Polarization in heavy quark decays

In this thesis I concentrate on the angular correlations in top quark decays and their next--to--leading order (NLO) QCD corrections. I also discuss the leading--order (LO) angular correlations in unpolarized and polarized hyperon decays. In the first part of the thesis I calculate the angular correlation between the top quark spin and the momentum of decay products in the rest frame decay of a polarized top quark into a charged Higgs boson and a bottom quark in Two-Higgs-Doublet-Models: $t(uparrow)rightarrow b+H^{+}$. The decay rate in this process is split into an angular independent part (unpolarized) and an angular dependent part (polar correlation). I provide closed form formulae for the ${mathcal O}(alpha_{s})$ radiative corrections to the unpolarized and the polar correlation functions for $m_{b}neq 0$ and $m_{b}=0$. The results for the unpolarized rate agree with the existing results in the literature. The results for the polarized correlations are new. I found that, for certain values of $tanbeta$, the ${mathcal O}(alpha_s)$ radiative corrections to the unpolarized, polarized rates, and the asymmetry parameter can become quite large. In the second part I concentrate on the semileptonic rest frame decay of a polarized top quark into a bottom quark and a lepton pair: $t(uparrow) to X_b + ell^+ + nu_ell$. I analyze the angular correlations between the top quark spin and the momenta of the decay products in two different helicity coordinate systems: system 1a with the $z$--axis along the charged lepton momentum, and system 3a with the $z$--axis along the neutrino momentum. The decay rate then splits into an angular independent part (unpolarized), a polar angle dependent part (polar correlation) and an azimuthal angle dependent part (azimuthal correlation). I present closed form expressions for the ${mathcal O}(alpha_{s})$ radiative corrections to the unpolarized part and the polar and azimuthal correlations in system 1a and 3a for $m_{b}neq 0$ and $m_{b}=0$. For the unpolarized part and the polar correlation I agree with existing results. My results for the azimuthal correlations are new. In system 1a I found that the azimuthal correlation vanishes in the leading order as a consequence of the $(V-A)$ nature of the Standard Model current. The ${mathcal O}(alpha_{s})$ radiative corrections to the azimuthal correlation in system 1a are very small (around 0.24% relative to the unpolarized LO rate). In system 3a the azimuthal correlation does not vanish at LO. The ${mathcal O}(alpha_{s})$ radiative corrections decreases the LO azimuthal asymmetry by around 1%. In the last part I turn to the angular distribution in semileptonic hyperon decays. Using the helicity method I derive complete formulas for the leading order joint angular decay distributions occurring in semileptonic hyperon decays including lepton mass and polarization effects. Compared to the traditional covariant calculation the helicity method allows one to organize the calculation of the angular decay distributions in a very compact and efficient way. This is demonstrated by the specific example of the polarized hyperon decay $Xi^0(uparrow) to Sigma^+ + l^- + bar{nu}_l$ ,($l^-=e^-, mu^-$) followed by the nonleptonic decay $Sigma^+ to p + pi^0$, which is described by a five--fold angular decay distribution.

NMR spectroscopy and imaging of hyperpolarized gases

Since the discovery of the nuclear magnetic resonance (NMR) phenomenon, countless NMR techniques have been developed that are today indispensable tools in physics, chemistry, biology, and medicine. As one of the main obstacles in NMR is its notorious lack of sensitivity, different hyperpolarization (HP) methods have been established to increase signals up to several orders of magnitude. In this work, different aspects of magnetic resonance, using HP noble gases, are studied, hereby combining different disciplines of research. The first part examines new fundamental effects in NMR of HP gases, in theory and experiment. The spin echo phenomenon, which provides the basis of numerous modern experiments, is studied in detail in the gas phase. The changes of the echo signal in terms of amplitude, shape, and position, due to the fast translational motion, are described by an extension of the existing theory and computer simulations. With this knowledge as a prerequisite, the detection of intermolecular double-quantum coherences was accomplished for the first time in the gas phase. The second part of this thesis focuses on the development of a practical method to enhance the dissolution process of HP 129Xe, without loss of polarization or shortening of T1. Two different setups for application in NMR spectroscopy and magnetic resonance imaging (MRI) are presented. The continuous operation allows biological and multidimensional spectroscopy in solutions. Also, first in vitro MRI images with dissolved HP 129Xe as contrast agent were obtained at a clinical scanner.

Polarization observables in virtual compton scattering

Virtual Compton Scattering (VCS) is an important reaction for understanding nucleon structure at low energies. By studying this process, the generalized polarizabilities of the nucleon can be measured. These observables are a generalization of the already known polarizabilities and will permit theoretical models to be challenged on a new level. More specifically, there exist six generalized polarizabilities and in order to disentangle them all, a double polarization experiment must be performed. Within this work, the VCS reaction p(e,e p)gamma was measured at MAMI using the A1 Collaboration three spectrometer setup with Q2=0.33 (GeV/c)2. Using the highly polarized MAMI beam and a recoil proton polarimeter, it was possible to measure both the VCS cross section and the double polarization observables. Already in 2000, the unpolarized VCS cross section was measured at MAMI. In this new experiment, we could confirm the old data and furthermore the double polarization observables were measured for the first time. The data were taken in five periods between 2005 and 2006. In this work, the data were analyzed to extract the cross section and the proton polarization. For the analysis, a maximum likelihood algorithm was developed together with the full simulation of all the analysis steps. The experiment is limited by the low statistics due mainly to the focal plane proton polarimeter efficiency. To overcome this problem, a new determination and parameterization of the carbon analyzing power was performed. The main result of the experiment is the extraction of a new combination of the generalized polarizabilities using the double polarization observables.

Noble-metal nanoparticles produced with colloidal lithography: fabrication, optical properties and applications

In this work, metal nanoparticles produced by nanosphere lithography were studied in terms of their optical properties (in connection to their plasmon resonances), their potential application in sensing platforms - for thin layer sensing and bio-recognition events -, and for a particular case (the nanocrescents), for enhanced spectroscopy studies. The general preparation procedures introduced early in 2005 by Shumaker-Parry et al. to produce metallic nanocrescents were extended to give rise to more complex (isolated) structures, and also, by combining colloidal monolayer fabrication and plasma etching techniques, to arrays of them. The fabrication methods presented in this work were extended not only to new shapes or arrangements of particles, but included also a targeted surface tailoring of the substrates and the structures, using different thiol and silane compounds as linkers for further attachment of, i.e. polyelectrolyte layers, which allow for a controlled tailoring of their nanoenvironment. The optical properties of the nanocrescents were studied with conventional transmission spectroscopy; a simple multipole model was adapted to explain their behaviour qualitatively. In terms of applications, the results on thin film sensing using these particles show that the crescents present an interesting mode-dependent sensitivity and spatial extension. Parallel to this, the penetrations depths were modeled with two simplified schemes, obtaining good agreement with theory. The multiple modes of the particles with their characteristic decay lengths and sensitivities represent a major improvement for particle-sensing platforms compared to previous single resonance systems. The nanocrescents were also used to alter the emission properties of fluorophores placed close to them. In this work, green emitting dyes were placed at controlled distances from the structures and excited using a pulsed laser emitting in the near infrared. The fluorescence signal obtained in this manner should be connected to a two-photon processes triggered by these structures; obtaining first insight into plasmon-mediated enhancement phenomena. An even simpler and faster approach to produce plasmonic structures than that for the crescents was tested. Metallic nanodiscs and nanoellipses were produced by means of nanosphere lithography, extending a procedure reported in the literature to new shapes and optical properties. The optical properties of these particles were characterized by extinction spectroscopy and compared to results from the literature. Their major advantage is that they present a polarization-dependent response, like the nanocrescents, but are much simpler to fabricate, and the resonances can be tailored in the visible with relative ease. The sensing capabilities of the metallic nanodiscs were explored in the same manner as for the nanocrescents, meaning their response to thin layers and to bio-recognition events on their surface. The sensitivity of these nanostructures to thin films proved to be lower than that of the crescents, though in the same order of magnitude. Experimental information about the near field extension for the Au nanodiscs of different sizes was also extracted from these measurements. Further resonance-tailoring approaches based on electrochemical deposition of metals on the nanodiscs were explored, as a means of modifying plasmon resonances by changing surface properties of the nanoparticles. First results on these experiments would indicate that the deposition of Ag on Au on a submonolayer coverage level can lead to important blue-shifts in the resonances, which would open a simple way to tailor resonances by changing material properties in a local manner.

The AMS-02 Experiment and the Dark Matter Search

AMS-02 is running after great scientific goals since one year and a half: a final setting up for dark matter searches has been achieved, allowing to study the so important antiparticle to particle ratios, which will probably be the first dark matter signals ever corroborated. Even if primary cosmic rays fluxes are subjected to a lot of uncertainties sources, some statements can be done and have been written down about dark matter properties: DM should be a heavy Majorana fermion or Spin 0 or 1 boson, with a mass from about 1 TeV to 10 TeV - unveiling a new TeV-ish search age - which could be able to originate antiparticle fluxes enhancements at high energies, both for positrons and antiprotons. All the observations, direct and indirect, point to these new paradigms or can be traced back to them quite easily. These enhancements perfectly fall into the research window of AMS-02, allowing the experiment to attack each today credible theory. Also an investigation of the Sommerfeld effect-associated dark boson will be possible, in terms of antiparticle to particle ratios substructures. The first great AMS-02 measurement is the positron fraction: an official paper is going to be submitted in few months, where the correct behavior of the apparatus will be reviewed and the full positron fraction rate will be analyzed up to 200 GeV. In this concern, one of the objectives of this work is to test the AMS-02 capability and versatility in doing these dark matter researches, thanks to an orbital temporal (and geomagnetic) stability. The goal has been accomplished: the experiment is very stable in time, so that the temporal error associated to the positron fraction measurement is compatible with zero, offering a beyond belief opportunity to measure CR antiparticle to particle ratios.

Methods for the Quantification of Motor Stability for the Assessment of Fall Risk

The research field of the Thesis is the evaluation of motor variability and the analysis of motor stability for the assessment of fall risk. Since many falls occur during walking, a better understanding of motor stability could lead to the definition of a reliable fall risk index aiming at measuring and assessing the risk of fall in the elderly, in the attempt to prevent traumatic events. Several motor variability and stability measures are proposed in the literature, but still a proper methodological characterization is lacking. Moreover, the relationship between many of these measures and fall history or fall risk is still unknown, or not completely clear. The aim of this thesis is hence to: i) analyze the influence of experimental implementation parameters on variability/stability measures and understand how variations in these parameters affect the outputs; ii) assess the relationship between variability/stability measures and long- short-term fall history. Several implementation issues have been addressed. Following the need for a methodological standardization of gait variability/stability measures, highlighted in particular for orbital stability analysis through a systematic review, general indications about implementation of orbital stability analysis have been showed, together with an analysis of the number of strides and the test-retest reliability of several variability/stability numbers. Indications about the influence of directional changes on measures have been provided. The association between measures and long/short-term fall history has also been assessed. Of all the analyzed variability/stability measures, Multiscale entropy and Recurrence quantification analysis demonstrated particularly good results in terms of reliability, applicability and association with fall history. Therefore, these measures should be taken in consideration for the definition of a fall risk index.

Sensitivity enhancement in NMR by using parahydrogen induced polarization

Enhancing the sensitivity of nuclear magnetic resonance measurements via hyperpolarization techniques like parahydrogen induced polarization (PHIP) is of high interest for spectroscopic investigations. Parahydrogen induced polarization is a chemical method, which makes use of the correlation between nuclear spins in parahydrogen to create hyperpolarized molecules. The key feature of this technique is the pairwise and simultaneous transfer of the two hydrogen atoms of parahydrogen to a double or triple bond resulting in a population of the Zeeman energy levels different from the Boltzmann equation. The obtained hyperpolarization results in antiphase peaks in the NMR spectrum with high intensities. Due to these strong NMR signals, this method finds arnlot of applications in chemistry e.g. the characterization of short-lived reaction intermediates. Also in medicine it opens up the possibility to boost the sensitivity of medical diagnostics via magnetic labeling of active contrast agents. Thus, further examination and optimization of the PHIP technique is of significant importance in order to achieve the highest possible sensitivity gain.rnrnIn this work, different aspects concerning PHIP were studied with respect to its chemical and spectroscopic background. The first part of this work mainly focused on optimizing the PHIP technique by investigating different catalyst systems and developing new setups for the parahydrogenation. Further examinations facilitated the transfer of the generated polarization from the protons to heteronuclei like 13C. The second part of this thesis examined the possibility to transfer these results to different biologically active compounds to enable their later application in medical diagnostics. Onerngroup of interesting substances is represented by metabolites or neurotransmitters in mammalian cells. Other interesting substances are clinically relevant drugs like a barbituric acid derivative or antidepressant drugs like citalopram which were investigated with regard to their applicability for the PHIP technique and the possibility to achievernpolarization transfer to 13C nuclei. The last investigated substrate is a polymerizable monomer whose polymer was used as a blood plasma expander for trauma victims after the first half of the 20th century. In this case, the utility of the monomer for the PHIP technique as a basis for later investigations of a polymerization reaction using hyperpolarized monomers was examined.rnrnHence, this thesis covers the optimization of the PHIP technology, hereby combining different fields of research like chemical and spectroscopical aspects, and transfers the results to applications of real biologally acitve compounds.

Advances in hardware and molecular design of polarizing agents for dynamic nuclear polarization

Die Kernmagnetresonanz (NMR) ist eine vielseitige Technik, die auf spin-tragende Kerne angewiesen ist. Seit ihrer Entdeckung ist die Kernmagnetresonanz zu einem unverzichtbaren Werkzeug in unzähligen Anwendungen der Physik, Chemie, Biologie und Medizin geworden. Das größte Problem der NMR ist ihre geringe Sensitivtät auf Grund der sehr kleinen Energieaufspaltung bei Raumtemperatur. Für Protonenspins, die das größte magnetogyrische Verhältnis besitzen, ist der Polarisationsgrad selbst in den größten verfügbaren Magnetfeldern (24 T) nur ~7*10^(-5).rnDurch die geringe inhärente Polarisation ist folglich eine theoretische Sensitivitätssteigerung von mehr als 10^4 möglich. rnIn dieser Arbeit wurden verschiedene technische Aspekte und unterschiedliche Polarisationsagenzien für Dynamic Nuclear Polarization (DNP) untersucht.rnDie technische Entwicklung des mobilen Aufbaus umfasst die Verwendung eines neuen Halbach Magneten, die Konstruktion neuer Probenköpfe und den automatisierten Ablauf der Experimente mittels eines LabVIEW basierten Programms. Desweiteren wurden zwei neue Polarisationsagenzien mit besonderen Merkmalen für den Overhauser und den Tieftemperatur DNP getestet. Zusätzlich konnte die Durchführbarkeit von NMR Experimenten an Heterokernen (19F und 13C) im mobilen Aufbau bei 0,35 T gezeigt werden. Diese Ergebnisse zeigen die Möglichkeiten der Polarisationstechnik DNP auf, wenn Heterokerne mit einem kleinen magnetogyrischen Verhältnis polarisiert werden müssen.rnDie Sensitivitätssteigerung sollte viele neue Anwendungen, speziell in der Medizin, ermöglichen.

Optical interface based on a nanofiber atom-trap

In this thesis, I present the realization of a fiber-optical interface using optically trapped cesium atoms, which is an efficient tool for coupling light and atoms. The basic principle of the presented scheme relies on the trapping of neutral cesium atoms in a two-color evanescent field surrounding a nanofiber. The strong confinement of the fiber guided light, which also protrudes outside the nanofiber, provides strong confinement of the atoms as well as efficient coupling to near-resonant light propagating through the fiber. In chapter 1, the necessary physical and mathematical background describing the propagation of light in an optical fiber is presented. The exact solution of Maxwell’s equations allows us to model fiber-guided light fields which give rise to the trapping potentials and the atom-light coupling in the close vicinity of a nanofiber. Chapter 2 gives the theoretical background of light-atom interaction. A quantum mechanical model of the light-induced shifts of the relevant atomic levels is reviewed, which allows us to quantify the perturbation of the atomic states due to the presence of the trapping light-fields. The experimental realization of the fiber-based atom trap is the focus of chapter 3. Here, I analyze the properties of the fiber-based trap in terms of the confinement of the atoms and the impact of several heating mechanisms. Furthermore, I demonstrate the transportation of the trapped atoms, as a first step towards a deterministic delivery of individual atoms. In chapter 4, I present the successful interfacing of the trapped atomic ensemble and fiber-guided light. Three different approaches are discussed, i.e., those involving the measurement of either near-resonant scattering in absorption or the emission into the guided mode of the nanofiber. In the analysis of the spectroscopic properties of the trapped ensemble we find good agreement with the prediction of theoretical model discussed in chapter 2. In addition, I introduce a non-destructive scheme for the interrogation of the atoms states, which is sensitive to phase shifts of far-detuned fiber-guided light interacting with the trapped atoms. The inherent birefringence in our system, induced by the atoms, changes the state of polarization of the probe light and can be thus detected via a Stokes vector measurement.

Magnetische Wechselwirkung Spin-Austauschgekoppelter Systeme mit Nitroxid und Nitronyl-Nitroxid Radikalen

Bei der Untersuchung molekularer magnetischer Materialien spielen Metall-Radikal Verbindungen eine bedeutende Rolle. Ein Forschungsschwerpunkt stützt sich auf die Familie der Nitronyl-Nitroxid (NIT) Radikale, die sich durch eine hohe chemische Stabilität auszeichnen. Im sogenannten „Metall-Radikal Ansatz“ wurden die starken Austauschwechselwirkungen zwischen stabilen Radikalen und Übergangsmetallionen in mehrdimensionalen Netzwerken ausgiebig untersucht. Um diese Netzwerke mit NIT Radikalen aufzubauen, müssen zusätzliche funktionelle Gruppen, mit einem Abstand zur spintragenden Einheit, in das Molekül eingebaut werden. Dies kann zu einer zusätzlichen schwachen Spinaustauschwechselwirkung führen. Um diese Wechselwirkung zwischen Metalldimeren mit einem einzelnen Benzoat annalogen NIT-Radikal zu untersuchen, wurden dimere Mangan(II), Kobalt(II) und Zink(II) Komplexe mit dem Chelatliganden N,N,N',N'-Tetrakis(2-benzimid-azolylalkyl)-2-hydroxy-1,3-diamino-propan synthetisiert und zusätzlich über eine periphere Carboxylat Gruppe eines NIT Radikals verbrückt.rnDie Messungen der magnetischen Suszeptibilität weisen auf eine dominante antiferromagnetische Wechselwirkung in der Metall-Radikal Verbindung hin, bei der es sich um die Spin-Austauschwechselwirkung innerhalb des Metalldimers handelt. Durch den Vergleich mit analogen Nitrobenzoat- verbrückten Mangan(II) und Kobalt(II) Verbindungen konnte gezeigt werden, dass keine Metall-Radikal Wechselwirkung beobachtet wird, obwohl eine Wechselwirkung der pi*-orbitale mit den delokalisierten pi-System des Phenylrings durch Spin-Polarisation grundsätzlich möglich ist. Auch ESR - Messungen bestätigen dies, da der Spingrundzustand das anisotrope Signal des freien NIT Radikals aufweist. Das Radikal verhält sich somit wie ein isoliertes S=1/2 Spin-Zentrum, was zusätzlich durch DFT-Rechnungen bekräftigt werden konnte. Zusammenfassend führt also die Koordination eines NIT-Benzoats an ein antiferromagnetisch gekoppeltes Metalldimer nur zur Anhebung des Spingrundzustandes und hat keinen signifikanten Effekt auf die Austauschwechselwirkung. Um trotzdem eine Metall-Radikal Wechselwirkung beobachten zu können, ist es notwendig Koordinationsverbindungen zu synthetisieren in denen hohe Spingrundzustände besetzt werden. Dies trifft auf das analoge Kupferdimer zu, wofür eine ferromagnetische Wechselwirkung zu beobachten ist.rnNach den Regeln der Spin-Polarisation müsste die Verkürzung des Austauschpfades um eine Bindung zu einer Umkehrung des Vorzeichens der magnetischen Wechselwirkung führen. Diese Verkürzung kann man durch die Verwendung des alternativen stabilen NOA-Radikals (tert-Butyl Nitroxid) erreichen. Sowohl das NIT als auch das NOA-Radikal werden an ein Kupfer(II)-dimer koordiniert, das durch die Verwendung des oben erwähnten N6O-Liganden gebildet wurde. In der Modellverbindung, ohne einen paramagnetischen Substituenten am Benzoat, zeigen die Kupferionen eine ferromagnetische Wechselwirkung mit einem Triplett Grundzustand, dessen Existenz durch die Messung der magnetischen Suszeptibilität und ESR-Spektroskopie belegt werden kann. Aufgrund der nahezu identischen Koordinationsumgebung bleibt bei allen synthetisierten Verbindungen die Kupfer-Kupfer Wechselwirkung dabei gleich. Die Daten von ESR und magnetischen Messungen zeigen weiterhin auf eine signifikante zusätzliche Metall-Radikal Wechselwirkung hin. Bei der NIT-Verbindung ist diese Austauschwechselwirkung schwach antiferromagnetisch, während die NOA-Verbindung eine schwache ferromagnetische Kopplung aufzeigt. Diese Resultate können durch DFT Rechnungen bekräftigt werden. Der Vorzeichenwechsel des Kopplungsparameters kann durch die Verkürzung des Austauschpfades vom NIT zum NOA-Benzoat um eine Bindung erklärt werden. Durch die Wahl von geeigneten Radikal- Liganden und Metallionen, zeigt sich die Möglichkeit, Systeme zu erzeugen, in denen die Radikal-Metall Wechselwirkung auch über größere Distanzen den Spin-Grundzustand des gesamten Systems signifikant beeinflussen kann. die Anwendung dieses Konzeptes auf Metall-Radikal Cluster System sollte Von großem Interesse sein.rn

Charge-transport simulations in organic semiconductors

In this thesis we have extended the methods for microscopic charge-transport simulations for organic semiconductors. In these materials the weak intermolecular interactions lead to spatially localized charge carriers, and the charge transport occurs as an activated hopping process between diabatic states. In addition to weak electronic couplings between these states, different electrostatic environments in the organic material lead to a broadening of the density of states for the charge energies which limits carrier mobilities.rnThe contributions to the method development includern(i) the derivation of a bimolecular charge-transfer rate,rn(ii) the efficient evaluation of intermolecular (outer-sphere) reorganization energies,rn(iii) the investigation of effects of conformational disorder on intramolecular reorganization energies or internal site energiesrnand (iv) the inclusion of self-consistent polarization interactions for calculation of charge energies.These methods were applied to study charge transport in amorphous phases of small molecules used in the emission layer of organic light emitting diodes (OLED).rnWhen bulky substituents are attached to an aromatic core in order to adjust energy levels or prevent crystallization, a small amount of delocalization of the frontier orbital to the substituents can increase electronic couplings between neighboring molecules. This leads to improved charge-transfer rates and, hence, larger charge-mobility. We therefore suggest using the mesomeric effect (as opposed to the inductive effect) when attaching substituents to aromatic cores, which is necessary for example in deep blue OLEDs, where the energy levels of a host molecule have to be adjusted to those of the emitter.rnFurthermore, the energy landscape for charges in an amorphous phase cannot be predicted by mesoscopic models because they approximate the realistic morphology by a lattice and represent molecular charge distributions in a multipole expansion. The microscopic approach shows that a polarization-induced stabilization of a molecule in its charged and neutral states can lead to large shifts, broadening, and traps in the distribution of charge energies. These results are especially important for multi-component systems (the emission layer of an OLED or the donor-acceptor interface of an organic solar cell), if the change in polarizability upon charging (or excitation in case of energy transport) is different for the components. Thus, the polarizability change upon charging or excitation should be added to the set of molecular parameters essential for understanding charge and energy transport in organic semiconductors.rnWe also studied charge transport in self-assembled systems, where intermolecular packing motives induced by side chains can increase electronic couplings between molecules. This leads to larger charge mobility, which is essential to improve devices such as organic field effect transistors, where low carrier mobilities limit the switching frequency.rnHowever, it is not sufficient to match the average local molecular order induced by the sidernchains (such as the pitch angle between consecutive molecules in a discotic mesophase) with maxima of the electronic couplings.rnIt is also important to make the corresponding distributions as narrow as possible compared to the window determined by the closest minima of thernelectronic couplings. This is especially important in one-dimensional systems, where charge transport is limited by the smallest electronic couplings.rnThe immediate implication for compound design is that the side chains should assist the self-assemblingrnprocess not only via soft entropic interactions, but also via stronger specific interactions, such as hydrogen bonding.rnrnrnrn

Proton magnetic resonance with parahydrogen induced polarization : Imaging strategies and continuous generation

A major challenge in imaging is the detection of small amounts of molecules of interest. In the case of magnetic resonance imaging (MRI) their signals are typically concealed by the large background signal of e.g. the tissue of the body. This problem can be tackled by hyperpolarization which increases the NMR signals up to several orders of magnitude. However, this strategy is limited for 1H, the most widely used nucleus in NMR andrnMRI, because the enormous number of protons in the body screen the small amount of hyperpolarized ones.Here, I describe a method giving rise to high 1H MRI contrast for hyperpolarized molecules against a large background signal. The contrast is based on the J-coupling induced rephasing of the NMR signal of molecules hyperpolarized via parahydrogen induce polarization (PHIP) and it can easily be implemented in common pulse sequences.rnrnHyperpolarization methods typically require expensive technical equipment (e.g. lasers or microwaves) and most techniques work only in batch mode, thus the limited lifetime of the hyperpolarization is limiting its applications. Therefore, the second part of my thesis deals with the simple and efficient generation of an hyperpolarization.These two achievements open up alternative opportunities to use the standard MRI nucleus 1H for e.g. metabolic imaging in the future.

Parahydrogen induced polarization on a clinical MRI system : polarization transfer of two spin order

Hyperpolarization techniques enhance the nuclear spin polarization and thus allow for new nuclear magnetic resonance applications like in vivo metabolic imaging. One of these techniques is Parahydrogen Induced Polarization (PHIP). It leads to a hyperpolarized 1H spin state which can be transferred to a heteronucleus like 13C by a radiofrequency (RF) pulse sequence. In this work, timing of such a sequence was analyzed and optimized for the molecule hydroxyethyl propionate. The pulse sequence was adapted for the work on a clinical magnetic resonance imaging (MRI) system which is usually equipped only with a single RF transmit channel. Optimal control theory optimizations were performed to achieve an optimized polarization transfer. A drawback of hyperpolarization is its limited lifetime due to relaxation processes. The lifetime can be increased by storing the hyperpolarization in a spin singlet state. The second part of this work therefore addresses the spin singlet state of the Cs-symmetric molecule dimethyl maleate which needs to be converted to the spin triplet state to be detectable. This conversion was realized on a clinical MRI system, both by field cycling and by two RF pulse sequences which were adapted and optimized for this purpose. Using multiple conversions enables the determination of the lifetime of the singlet state as well as the conversion efficiency of the RF pulse sequence. Both, the hyperpolarized 13C spin state and the converted singlet state were utilized for MR imaging. Careful choice of the echo time was shown to be crucial for both molecules.

Light harvesting and orbital tuning : perylene monoimides and diimides for photovoltaic purposes

Die vorliegende Dissertation beschaftigt sich mit der Steuerung der Absorption und Orbitalenergien von Perylenmonoimiden und Perylendiimiden fur die Anwendung in organischer Photovoltaik (OPV). Eine breite Absorption spielt hier eine wichtige Rolle, um moglichst viel Licht zu ernten, das dann in elektrische Energie umgewandelt wird. Um sicher zu stellen, dass die Zelle ezient arbeiten kann, ist die Abstimmung von Orbitalenergien eine zweite wichtige Voraussetzung. Es werden drei neue Design-Konzepte fur Perylenmonoimid-Sensibilatoren fur Festk orper-Farbstosolarzellen (solid-state dye-sensitised solar cells - sDSSCs) untersucht. Die Synthese, die optischen und elektronischen Eigenschaften der neuen Sensibilisator- Verbindungen sowie ihre Leistungsdaten in sDSSCs werden beschrieben und diskutiert. Die in dieser Arbeit vorgestellten Konzepte reichen von der Einfuhrung von - Abstandhaltern uber neue Funktionalisierungen bis hin zur Erweiterung der Perylenmonimid Grundkorper. Der Push-Pull-Charakter der Systeme variiert von starker Kopplung bis zu kompletter Entkopplung des Donors vom Akzeptor. Dies hat einen starken Ein uss sowohl auf die Absorptionseigenschaften, als auch auf die HOMO/LUMO Energie-Niveaus der Verbindungen. Einige der Konzepte konnen auf Perylendiimide ubertragen werden. Ein Beispiel von Perylendiimid (PDI)-Farbsteuerung wird an einer Reihe von drei Terthiophen-PDIs gezeigt

Hunting for warped extra dimensions via loop-induced processes

This thesis is on loop-induced processes in theories with warped extra dimensions where the fermions and gauge bosons are allowed to propagate in the bulk, while the Higgs sector is localized on or near the infra-red brane. These so-called Randall-Sundrum (RS) models have the potential to simultaneously explain the hierarchy problem and address the question of what causes the large hierarchies in the fermion sector of the Standard Model (SM). The Kaluza-Klein (KK) excitations of the bulk fields can significantly affect the loop-level processes considered in this thesis and, hence, could indirectly indicate the existence of warped extra dimensions. The analytical part of this thesis deals with the detailed calculation of three loop-induced processes in the RS models in question: the Higgs production process via gluon fusion, the Higgs decay into two photons, and the flavor-changing neutral current b → sγ. A comprehensive, five-dimensional (5D) analysis will show that the amplitudes of the Higgs processes can be expressed in terms of integrals over 5D propagators with the Higgs-boson profile along the extra dimension, which can be used for arbitrary models with a compact extra dimension. To this end, both the boson and fermion propagators in a warped 5D background are derived. It will be shown that the seemingly contradictory results for the gluon fusion amplitude in the literature can be traced back to two distinguishable, not smoothly-connected incarnations of the RS model. The investigation of the b → sγ transition is performed in the KK decomposed theory. It will be argued that summing up the entire KK tower leads to a finite result, which can be well approximated by a closed, analytical expression.rnIn the phenomenological part of this thesis, the analytic results of all relevant Higgs couplings in the RS models in question are compared with current and in particular future sensitivities of the Large Hadron Collider (LHC) and the planned International Linear Collider. The latest LHC Higgs data is then used to exclude significant portions of the parameter space of each RS scenario. The analysis will demonstrate that especially the loop-induced Higgs couplings are sensitive to KK particles of the custodial RS model with masses in the multi tera-electronvolt range. Finally, the effect of the RS model on three flavor observables associated with the b → sγ transition are examined. In particular, we study the branching ratio of the inclusive decay B → X_s γ