OPUS 4 | Physik

A Detailed Treatment of the Measurement of Transport Coefficients in Transient Grating Experiments (2007)

Marianne Hartung

This thesis treats the measurement of transport coefficients in transient grating experiments and is organized into 3 parts. Part 1 provides a brief review of the thermodynamic-phenomenological theory relevant for a correct description of the Soret effect. It comprises the formulation of the first law in open systems, the calculation of the entropy production, and the derivation of the phenomenological equations. This part is based on the books by de Groot and Mazur and by Haase and contains also some own results. We have explicitely derived a relation between reversible work and dissipation function, if heat and mass are exchanged reversibly and irreversibly between the two homogenous phases of a non-isothermal heterogenous system. Moreover we have discussed in detail, whether Onsager coefficients are invariant against shifts of enthalpy or entropy zero. Furthermore some comments on recent literature work have been made, since thermodynamic principles are not always correctly incorporated. In parts 2 and 3 we have treated the measurement of heat, mass and thermal diffusion in transient grating experiments. In part 2 we have presented a two-dimensional model to account for the role of heat conducting walls in the measurement of heat transport and Soret effect driven mass transport in transient holographic grating experiments. Heat diffusion into the walls leads to non-exponential decay of the temperature grating. Under certain experimental conditions it can be approximated by an exponential function and assigned an apparent thermal diffusivity D_{th,app} <D_{th,s}, where D_{th,s} is the true thermal diffusivity of the sample. The ratio D_{th,app}/D_{th,s} depends on only three dimensionless parameters, d/l_s, k_s/k_w, and D_{th,s}/D_{th,w}. d is the grating period, l_s the sample thickness, k_s and k_w the thermal conductivities of sample and wall, respectively, and D_{th,w} the thermal diffusivity of the wall. If at least two measurements are performed at different d/l_s, both D_{th,s} and k_s can be determined. Instead of costly solving partial differential equations, the unknown parameters can be obtained by finding the zero of an analytic function. For thin samples and large grating periods, heat conduction into the walls plays a predominant role and consequently the concentration grating in binary mixtures is no longer one-dimensional. Nevertheless, the normalized heterodyne diffraction efficiency of the concentration grating remains unaffected and the true mass and thermal diffusion coefficient and the correct Soret coefficient are still obtained from a simple one-dimensional model. All theoretical predictions have been tested by experiments on pure and binary liquids over a wide range of grating periods and sample thicknesses. Excellent agreement has been found in all cases. A new transient grating technique for the measurement of heat, mass and thermal diffusion in liquids has been introduced in part 3. Similar to holographic grating experiments, a temperature grating is created in the sample. Thermal expansion transforms the temperature into a refractive-index grating, which is read by diffraction of a readout laser beam. In a multicomponent mixture an additional concentration grating is formed by thermal diffusion driven by the temperature gradients of the temperature grating. Differently to laser induced dynamic grating experiments we use Joule heating instead of optical heating. For that purpose we have built cuvettes which have a grating of transparent conducting strips on the inner side of one of their windows. If heated by an electric current a temperature grating will build up in the sample. Both, the heat equation and the extended diffusion equation, have been solved in two dimensions to allow for quantitative data analysis. Our apparatus and method of analysis have been validated by measurements of heat, mass and thermal diffusion in pure and binary liquids. Heat diffusion can be correctly determined as was shown for pure toluene, pure dodecane and the symmetric mixture of isobutylbenzene-dodecane. Mass and thermal diffusion was studied in the three symmetric mixtures of dodecane, isobutylbenzene and tetralin. The obtained diffusion and Soret coefficients agree with the literature values within the experimental errors. Uncompensated transient heating effects limit the resolution of the experimental technique.

A Study of Magnetic Helicity in Decaying and Forced 3D-MHD Turbulence (2009)

Shiva Kunar Malapaka

This thesis presents a numerical study of a property of three dimensional magnetohydrodynamic (3D-MHD) turbulence, namely, inverse cascade (spectral transport from small scales to large scales) of magnetic helicity. Magnetic helicity is defined as the volume integral of the dot product of the magnetic field and the magnetic vector potential. It characterizes the linkage and twists of the magnetic field lines. The inverse cascade is believed to be one of the causes of large-scale magnetic structure formation in the universe. This numerical studies is aimed at understanding how the inverse cascade of magnetic helicity effects other quantities of the turbulent flow. Two setups, namely, forced turbulence and decaying turbulence are studied. In the forced case, the numerical simulation setup consists of an initial energy distribution and a forcing localized in the small scales. The decaying setup consists of an initial energy distribution in the intermediate scales, which is allowed to decay naturally. The analysis of the results shows that several quantities in the turbulent flow, show self-similar behavior in their spectra, giving rise to power laws, which were hitherto unknown. Some of the quantities which are known to show power law behaviors exhibit different values to the power law exponents. These power law behaviors are analyzed together with the dimensional analysis of the eddy damped quasi normal Markovian (EDQNM) approximation equations, to attain a new relation which explains the evolution of large-scale magnetic structures in both the turbulent setups. The results are substantiated by the analysis of structure functions, probability density functions and correlation functions. Visualization of real space structures is also carried out. A mechanism to achieve large-scale magnetic structures from random small-scale magnetic fluctuations involving both the forced and decaying turbulences, is suggested.

Accurate charge densities of amino acids and peptides by the Maximum Entropy Method (2008)

Jeanette Netzel

In the present thesis accurate charge densities of several amino acids and peptides have been reconstructed by the Maximum Entropy Method (MEM) to study chemical bonds. The MEM model-independently determines the most probable electron density, which simultaneously maximizes the information entropy and fits the X-ray diffraction data. The quality of the MEM densities is enhanced by several extensions to the MEM such as the use of a non-uniform prior-density, the employment of the method of prior-derived F-constraint, static weighting and the choice of the optimal stopping criterion for the MEM iterations. The latter is achieved by inspection of difference Fourier maps and dynamic deformation maps. The reconstructed electron densities have been analyzed according to Baders Atoms in Molecules (AIM) theory to derive information about chemical bonds, in particular covalent bonds and hydrogen bonds. Local maxima of the densities, their associated atomic basins and charges, bond critical points (BCPs) and their densities and second derivatives, i.e. the eigenvalues and the Laplacians, have been determined according to the AIM theory. For all studied compounds it is shown that, providing the employment of the extensions to the MEM, the densities obtained from the MEM exhibit similar properties as those obtained from the multipole method. However, it is demonstrated that the features of hydrogen bonds are described more convincingly by the MEM than by the multipole method. By comparison of densities at BCPs from the MEM with the corresponding values from multipole refinement and from quantum chemical calculations it is shown that each method produces similar densities at BCPs. For all studied compounds it is demonstrated that for MEM densities, densities at BCPs of hydrogen bonds possess a larger magnitude than corresponding values from densities by the multipole method, while the opposite is true for covalent bonds. The values of the Laplacians at BCPs, especially of C–O bonds, show larger discrepancies between values from the MEM and from multipole refinement. These differences are caused by thermal motion which is present in dynamic MEM densities, but absent in static densities produced by the multipole method. The results of the extensive study of electron densities of amino acids and tripeptides show that densities and energetic properties at BCPs of covalent bonds and hydrogen bonds depend exponentially on their bond lengths. The functions of the dependencies of the densities on the bond lengths differ from the corresponding functions fitted to values from the multipole method. It is demonstrated that the ratio of the potential and kinetic energy densities at BCPs of hydrogen bonds reveals the possibility to classify them according to the distance between hydrogen atom and acceptor atom. Short hydrogen bonds possess covalent character, hydrogen bonds with intermediate distance have a mixed covalent-ionic character and long hydrogen bonds are ionic. This classification coincides with the usual classification of strong, intermediate and weak hydrogen bonds as proposed in the literature. The studied hydrogen bonds are classified as possessing mainly a mixed covalent-ionic character. However, for covalent bonds a classification according to the bond lengths does not suffice to characterize them. The results of the amino acids and tripeptides indicate that the prior density contributes a large part to the densities at BCPs. However, for the Laplacians and the energy densities at BCPs the differences between MEM and prior densities are larger than for the densities at BCPs. Densities at BCPs of hydrogen bonds from MEM densities show a different trend in their dependence on the bond distance than corresponding trends from prior densities. Thus, it is demonstrated that the analysis of the true density instead of the prior or procrystal density is recommended in order to extract information about chemical bonding. It is concluded from the results of the Accurate Charge Density studies reported in the present thesis, that the MEM allows a good characterization of chemical bonds and describes the electron density of hydrogen bonds more realistic than the multipole model. Chapter 3 of the present thesis has been published in Acta Crystallogr. B, 63, 285–295 (2007) and is reproduced with permission of the International Union of Crystallography (http://journals.iucr.org). Chapter 4 of the present thesis has been published in CrystEngComm, 10, 335–343 (2008) and is reproduced with permission of The Royal Society of Chemistry (RSC) (http://www.rsc.org).

Active and Passive Transport at Interfaces (2011)

Saeedeh Aliaskarisohi

In this thesis we studied different forms of transport at interfaces. Four different interfacial transport mechanisms have been investigated. In each of them one physical aspect of active and passive transport is discussed. The four systems are arranged and discussed in four separate chapters. In chapter 3 and 4 we study the effect of static or hydrodynamic interactions on the cross over from individual diffusion towards collective diffusion. In chapter 3 the diffusion of circular domains on a giant unilamellar vesicle is measured. By tracking the motion of hydrodynamic interacting domains on a curved membrane we determined whether it is possible to extract rheological properties of the bilayer membrane. A similar two dimensional system interacting via static dipole interactions is studied in chapter 4. A mixture of paramagnetic and nonmagnetic colloidal particles immersed into a diluted ferrofluid is self assembled into colloidal flowers. In this experiment the effect of static interactions on the modes of diffusion of the petals of the colloidal flower is investigated in a one dimensional system. The results are compared with the single file diffusion of a hard core interacting one dimensional system. In chapter 5, the effect of actively directing particles with fluctuating active forces in a symmetry broken environment is studied. We address the question how to competing symmetry breaking effects decide on the direction of motion. The system consists of paramagnetic colloidal particles placed into an aqueous solution above the liquid-solid interface of a magnetic garnet film. An external modulated field supplies the fluctuations and the symmetry is broken by tilting the external field with respect to the magnetic film and/or by a magnetic symmetry broken pattern of the magnetic film. The direction of motion of the paramagnetic colloids is measured and we give a theoretical explanation of why which symmetry breaking wins. The fluidization of a two dimensional solid to a two dimensional liquid via the yielding of the monolayer is studied in chapter 6. The monolayer is locally yielded with thermo capillary interactions by focusing a laser onto it. We investigate the yielding as a function of the chemical nature of the monolayer and determine the thermodynamic requirements necessary for thermo capillary yielding.

Bogoliubov Excitations of Inhomogeneous Bose-Einstein Condensates (2010)

Christopher Gaul

In this thesis, different aspects of interacting ultracold bosons in presence of inhomogeneous external potentials are studied. The first part deals with repulsively interacting Bose-Einstein condensates in speckle disorder potentials. In the Bogoliubov approach, the many-body problem is split into the Gross-Pitaevskii condensate (mean-field) and the Bogoliubov excitations, which are bosonic quasiparticles. The disorder potential causes an imprint in the condensate, which makes the Hamiltonian for the Bogoliubov excitations inhomogeneous. The inhomogeneous Bogoliubov Hamiltonian is the starting point for a diagrammatic perturbation theory that leads to the renormalized Bogoliubov dispersion relation. From this effective dispersion relation, physical quantities are derived, e.g. the mean free path and disorder corrections to the speed of sound and the density of states. The analytical results are supported by a numerical integration of the Gross-Pitaevskii equation and by an exact diagonalization of the disordered Bogoliubov problem. In the second part, Bloch oscillations of Bose-Einstein condensates in presence of time-dependent interactions are considered. In general, the interaction leads to dephasing and destroys the Bloch oscillation. Feshbach resonances allow the atom-atom interaction to be manipulated as function of time. In particular, modulations around zero are considered. Different modulations lead to very different behavior: either the wave packet evolves periodically with time or it decays rapidly. The former is explained by a periodic time-reversal argument. The decay in the other cases can be described by a dynamical instability with respect to small perturbations, which are similar to the Bogoliubov excitations in the first part.

Characterization of Oligonucleotide Microarray Hybridization: Microarray Fabrication by Light-Directed in situ Synthesis – Development of an Automated DNA Microarray Synthesizer, Characterization of Single Base Mismatch Discrimination and the Position-Dependent Influence of Point Defects on Oligonucleotide Duplex Binding Affinities (2008)

Thomas Naiser

The present thesis focuses on nucleic acid hybridization between free-floating target sequences and complementary end-tethered oligonucleotide probes on the surface of DNA microarrays. Hybridization experiments were performed on oligonucleotide microarrays (DNA Chips) which were fabricated with an automated synthesis apparatus (developed in the framework of the present thesis). The working principle of the microarray synthesizer is based on a photochemically controlled in situ synthesis process. By means of the combinatorial approach up to 25000 different (arbitrary) probe sequences can be fabricated in parallel - starting from nucleotide building blocks (NPPOC-phosphoramidites) - directly on the surface of the microarray. Great flexibility with regard to the choice of probe sequences is achieved by use of virtual photomasks on the basis of a spatial light modulator (Digital Micromirror Device, DMD, Texas Instruments Inc.). A microscope projection photolithography system is employed to project the virtual masks (i.e. the photomask images shown on the DMD) onto the surface of the microarray substrate. Spatially controlled photodeprotection of photolabile NPPOC protective groups (followed by coupling of a further nucleotide building block) enables massively parallel synthesis of DNA probe sequences. In the automated synthesis process microarrays are routinely fabricated over night. Comparable in situ synthesis systems are currently operated only at very few institutions around the world. We first report the application of phosphorus dendrimer substrates in the in situ synthesis of DNA microarrays. With the phosphorus dendrimer functionalization we obtained superior results in regard to sensitivity, surface homogeneity, signal/background-ratio and reusability of the microarrays. We performed microarray hybridization experiments to investigate the impact of single base defects (deliberately introduced single base mismatches and single base bulges) on the binding affinity of oligonucleotide duplexes. This is particularly interesting with regard to genotyping microarrays which are increasingly employed as a molecular diagnostics tool for the detection of single nucleotide polymorphisms (SNPs). In a number of experiments we investigated the large influence of the single-defect position on duplex binding affinity. The origin of this positional dependence - which is apparently not in agreement with the (two-state) nearest-neighbor model - had not been identified so far. We discovered that the influence of the defect position is not restricted to single base mismatches but can also be observed for single base bulge defects. On the basis of the double-ended zipper model (assuming fluctuating end-domain-opening of the oligonucleotide duplex) we could reproduce the experimentally observed positional influence. Moreover, our theoretical investigations on the zipper model indicate a significant positional influence in regard to the contributions of the individual Watson-Crick nearest-neighbor pairs to the Gibbs free energy of oligonucleotide duplex formation. The present work provides for the first time a theoretical approach for the positional-dependent nearest-neighbor model (PDNN) of Zhang et al.. In the in situ synthesis process of DNA microarrays random point-mutations are introduced into the microarray probe sequences. We have shown - experimentally and by means of a numerical model - that synthesis-related defects significantly affect microarray hybridization characteristics. With regard to single base mismatch discrimination, we discovered significant differences between DNA/DNA- and RNA/DNA hybridization: experimental results indicate an improved discrimination of purine-purine mismatch base pairs in RNA/DNA-duplexes. For the experimentally observed, unexpectedly high stability of Group II single bulges we provide an explanatory approach on the basis of the zipper model. The selection of appropriate (specific and sensitive) probe sequences is of crucial importance for successful application of DNA microarray technology. Our experimental results confirm previous results which show that only a small fraction (in piecewise sections about 20-30%) of a long cRNA target sequence is available for hybridization with the complementary microarray probes. Reduced binding affinities are assumed to originate from the influence of target secondary structure. Using software tools for antisense oligonucleotide design (accounting for target accessibility) we were able to predict efficient microarray probes. We discovered evidence that mechanically stable secondary structures (e.g. double-helical sections) interfere with the microarray surface (sterical hindrance) and thus result in reduced microarray binding affinities.

Characterization of phase transitions by the analysis of crystal structures (2009)

Joachim Angelkort

In this thesis results of the investigations of the mechanisms of solid-solid phase transitions are reported on basis of the exemplary characterization of the phase transition of the metalorganic compound Eu(SC36H49)2 and of the inorganic transition-metal compounds TiI3 and CrOCl. The phase transitions were surveyed temperature dependently by the performance of single-crystal X-ray diffraction experiments and measurements of the magnetic susceptibility. The X-ray diffraction experiments were carried out as data collections of integrated intensities of reflections and as measurements of profiles on selected reflections in so-called omega-2theta maps. The data sets of the integrated intensities were used to determine the crystal structures at different temperatures. By the comparison of the high- and the low-temperature crystal structures the mechanisms of the phase transitions of the compounds Eu(SC36H49)2 and TiI3 were determined. Furthermore the transition temperatures of all three compounds were determined by temperature-dependent measurements of intensities of superstructure reflections. From the omega-2theta maps the monoclinic lattice distortion of the low-temperature phase of CrOCl was determined.

Coherent Transport of Matter Waves in Disordered Optical Potentials (2007)

Robert Kuhn

The development of modern techniques for the cooling and the manipulation of atoms in recent years, and the possibility to create Bose-Einstein condensates and degenerate Fermi gases and to load them into regular optical lattices or disordered optical potentials, has evoked new interest for the disorder-induced localization of ultra-cold atoms. This work studies the transport properties of matter waves in disordered optical potentials, which are also known as speckle potentials. The effect of correlated disorder on localization is first studied numerically in the framework of the Anderson model. The relevant transport parameters in the configuration average over many different realizations of the speckle potential are then determined analytically, using self-consistent diagrammatic perturbation techniques. This allows to make predictions for a possible experimental observation of coherent transport phenomena for cold atoms in speckle potentials. Of particular importance are the spatial correlations of the speckle fluctuations, which are responsible for the anisotropic character of the single scattering processes in the effective medium. Coherent multiple scattering leads to quantum interference effects, which entail a renormalization of the diffusion constant as compared to the classical description. This so-called weak localization of matter waves is studied as the underlying mechanism for the disorder-driven transition to the Anderson-localization regime, explicitly taking into account the correlations of the speckle fluctuations.

Convection and Magnetic Field Generation in Rotating Spherical Fluid Shells (2004)

Radostin D. Simitev

The dissertation reports results from numerical and analytical studies of convection and dynamo action in rotating fluid spheres and spherical shells. This research is motivated by the geophysical problem of the origin and properties of the Earth's magnetism. Extensive numerical simulations are performed in order to advance the understanding of the basic physical components and mechanisms believed to be responsible for the generation and the variations in time of the main geomagnetic field. Questions such as linear onset and nonlinear finite-amplitude properties of rotating convection, generation and equilibration of magnetic fields in electrically conducting fluids, nonlinear feedback effects of the generated magnetic fields on convection, spatio-temporal structures of magnetic and velocity fields, oscillations and coherent processes in turbulent regimes and other questions are studied in dependence on all basic parameters of the problem, as well as for various choices of the magnetic, thermal and velocity boundary conditions and for some secondary assumptions such as a finitely-conducting inner core and various basic temperature profiles. Because of the lack of knowledge of the properties of the Earth's core and the uncertain details of the processes that take place there, this research is necessary in order to provide the tools for extrapolation to realistic models of the geodynamo. Of particular interest are various types of oscillations of dipolar fields. In contrast to quadrupolar and hemispherical dynamos dipolar dynamos have been originally considered to be non-oscillatory. But the six different types of dipolar oscillations, among which is the ``invisible'' one, reported in this dissertation alter this view. Generation of magnetic fields by convection shows a strong dependence on the Prandtl number P of the fluid. But this fact has received little attention in the past. Convection-driven dynamo action at Prandtl numbers larger than unity is studied with the goal to test the validity of the magnetostrophic approximation. The latter is found to be poorly satisfied for P < 300. Dynamos in this regime require magnetic Prandtl numbers Pm which increase with P. The same trend continues to hold for values of P less then unity and this regime thus seems to be best suited to reach the goal of minimal values of Pm. For Pm=P=0.1 a hemispherical dynamo is obtained in the case of a rotation parameter tau=10**5. A further reduction of Pm leads to a decay of magnetic field irrespective of the Rayleigh numbers used. Apart from numerical simulations and parameter studies of basic physical mechanisms, the dissertation includes an analytical study of inertial convection in rotating spheres in the limit of small Prandtl numbers and large rotation rates. Explicit expressions for the dependence of the Rayleigh number on the azimuthal wavenumber and on the product of P tau are derived and new results for the case of a nearly thermally insulating boundary are obtained. Limited comparisons with actually observed features of the geomagnetic field are also presented. An example are the torsional Alfven waves found in the numerical simulations of this dissertation. They are geophysically relevant as a possible cause for the observed secular variation impulses of the Earth's magnetic field. Reversals of the magnetic field polarity have also been observed in our simulations. Dynamo intermittency and interaction between dipolar and quadrupolar components are preconditions for aperiodic dipolar reversals similar to those of the Earth's main field. However, the opportunities for quantitative comparisons with geophysical observations are rather limited by the complexity of the self-consistent dynamo problem and by the computational restrictions of our numerical simulations.

Correlated electron dynamics and memory in time-dependent density functional theory (2009)

Mark Thiele

Korrelierte Elektronendynamik ist für nichtlineare und lineare Prozesse in Atomen und Molekülen von großer Bedeutung. Dies betrifft insbesondere die Wechselwirkung mit starken Feldern und die Photoabsorptionsspektren. Die theoretische Beschreibung der Korrelationen gestaltet sich jedoch im Allgemeinen schwierig. Außerdem erfordern Anwendungen im Bereich starker Felder einen nicht-perturbativen Zugang. Im Prinzip liefert die zeitabhängige Vielteilchen-Schrödingergleichung (TDSE) die exakte Lösung für beliebige Prozesse. Allerdings wird der numerische Rechenaufwand bereits für kleine Systeme in starken Feldern zu groß. Eine Alternative bietet hier die zeitabhängige Dichtefunktionaltheorie (TDDFT), die sowohl die Berücksichtigung von Korrelationseffekten als auch einen nichtperturbativen Zugang bei starken Feldern erlaubt. Bei der TDDFT handelt es sich um eine exakte Umformulierung der TDSE, bei der das Problem vieler wechselwirkender Elektronen auf das Kohn-Sham-System nicht-wechselwirkender Teilchen abgebildet wird, das die exakte Elektronendichte reproduziert. Da dieses Hilfssystem auf Einteilchengleichungen beruht, können numerische Berechnungen wesentlich effizienter durchgeführt werden als auf Basis der TDSE. Die gesamten nicht-klassischen Vielteilcheneffekte werden im Kohn-Sham-System über das Austausch-Korrelations-Potential berücksichtigt, das jedoch im Allgemeinen unbekannt ist und daher angenähert werden muss. Dieses Vorgehen stellt einen wohldefinierten Zugang zur Beschreibung des Vielteilchen-Problems dar. Ein wichtiger Aspekt dieser notwendigen Näherung betrifft die Berücksichtigung sogenannter Gedächtniseffekte im Austausch-Korrelations-Potential. Letzteres ist nämlich im Allgemeinen ein kompliziertes Funktional, das nichtlokal im Raum von der gesamten Vorgeschichte der Elektronendichte abhängt. Werden die Nichtlokalität in der Zeit und damit die Gedächtniseffekte vernachlässigt, spricht man von der adiabatischen Näherung. Diese wird in der Regel mit einer Näherung der räumlichen Nichtlokalität kombiniert. Durch diese Verknüpfung wird die Interpretation von TDDFT-Ergebnissen häufig erschwert. Insbesondere bei der Beschreibung starker äußerer Felder treten im Rahmen der TDDFT Probleme auf, deren Beziehung zu den Gedächtniseffekten bisher unklar ist. Aber auch im Falle der linearen Anregungsspektren spielen die Gedächtniseffekte eine wichtige Rolle. Ziel dieser Arbeit ist es daher, den Zusammenhang zwischen den Gedächtniseffekten und der korrelierten Elektronendynamik in starken und schwachen Feldern zu untersuchen. Zu diesem Zweck werden eindimensionale Zwei-Elektronen-Singulett-Systeme untersucht, da hier sowohl die Lösung der TDSE als exakte Referenz als auch die Berechnung der relevanten TDDFT-Größen möglich ist. Gleichzeitig schließen diese Systeme das eindimensionale Helium-Atom-Modell ein, das ein etabliertes System zur Untersuchung der charakteristischen Effekte korrelierter Elektronendynamik in äußeren Feldern darstellt. Bei diesen Untersuchungen hat sich gezeigt, dass Gedächtniseffekte für Starkfeld-Prozesse nur eine untergeordnete Rolle spielen. Hier ist vielmehr die korrekte Näherung der räumlichen Nichtlokalität entscheidend. Bei den Photoabsorptionsspektren hingegen führt die Vernachlässigung der Gedächtniseffekte zu qualitativen und quantitativen Fehlern. Es zeigt sich, dass diese Probleme mit dem Auftreten von Doppelanregungen zusammenhängen. Um ein besseres Verständnis zu entwickeln, unter welchen Umständen Gedächtniseffekte wichtig werden, hat sich die sogenannte Quanten-Hydrodynamik als äußerst nützlich erwiesen. Hierbei handelt es sich um eine weitere Darstellungsmöglichkeit des quantenmechanischen Vielteilchen-Problems, die auf hydrodynamischen Größen wie Dichte und Geschwindigkeit basiert. Man findet, dass Gedächtniseffekte immer dann wichtig werden, wenn das Geschwindigkeitsfeld starke Gradienten ausbildet und Dissipationseffekte auftreten. Daraus ergeben sich interessante Schlussfolgerungen für die Interpretation der Elektronen als viskoelastische Flüssigkeit. Diese und weitere Ergebnisse sind in vier Publikationen enthalten, die sich am Ende dieser Arbeit finden.

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