Universität Stuttgart
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Item Open Access Ambient pressure oxidation of Ag(111) surfaces : an in-situ X-ray study(2008) Reicho, Alexander; Dosch, Helmut (Prof. Dr.)The oxidation of metals plays an outstanding role in everyday life. Typical phenomena are the formation of rust on steel or oxide scales on copper, showing up as a green patina. The formation of metal oxides is not always an unwanted process. The functionality of many materials is directly related to their controlled oxidation. The most prominent examples are passivating oxide layers on stainless steel. Relevant for this thesis are industrially applied heterogeneous catalytic reactions for the synthesis of many chemical products, where gaseous reactants are in contact with the solid surface of the catalyst. Oxidation reactions are very important in this context, leading to a big need of understanding of these processes in research and development. Thereby, the active oxygen species on the surface and selectivity and poisoning of the catalyst have to be studied on an atomic scale. The high temperature and high pressure oxidation of the 4d transition metals Ru, Rh, Pd and Ag is a matter of particular interest, because these metals are widely used as oxidation catalysts. On Ruthenium one observes the formation of RuO2(110) bulk oxide islands at elevated temperatures and oxygen pressure. In the case of the Pd(100) and Rh(111) surface oxidation can lead to the formation of so-called surface oxides. These oxides are structurally related to the bulk oxide of the respective element. Furthermore, surface oxides are ultra thin oxides containing one metallic layer surrounded by two oxygen layers, giving rise to an oxygen-metal-oxygen sequence perpendicular to the surface plane. A future vision is to get a direct microscopic control of the emerging surface structures and ultimately of the real-time oxidation/reduction dynamics allowing one to tailor such catalytic reactions to better performance. A necessary prerequisite to the microscopic control is the full atomistic understanding of the surface structures which form at high temperature and at high oxygen pressures. Silver plays a unique role in heterogeneous catalysis. Supported Ag catalysts are used for the selective oxidation ('epoxidation') of ethylene and for the partial oxidation of methanol to formaldehyde. Ethylene oxide and its derivates are basic chemicals for industry, used in a many technologies with a world-wide production of more than 10 million tons as in medicine for disinfection, sterilization, or fumigation, or in transport and energy technologies for engine antifreeze and heat transfer. Because of its ability to kill most bacteria, formaldehyde is extensively used as disinfectant and as preservative in vaccinations. Therefore, the optimisation of these two Ag-supported catalytic reactions is of paramount importance. Current strategies employed in the industrial process to enhance selectivity include the empirical use of inhibitors (Cl) and promoters (Cs), however, on the way to a knowledge-based control of these reactions one has first to understand the surface structure of oxidized silver under relevant conditions in full detail. The formation of extended Ag(111) facets is observed on polycrystalline silver during the above industrial catalytic oxidation reactions, in turn fundamental research (experiment and theory) has been devoted to the detailed understanding of oxidation of this surface. The formation of an oxygen induced p(4x4) reconstruction on the Ag(111) surface is known since the early 70s. A surface oxide trilayer model, based on a three-layer slab of Ag2O(111), was proposed. Accordingly, the Ag(111) surface seemed to show a similar behaviour like Pd and Rh, being neighbours in the periodic table. Further theoretical calculations predicted the stability of this reconstruction under industrially relevant conditions. Nevertheless, several questions remained unsolved: the stability of the p(4x4) reconstruction under industrially relevant conditions was not checked experimentally, the structural model of the p(4x4) structure was not proven by a crystallographic method and previously unknown structures might play an important role for the catalytic activity of Ag(111) facets. Our experimental approach is based on the nowadays routinely available highly brilliant x-ray radiation produced by third generation synchrotron light sources. This radiation is used by us in three surface sensitive x-ray techniques. In-situ surface x-ray diffraction (SXRD) allows the identification and determination of structural models of surface reconstructions under industrially relevant conditions. This technique is combined with high resolution core level spectroscopy (HRCLS) and normal incidence x-ray standing wave absorption (NIXSW), giving insight into the local binding geometry of the oxygen and silver atoms.Item Open Access Resonant Raman scattering from superconducting single crystals of (BEDT-TTF)2I3(1993) Pokhodnia, Konstantin I.; Graja, Andrzej; Weger, Meir; Schweitzer, DieterA study of low energetic resonant Raman scattering of (BEDT-TTF)2I3 superconducors was performed. The vanishing of phonon bands accompanied by a decrease of the electronic background was observed below Tc. We propose a theoretical explanation for this novel effect in terms of the Balseiro-Falicov model of phonon-superconducting amplitude mode interaction.Item Open Access Colloidal monolayers on quasiperiodic laser fields(2010) Mikhael, Jules; Bechinger, Clemens (Prof. Dr.)Quasicrystals are somewhat paradoxical structures which exhibit many amazing properties distinguishing them from ordinary crystals. Although the atoms are not localized at periodic positions, quasicrystals posses perfect long-range order. Until the early 1980s it was unanimously established that ordered matter is always periodic. Accordingly, the rotational symmetry in real space was thought to be limited to n=2,3,4 and 6. However more than a hundred complex metal alloys, for instance the discretely diffracting icosahedral AlPdMn or decagonal AlNiCo, have defied these crystallographic rules and self-organized into quasicrystals. Although the majority of the identified quasicrystals are complex metal alloys synthesized in the laboratory, recent experimental results proved that quasiperiodic order is not limited to metals. Matter also organizes itself aperiodically at larger length scales where thermal fluctuations play an important role. Recent experiments have shown that quasiperiodic order is also oberved in soft matter systems, such as micellars, polymers, and binary nanoparticles. Quasicrystals show many interesting properties which are quite different from that of periodic crystals. Accordingly, they are considered as materials with high technological potential e.g. as surface coatings, thermal barriers, catalysts or photonic materials. Quasicrystalline structures have been theoretically predicted also in systems with a single type of particles. Nevertheless, experimentally their spontaneous formation has been only observed in binary, ternary or even more complex alloys. Accordingly, their surfaces exhibit a high degree of structural and chemical complexity and show intriguing properties. In order to understand the origin of those characteristics it would be helpful to disentangle structural and chemical aspects which can be achieved by growing single-element monolayers to quasicrystalline surfaces. Apart from understanding how quasicrystalline properties can be transferred to such monolayers, this approach might allow fabrication of materials with novel properties. First heteroepitatic growth experiments on decagonal and icosahedral surfaces indeed demonstrate the formation of Pb, Bi and Sb monolayers with a high degree of quasicrystalline order as determined by low-energy electron diffraction and elastic helium atom scattering experiments. Compared to reciprocal space studies, only recently atomically resolved scanning tunneling microscopy investigations of the adsorbate morphology became possible. Even then, however, it is difficult to relate the structure of the adsorbate to that of the underlying substrate. In that respect, the study of the phase behaviour of colloidal particles interacting with quasiperiodic laser fields can throw new light on fundamental problems of broad interest in the physics of quasicrystals and in condensed matter physics. In fact colloidal systems are meanwhile established as excellent model for atomic systems and colloidal physics have demonstrated that such systems can give answers to many basic physics questions. Depending on the pair-interaction and the concentration, colloidal systems show analogues of all the states of atomic systems: gas, liquid and solid states. The mesoscopic size (nm-µm), the time scales (ms-s) and the tunability of the pair interaction in colloidal systems make them a convenient model system for experimental and theoretical studies. As a consequence, real space analysis by means of video microscopy allows tracking the trajectories of the individual particles and makes the time evolution of the system accessible in detail. Such information is inaccessible in systems investigated by diffraction experiments, as the scattering information is available only averaged over the scattering area. Because in a colloidal system there is direct access to real space information, the strength and nature of the different interactions, the origins of the complex phase behavior could be in different examples identified. In conclusion, the study of the rich phase behavior of colloidal suspensions provides ideal conditions for experimental and theoretical studies. In this Thesis, we report on a real-space investigation of the phase behaviour of charged colloidal monolayers interacting with quasicrystalline decagonal or tetradecagonal substrates created by interfering five or seven laser beams. Different starting configurations, such as dense fluid and triangular crystals with different densities, are prepared. At low intensities and high particle densities, the electrostatic colloidal repulsion dominates over the colloid-substrate interaction and the crystalline structure remains mainly intact. As expected, at very high intensities the colloid-substrate interaction dominates and a quasiperiodic ordering is observed. Interestingly, at intermediate intensities we observe the alignment of crystalline domains along the 5 directions of the quasicrystalline substrate. This is in agreement with observations of Xenon atoms adsorbed on the ten-fold decagonal Al-Ni-Co surface and numerical simulations of weakly adsorbed atomic systems. Intermediate phases are observed for colloid-substrate interactions strong enough to produce defects in the crystal. These defects adapt the form of rows of quadratic tiles. Surprisingly, for specific particle densities (at which the colloid-substrate interaction is minimized) we identify a novel pseudomorphic ordering. This intermediate phase which exhibits likewise crystalline and quasicrystalline structural properties can be described by an Archimedean-like tiling consisting of alternating rows of quadratic and triangular tiles. The calculated diffraction pattern of this phase is in agreement with recent observations of copper adsorbed on icosahedral AlPdMn surfaces. Interestingly, we also observe the formation of the same phase on tetradecagonal substrates also at densities for which the potential energy of the colloidal system is minimized. Although the structure can also be described by rows of triangles and rows of squares, a closer analysis reveals substantial differences. Here, large domains with almost periodic ordering are found. We show that this behavior is closely related to the low density of highly symmetric local motifs in the substrate potential. In the second part of this Thesis the conditions under which quasicrystals form are investigated. Currently, it is not clear why most quasicrystals hold 5- or 10-fold symmetry but no single example with 7 or 9-fold symmetry has ever been observed. Since the properties of quasicrystals are strongly connected to their atomic structure, a better understanding of their growth mechanisms is of great importance. In contrast to crystals which are periodic in all three dimensions, quasiperiodicity is always (except for icosahedral quasicrystals) restricted to two dimensions. Accordingly, three-dimensional quasicrystals are comprised of a periodic stacking of quasiperiodic layers and any hurdle in the formation of quasiperiodic order within a single layer will eventually prohibit their growth along the periodic direction. In this Thesis, we also report on geometrical constraints which impede the formation of quasicrystals with certain symmetries in a colloidal model system. This is achieved by subjecting a colloidal monolayer to N=5- and 7-beam quasiperiodic potential landscapes. Our results clearly demonstrate that quasicrystalline order is much easier established for N = 5 compared to N = 7. With increasing laser intensity we observe that the colloids first adopt quasiperiodic order at local areas which then laterally grow until an extended quasicrystalline layer forms. As nucleation sites where quasiperiodicity originates, we identify highly symmetric motifs in the laser pattern. We find that their density strongly varies with n and surprisingly is smallest exactly for those quasicrystalline symmetries which have never been observed in atomic systems. Since such high symmetry motifs also exist in atomic quasicrystals where they act as preferential adsorption sites, this suggests that it is indeed the deficiency of such motifs which accounts for the absence of e.g. materials with 7-fold symmetry. In addition to the fundamental aspects, we report in this Thesis on the fabrication of large colloidal quasiperiodic layers incorporated in a polymer hydrogel matrix. Because quasicrystals have higher point group symmetry than ordinary crystals, micrometer-scale quasicrystalline materials are expected to exhibit large and isotropic photonic bandgaps in the visible range. In our case, the quasiperiodic symmetries are induced using extended light fields. The reported gelled colloidal quasicrystals are unique in that they have large sizes as well as good optical uniformity. With laser diffraction the in situ variable length scale of such materials is demonstrated. In conclusion, we have studied the phase behavior of charged colloidal particles interacting with quasiperiodic laser fields. We showed that novel pseudomorphic growth can lead to the formation of a phase which exhibits likewise crystalline and quasicrystalline structural properties. We also performed unconventional measurements in order to understand why the formation of quasicrystals is limited to specific rotational symmetries. We have found that geometrical hurdles play a crucial role in the proliferation of quasiperiodicity and that such hurdles can hindered or even prohibited the formation of e.g. 7- or 9-fold symmetry. And finally, we have shown that the combination of extended light fields and hydrogel matrices leads to the formation of large quasiperiodically ordered colloidal materials.Item Open Access Towards spin injection into silicon(2007) Dash, Saroj Prasad; Carstanjen, Heinz Dieter (Prof. Dr.)The efficient spin injection into semiconductors could pave the way to a new generation of electronics devices such as spin memories, spin transistors, and spin quantum computers. The most important semiconductor for industrial application, Si has been studied for the purpose of spin injection extensively in this thesis. Three different concepts for spin injection into Si have been addressed: (1) spin injection through a ferromagnet-Si Schottky contact, (2) spin injection using MgO tunnel barriers in between the ferromagnet and Si, and (3) spin injection from Mn-doped Si (DMS) as spin aligner. (1) FM-Si Schottky contact for spin injection: In a heterostructure of a ferromagnetic thin film on a Si substrate, any structural disorder at the interface would drastically reduce the spin polarization at the interface and, hence, the spin injection efficiency. To be able to improve the interface qualities one needs to understand the atomic processes involved in the formation of such silicide phases. In order to obtain more detailed insight into the formation of such silicide phases the initial stages of growth of Co and Fe were studied in situ by HRBS with monolayer depth resolution. As understood, it was important to prohibit the in-diffusion of Co into interstitial sites at the initial stages of growth and the out-diffusion of Si atoms in the latter stages. So in order to control and improve the interface, equilibrium growth conditions were followed (i) by lowering the growth temperature and (ii) by surfactant-mediated growth. Low temperature growth of Co on Si (100): Already at very low coverage Co diffusion into the bulk Si has been observed. The amount of in-diffused Co is, however, less than at room temperature. In contradiction to room temperature growth, Co atoms form layers of pure Co on top of the Si surface already at very low coverage. Every second Si layer, starting with the first Si layer, is Co depleted. This leads to an oscillatory Co distribution in the Si lattice which is preserved up to higher coverages (1.3 ML). Surfactant-mediated growth of Co on Si (100) : The lower surface free energy of Sb in comparison to Co and Si, makes it a potential candidate for surfactant mediated growth. By the use of one monolayer of Sb adsorbed on a Si (100) surface, Co-Si intermixing at the interface is strongly reduced in comparison to the interface without Sb as surfactant. The improved interface quality with Sb-mediated growth is also reflected in magnetic measurements. Co with Sb-mediated growth shows a higher magnetic moment. It was shown that simple solutions can reduce the FM-Si inter diffusion at the interface and improve the interface quality. However these non-equilibrium growth conditions could not stop the silicide formation completely. (2) MgO tunnel barrier for spin injection into Si: On the other hand, using an ultra-thin tunnel barrier between FM and Si will have three advantages: (i) form a chemical barrier between the FM and Si, (ii) circumvent the conductivity mismatch problem, and (iii) in addition act as a spin filter. The fabrication and characterization of ultra-thin crystalline MgO tunnel barriers on Si (100) was presented. Some of the important properties required for tunnel barriers on Si have been addressed. Ultra-thin stoichiometric MgO tunnel barriers with sharp interface with Si (100), very homogeneous, without pin-holes, and crystalline in structure could be fabricated by reactive molecular beam epitaxy. Co and Fe on an ultra thin MgO tunnel barrier were found to have island-like growth with a rough surface. Ultra-thin Co and Fe films are found to be thermally quite stable up to 450 °C. (3) Mn doped Si for spin injection: For spin injection purpose, instead of contacting the Si with a ferromagnetic metal, the contact could be made with another semiconductor, one with ferromagnetic properties. This solves the conductivity mismatch problem by ensuring that the resistivities of the materials on both side of the interface are comparable in magnitude. Si-based diluted magnetic semiconductor samples were prepared by doping Si with Mn by two different methods i) by Mn ion implantation and ii) by in-diffusion of Mn atoms (solid state growth). In the case of implanted samples, Mn atoms do not substitute Si sites. The implanted samples show room temperature ferromagnetism as measured by a SQUID magnetometer. The magnetic moment per Mn atom is found to decrease with increasing implantation dose. It has been observed that the implanted samples show carrier mediated ferromagnetism and, more importantly, mediated by both holes and electrons in contrast to statements in the literature. Solid state growth of Mn doped Si : For evaporation of Mn on Si (100), Mn atoms diffuse deep into the Si bulk already at room temperature, even for very low coverage (0.25 ML) with an oscillatory concentration depth profile as observed by HRBS with monolayer depth resolution. This results in natural MnxSi1-x/Si digital layers on the surface. Surprisingly, the samples prepared by this solid state diffusion process show room-temperature ferromagnetism having a magnetic moment of 1.8 µB per Mn atom, which is much higher than that of the ion-implanted samples. In contrast to ion-implanted samples the ferromagnetism in these samples does not show any carrier mediation.Item Open Access Die wichtigsten Gesetzmäßigkeiten des Wachstums von Filmen und epitaktischen Schichten aus Ionenmolekularströmen(1999) Ljutovic, Abram Srulevic; Ludwig, Sören (Übersetzerin); Pertschi, Ottmar (Übersetzer)Hier werden charakteristische Forschungsergebnisse verschiedener Autoren dargestellt, die die Vorgänge der Metallschichtabscheidung und Kristallisation von Halbleiter-Epitaxieschichten aus Ionenmolekularströmen untersucht haben. Dabei widmen wir den Untersuchungen zum Wachstum von Halbleiter-Einkristallschichten besondere Aufmerksamkeit.Item Open Access Kinetics of the triplet state of 2,3-dichloroquinoxaline from microwave-induced phosphorescence transients(1973) Schweitzer, Dieter; Zuclich, Joseph A.; Maki, August H.The phosphorescent state of 2,3-dichloroquinoxaline doped into single crystals of durene and 1,2,4,5-tetrachlorobenzene has been studied using several methods based upon optical detection of magnetic resonance (ODMR). Flash excitation and continuous optical pumping methods are described and analysed. Phosphorescent transient effects caused by spin-lattice relaxation and variable intersystem crossing rates are observed and described using first-order solutions of the appropriate rate equations. The effects of spatial polarization of the phosphorescence (anisotropic spatial distribution of phosphorescence intensity) of single crystals on ODMR signals is observed and discussed. The spatial polarization of emission can cause difficulties in determining relative radiative rate constants of the triplet sublevels in oriented samples, but can yield information about the linear polarization of the emission analogous to that obtained by conventional means using polarizers.Item Open Access Macroscopic quantum tunneling in Bose-Einstein condensates(2013) Schwidder, Torsten; Main, Jörg (Prof. Dr.)The decay of Bose-Einstein condensates from a metastable ground state into collapse due to macroscopic quantum tunneling is investigated using a semiclassical approximation to Feynman’s path integral formalism. Applying a variational ansatz of a single Gaussian to the wave function determined by the Gross-Pitaevskii equation, a special choice of the Gaussian width parameters yields a mean-field energy functional in Hamiltonian form. The temporal spatial extension of the condensate then is described in the picture of a particle moving in an external potential. In this picture the decay of the condensate wave function is investigated using the bounce trajectory method, accounting for the action of the tunneling orbit in imaginary time and quadratic fluctuations around it, the latter described by the Gelfand-Yaglom differential equation. Additionally, the tunneling formalism is used for describing the condensate wave function by a trial function of a superposition of several Gaussians. Using this ansatz a Hamiltonian form of the mean-field energy does not exist and the formalism of describing the tunneling process has to be extended. The bounce trajectory is described in the parameter space of the Gaussian width parameters and the equations of motion for the imaginary time evolution are obtained by applying a time-dependent variational principle. The action of the bounce trajectory is investigated as a function of the number of Gaussians taken into account and the contribution of the fluctuations is evaluated using the monodromy matrix.Item Open Access Funktionalmethoden und Abbildungen dissipativer Quantensysteme(2007) Baur, Holger; Weiß, Ulrich (Prof. Dr.)Im ersten Abschnitt dieser Arbeit versuchen wir, die algebraische Struktur, welche im Rahmen der dissipativen Quantenmechanik unter Verwendung des Influenzfunktionals auftritt, herauszuarbeiten. Dies erlaubt uns einen tieferen Einblick in ansonsten unübersichtliche und langwierige Rechenschritte, speziell im Realzeitformalismus, und ermöglicht uns eine leichtere Identifikation der dabei auftretenden Terme und deren Ursprung aus dem zugrunde liegenden Modell als auch deren physikalische Bedeutung. Die verwendeten Methoden haben wir soweit als möglich in konsistenter und anschaulicher Form eingeführt, so dass diese Arbeit ohne spezielle Kenntnis des Gebietes der dissipativen Quantenmechanik gelesen werden kann. Besonderen Wert haben wir auf den Übergang von der quantenmechanischen auf die klassische Beschreibung von dissipativen Vorgängen gelegt, da ein Verständnis dieses Übergangs eine tiefere Einsicht in den Messprozess liefert. In der selben Weise ist damit auch der Übergang von der mikroskopischen - durch die Quantenmechanik beschriebenen - Welt in die makroskopische Welt verbunden, welche den Gesetzen der klassischen Mechanik folgt. Zusätzlich zeigen wir, wie die Resultate der Influenzfunktionalmethode stochastisch interpretiert werden können, was einen leichteren Vergleich mit der bekannten quantenmechanischen Zeitentwicklung durch die Schrödingergleichung erlaubt. Im weiteren betrachten wir die sogenannte Tight-Binding Näherung von Modellen, bei welchen sich der Hamiltonraum für die Systembeschreibung im wesentlichen durch diskrete Eigenzustände des Ortsoperators ausdrücken lässt und Übergänge zwischen diesen Zuständen unterdrückt sind, wodurch eine Propagation entweder durch ein Tunneln oder durch thermische Anregung erfolgt. Im Rahmen der dissipativen Quantenmechanik bringt diese Methode eine immense Vereinfachung in der effektiven Beschreibung des Systems, da Anstelle einer ganzen Historie von Systempfaden nur noch die Übergangszeiten mit den entsprechenden Übergängen berücksichtigt werden müssen. Im Bild des Pfadintegralformalismus bedeutet dies, dass Anstelle des Integrals über alle Systempfade ein Produkt von Integrationen über alle möglichen Sprungzeiten mit Sprunggewichten entsprechend des Übergangs rückt, welches analytisch als auch numerisch wesentlich einfacher handzuhaben ist. Innerhalb dieser Näherung wurden dadurch in der Vergangenheit viele beeindruckende analytische Resultate abgeleitet. Darüber hinaus beschäftigen wir uns mit der Abbildung und dem Zusammenhang von dissipativen Modellen mit Feldmodellen aus der Quantenfeldtheorie und im besonderen der Feldtheorie statistischer Systeme. Der Reiz dieser Abbildungen liegt besonders darin, dass in den letztgenannten Gebieten schon seit Jahrzehnten sehr intensiv die grundlegenden Modelle bearbeitet wurden und vor allem auch nach neuen Methoden gesucht und Forschung dafür betrieben wurde und noch immer Gegenstand der aktuellen Forschung darstellt. Als Beispiel sei in zwei Dimensionen die Invarianz unter konformen Abbildungen genannt, welche immer dann Anwendung findet, wenn Systeme nur lokal wechselwirken und eine Invarianz unter lokaler Umskalierung der Felder zeigen. Bei statistischen Systemen mit lokaler Wechselwirkung zeigt sich dieses Verhalten immer beim Erreichen eines kritischen Punktes, da hier per Definition keine Längenskala ausgezeichnet ist. In zwei Dimensionen führt dies zu einer immensen Einschränkung der möglichen Form von Korrelationsfunktionen und hat zu dem eigenständigen Gebiet der Konformen Feldtheorie geführt, da konforme Abbildungen per Definition die lokale Struktur erhalten (Winkeltreue) und lokal nur zu einer Unskalierung führen. Während in D>2 Dimensionen nur endlich viele Generatoren für konforme Abbildungen existieren, ist deren Anzahl in 2 Dimensionen unendlich. Dies resultiert in einer unendlichen Anzahl von lokalen Erhaltungsgrößen mit den entsprechenden Folgen. Während solche Techniken sehr schnell unanschaulich werden, erlaubt die Abbildung auf dissipative Modelle hier oftmals eine sehr anschauliche Interpretation.Item Open Access Experimental results on absolute phonon detection sensitivity of superconducting tunneling junctions(1972) Trumpp, Hans-Joachim; Epperlein, Peter W.; Lassmann, KurtSuperconducting tin tunnelling junctions are used for generating and detecting 300 GHz phonons. The absolute phonon detection sensitivity can be obtained, making possible a comparison of the number of phonons detected to the number of phonons generated. This, together with measurements of the dependence of junction time constant on its thickness, gives an indication that far more phonons are radiated into liquid He than is expected from a simple acoustic model.Item Open Access Ultrafast spectroscopy of single quantum dots(2012) Wolpert, Christian; Lippitz, Markus (Juniorprofessor Dr.)In this thesis, the coherent interaction of single semiconductor quantum dots and ultrafast optical pulses is studied. Under certain conditions, localized exciton transitions in quantum dots can be seen as semi-isolated two-level systems. While this description is sufficient for the explanation of some observations in coherent experiments, it is sometimes necessary to explicitly consider coupling of the discreet quantum states confined to the dot with the environment. We start out from simple, classical examples of coherent spectroscopy and then turn towards experiments where the interaction with the vicinity of the dot becomes an important factor. First, a novel method for transient differential reflectivity spectroscopy of single quantum systems is introduced. It is a pure far-field optical technique which does not require any sophisticated sample preparation steps which makes it applicable to a broad range of structures. Pump pulses excite the sample structure and probe pulses read out the pump-induced changes in the system after a variable delay time. In the case of a single dipole, the signal is given in the form of the spectral inteferogram between the backscattered wave from the particle and the probe light which is reflected at the sample surface. This form of homodyne detection amplifies the weak scattered wave from the particle and thus makes this kind of spectroscopy for single quantum dots feasible. In the remainder of this thesis our spectroscopic method is applied to either characterize the coherent properties of single quantum dots, to prepare and read-out a desired quantum state or to deliberately manipulate them. Coherence times and oscillator strengths are determined for localized exciton transitions. Arbitrary population states can be written by driving coherent population oscillations using resonant pulses, while entangled superpositions of two exciton states in a single dot are investigated by quantum beats on transient differential spectra. We finally exploit the interaction between the dot and a nearby absorbing layer to switch the dot's absorption spectrum on ultrafast timescales via light-induced transient electric fields.