experiments to calculate an aggregation number if demonstrated in measurements on a microemulsion (detergent-pentanol-dodecane). The product of the quenching constant, 0.2 × 107 sec−1, and the lifetime, 309 nsec, of the fluorescent molecule is too small to justify the use of steady-state data only. The estimated aggregation number for this solution is 306, a factor of three higher than earlier reported. For pentanol-swollen micelles, data are presented where there is a need for deconvolution, the quenching constant, 6 × 107 sec−1, being too large to ignore the finite width of the excitation pulse. For this system it is better to calculate the aggregation numbers from steady-state data than from single-photon counting data once the magnitude of the quenching constant and the lifetime have been deduced. The fluidity of the micellar aggregates are discussed by defining local second-order rate constant. These were calculated to 0.4 × 109M−1 sec−1 in micelles, to 1.0 × 109M−1 sec−1 in pentanol-swollen micelles, and to 1.5 × 109M−1 sec−1 in microemulsion particles, compared to a measured value of 4.2 × 109M−1 sec−1 in a homogenous ethanol solution.

Infrared optical properties of evaporated alumina films

Abstract: The dielectric function ∊ = ∊1 + i∊2 has been determined for Al2O3 films prepared by electron beam evaporation, in the 5–50-μm wavelength range. The data were extracted from spectrophotometric recordings of transmittance and reflectance by use of a novel technique. Supplementary measurements were made of the refractive index for visible and near-infrared wavelengths and of the dielectric constant at 1 MHz. Kramers-Kronig analysis was employed to check the consistency of our results for ∊1 and ∊2.

An Analysis of some Finite Element Methods for Advection-Diffusion Problems

Abstract: We analyze two ways of improving the performance of the usual Galerkin finite element method when applied to a stationary advection-diffusion problem with small diffusion having a non-smooth solution. The first method consists in postprocessing the usual Galerkin solution by smoothing which increases the accuracy in regions where the exact solution is smooth. The second method is the result of applying the usual Galerkin method to a modified advection-diffusion problem obtained by adding extra diffusion in the streamline direction and compensating by modifying the right hand side. We prove error estimates and give the results of some numerical experiments.

Global Classical Solutions of Nonlinear Wave Equations.

Abstract: is a formally selfadjoint positive elliptic operator and f is a real C2-function with non-negative primitive function. We treat both the homogeneous initialboundary value problem over IR+ x f2 for a smooth open set ~2clR" and the Cauchy problem over IR+ x 1R ~ For the initial-boundary value problem we show that any strong solution of (0.1), known to exist according to Heinz and yon Wahl [4] if 2rn f (J) (u2)=O(lul~-2J) , j = 0 , 1 , [u[~oo,

Superconvergence of a finite element approximation to the solution of a Sobolev equation in a single space variable

Abstract: A standard Galerkin method for a quasilinear equation of Sobolev type using continuous, piecewise-polynomial spaces is presented and analyzed. Optimal order error estimates are established in various norms, and nodal superconvergence is demonstrated. Discretization in time by explicit single-step methods is discussed.

Determination of Optical Constants of Absorbing Materials Using Transmission and Reflection of Thin Films on Partially Metallized Substrates: Analysis of the New (T,R(m)) Technique

Abstract: A new accurate technique for determining the optical constants of thin absorbing films is presented. The configuration used includes a conventional spectrophotometer and a transparent substrate, half of which is covered with an opaque highly reflecting metal layer prior to the deposition of the film to be studied. The normal incidence transmission T of the thin film on the transparent substrate is then combined with a measurement of the near normal reflection R(m) of the same film on the metal covered part of the substrate. The combination (T,R(m)) yields an accurate and experimentally simple technique for determining the optical constants of thin films. A detailed evaluation of the accuracy of the extracted optical constants n and k of the thin film is obtained from numerical computations using realistic assumptions of various experimental uncertainties. Comparison with conventional techniques shows a greatly improved accuracy of the new technique.

Experience with Swedish multifunctional prosthetic hands controlled by pattern recognition of multiple myoelectric signals.

Abstract: Clinical experience with two types of multifunctional prosthetic hand, controlled by pattern recognition of multiple myoelectric signals is reported. The prostheses have been used for between one and five years by five patients. The pattern recognition control system enabled the patients to control six separate movements accurately after a short period of training. One of the tested prostheses, the SVEN-hand, was not reliable enough to allow clinical use outside the laboratory. The ES-hand, a second generation multifunctional prosthesis, has promising features, being self-contained and fast moving. It is concluded that multifunctional prosthetic hands help amputees to avoid tiresome and awkward compensatory movements. Their scope, however, does not extend beyond that of conventional myoelectric prostheses. Their combined movements are cosmetically more appealing than a single three-point grip. In order to gain wider acceptance, multifunctional prosthetic hands must reach a stage of development comparable to conventional myoelectric devices particularly with regard to weight and compactness. A pattern recognition control system is essential to the design.

A tone oriented voice excited vocoder

Abstract: An LPC base-band vocoder is developed. The novel feature concerns the coding of the base-band. A model is set up for the base-band as a set of modulated tones. Algorithms are presented for the extraction of amplitude and phase/frequency of the tones. Implementation aspects as well as simulation results are discussed. Total bit rates in the order of 3,2-4.8 kbits are possible where approximately one half of the bits represents the base-band coding. Experiments have shown the coder to be robust to background noise.

Mobile lidar system for environmental probing

Abstract: A fully mobile remote-sensing system based on the lidar principle is described. With this system, atmospheric probing using Mie scattering, differential absorption, or Raman techniques can be performed yielding information on atmospheric pollutants or general atmospheric parameters. The system incorporates a powerful Nd:YAG laser pumping a dye laser and is equipped with a fixed Newtonian telescope used in conjunction with a flat steering mirror. The lidar signals are electrically recorded using a fast-transient digitizer and are processed by a minicomputer, which also controls the laser, the chosen measuring direction, and the output media. Examples of measurements on atmospheric NO(2) and SO(2) are given.

Assessment of renal parenchymal thickness in normal children.

Abstract: The normal range of variation of the parenchymal thickness in well defined parts of the kidney was determined in children (0 to 15 years). The values were correlated with the distance between the superior surface of L1 and the inferior surface of L3. The ratio between the parenchymal thickness on the right side and the left side was also determined. The results are compiled in two nomograms.

Programming in Constructive Set Theory: Some examples

Abstract: @y e List(A) (Perm(x,y) & Sorted(y)) which is read “for all lists x, there is a sorted permutation y of x”. We can prove that this proposition is true, using the rules of the language to construct a program for the task. If the proposition were not true, it would be impossible to find a program for it and we would have had an impossible task. The types of Constructive Set Theory can be seen as a specification language for the programs, but of course there is only one language, avoiding the complexity of mixing a programming language with a logical language.The similarity (or rather: identity) between a mathematical proof of a given proposition and a program for a given task suggests that programming should be similar to the mathematicians activity of finding proofs. We have illustrate d this with an example of how a proof of the Euclidean division theorem yields a program to compute the quotient and the remainder between two natural numbers.The paper contains a description of the language. Since all programs in the language terminate, the proof rules and the semantics are simple. We give some examples of programming with lists and reasoning about the programs. We also define the Ackermann function.

Photoacoustic study of layered samples

Abstract: The application of photoacoustic spectroscopy to the spectroscopy of layered samples with thicknesses of tens of microns is demonstrated both theoretically and experimentally. Theoretical expressions for the signal damping and phase delay are developed in terms of the thermal diffusion length, which is determined by the chopper frequency and the thermal properties of the sample. The theory is supported by measurements of both amplitude and phase on a color photographic film containing three light absorbing layers. The application of the method is further illustrated by a simple study of fading leaves.

Flow Properties of Nematic 8CB: An Example of Diverging and Vanishing α3

Abstract: The temperature dependence of the Leslie viscosities α2 and α3 has been measured in the nematic liquid crystal 8CB (octyl-cyano-biphenyl), using a trosional shear flow apparatus. In most nematics studied so far α3 is found to be everywhere negative, but if the nematic has an adjacent semectic. A phase at lower temperatures, it can be expected that α3 changes sign and becomes positive in the vicinity of TNA as a precursory phenomenon of the transition. In fact this influence is dominating the major part of the nematic phase. We have found that for 8CB α > 0 in the whole nematic range (33.5°C-40.1°C), except for a small temperature interval of one degree just below the clearing point. This leads to intrinsic flow instabilities, but the flow can be stabilized by applied electric fields. The results further contain the first measurement of a diverging α3 in a flow experiment.