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Aufbau und Inbetriebnahme einer Versuchsapparatur zur Bestimmung von Ionisationspotentialen von Clustern im Molekularstrahl
Get PDFDie hier vorgelegte Arbeit beschreibt den Aufbau einer Versuchsapparatur und die ersten daraus resultierenden, experimentellen Ergebnisse zur Bestimmung der Ionisationspotentiale von Clustern im Molekularstrahl. Der Fokus der Arbeit liegt auf der Konstruktion der Apparatur und hierbei vor allem auf der Erzeugung von vakuum-ultraviolett (engl. vacuum-ultraviolet) (VUV)-Licht, welches für die Ionisation von Metallclustern notwendig ist. Der Versuchsaufbau wird anschließend anhand von mehreren, unterschiedlichen Experimenten validiert und überprüft. Ebenso werden verschiedene Möglichkeiten präsentiert, wie der apparative Aufbau in Zukunft verbessert oder erweitert werden kann, um ein tieferes Verständnis der elektronischen Struktur der Cluster anhand ihres Ionisationsverhaltens zu bekommen. Der Aufbau der Apparatur erfolgte in zwei Stufen, welche nacheinander durchgeführt wurden: Zum einen der Aufbau eines möglichst kompakten Molekularstrahlexperiments bestehend aus einer Clusterquelle, dem Massenspektrometer zur Detektion und einer Xenon-Blitzlampe mit dazugehörigem Monochromator als Ionisationsquelle. Zum anderen aus die Vereinigung dieser Einheit mit einer bestehenden Anlage zum laserspektroskopischen Studium von Dissoziationsprozessen. Zusätzlich wurde bei dem finalen Aufbau die Blitzlampe durch ein Lasersystem ersetzt, das die hoch genaue Bestimmung von Ionisationspotentialen ermöglicht. Auch wenn die energetische Auflösung des monochromatisierten Lichts der Blitzlampe nur ausreicht, um Ionisierungspotentiale mit einer Genauigkeit von typischerweise 0,1 eV zu bestimmen, hat dieser Aufbau den großen Vorteil, dass Energien von 5,0 bis 8,5 eV angefahren werden können ohne Veränderungen an der Photoionisationsquelle vornehmen zu müssen. Um diesen Energiebereich mit dem Lasersystem abzudecken, sind verschiedene nichtlineare optische Prozesse, wie die Summenfrequenz- oder die Vierwellenmischung, notwendig und es muss eine ganze Palette an Laserfarbstoffen verwendet werden. Allerdings wird dadurch die energetische Auflösung stark verbessert, sodass Ionisierungspotentiale mit einer Genauigkeit besser 0,01 eV bestimmt werden können. Damit ist es auch möglich, Auswirkungen thermisch angeregter Schwingungszustände auf das Ionisierungsverhalten zu verfolgen.
Die Untersuchung von verschiedenen Metall-Atomen im Energiebereich von 5,5 bis 7,5 eV zeigt, dass mit Hilfe der Blitzlampe in Kombination mit dem Monochromator bereits genaue Werte bei der Bestimmung von Ionisationspotentialen erzielt werden können. Außerdem beweisen diese Messungen, dass die durchgeführte Kalibrierung des Monochromators sinnvoll und ausreichend präzise ist. Mit dem Versuchsaufbau wurden die Ionisierungspotentiale von Zinn-Clustern mit acht bis zwölf Atomen bestimmt. Die gemessene Photoionenausbeute wurde dazu mit Hilfe von quantenchemischen Simulationen analysiert. Zusätzlich war es möglich, Strukturisomere einer einzelnen Clustergröße anhand des Ionisierungsverhaltens voneinander zu unterscheiden, sofern der Unterschied im Wert der Ionisationspotentiale größer ist als die experimentelle erreichte Bandbreite der Lichtquelle. Neben reinen Zinnclustern wurde auch die Photoionenausbeute von mit einem Eisenatom dotierten Zinncluster erstmals untersucht. Hierbei zeigt sich, dass die Dotierung mit Eisen zu einer deutlichen Erhöhung des Ionisierungspotentials führt.
Nach dem Aufbau des Lasersystems wird zunächst die Summenfrequenzmischung in nichtlinearen optischen Kristallen verwendet, um Energien zwischen 5,0 und 6,5 eV zu erzeugen. Damit wurde beispielhaft das Ionisationsverhaltens des Eisen-Dimers genauer studiert. Hierbei gelang es nicht nur den Wert des adiabatischen Ionisierungspotentials mit einer Genauigkeit von 2,5 meV zu bestimmen, sondern auch den Einfluss der thermisch angeregten Schwingung auf das Ionisationsverhalten mit Hilfe eines Isotopeneffekts eindeutig nachzuweisen und die Schwingungswellenzahl des neutralen ⁵⁶Fe₂ zu (294 ± 3) cm⁻¹ zu ermitteln. Schließlich können durch die Verwendung der Vierwellenmischung in einer Xe-Gaszelle auch Energien zwischen 5,5 und 8,0 eV erzeugt werden. Damit erfolgte die erneute Messung des Ionisierungspotentials des Zinn-Atoms, nun allerdings mit einer um den Faktor 15 verbesserten Genauigkeit.
Außerdem wurde auch das Ionisierungsverhalten von Sn₁₀ erstmalig mit hoher Auflösung untersucht. Abschließend werden sowohl apparative als auch quantenchemische Probleme diskutiert und Verbesserungsvorschläge entwickelt, welche die Durchführung und Analyse der Experimente in Zukunft noch leistungsfähiger und aussagekräftiger machen werden
Silicon Drift Detectors for the Measurement and Reconstruction of Beta Spectra
Get PDFThe ASPECT-BET project, or An sdd-SPECTrometer for BETa decay studies, aims to develop a novel technique for the precise measurement of forbidden beta spectra in the 10 keV–1 MeV range. This technique employs a Silicon Drift Detector (SDD) as the main spectrometer with the option of a veto system to reject events exhibiting only partial energy deposition in the SDD. A precise understanding of the spectrometer’s response to electrons is crucial for accurately reconstructing the theoretical shape of the beta spectrum. To compute this response, GEANT4 simulations optimized for low-energy electron interactions are used and validated with a custom-made electron gun. In this article we present the performance of these simulations in reconstructing the electron spectra measured with SDDs of a ¹⁰⁹Cd monochromatic source, both in vacuum and in air. The allowed beta spectrum of a ¹⁴C source was also measured and analyzed, proving that this system is suitable for the application in ASPECT-BET
Scalable Structure Learning of Continuous-Time Bayesian Networks from Incomplete Data
Get PDFContinuous-time Bayesian Networks (CTBNs) represent a compact yet powerful framework for understanding multivariate time-series data. Given complete data, parameters and structure can be estimated efficiently in closed-form. However, if data is incomplete, the latent states of the CTBN have to be estimated by laboriously simulating the intractable dynamics of the assumed CTBN. This is a problem, especially for structure learning tasks, where this has to be done for each element of a super-exponentially growing set of possible structures. In order to circumvent this notorious bottleneck, we develop a novel gradient-based approach to structure learning. Instead of sampling and scoring all possible structures individually, we assume the generator of the CTBN to be composed as a mixture of generators stemming from different structures. In this framework, structure learning can be performed via a gradient-based optimization of mixture weights. We combine this approach with a new variational method that allows for a closed-form calculation of this mixture marginal likelihood. We show the scalability of our method by learning structures of previously inaccessible sizes from synthetic and real-world data
Nanoscale pores introduced into paper via mesoporous silica coatings using sol–gel chemistry
Get PDFMesopores, with diameters between 2 and 50 nm, not only increase the specific surface area, but also generate hierarchically porous materials with specific properties such as capillary fluid transport, ion specific pore accessibility, or size exclusion. Paper is a strongly hierarchical, porous material with specific properties, such as capillary force-driven fluid transport. However, paper fibers change their morphology during the initial step of wood disintegration. This results in changes of the porous fiber structure. In particular paper fibers loose their mesopores during the final drying step in the fabrication process. Here, we investigate silica mesopore formation in paper by sol–gel chemistry and evaporation induced self-assembly to specifically introduce and rationally design mesopore formation and distribution in cotton linter and eucalyptus sulfate paper sheets. We demonstrate the importance of synchronizing the solvent evaporation rate and capillary fluid velocity to ensure mesopore formation as well as the influence of the fiber type and sol–gel solution composition. The combination of argon and krypton sorption, SAXS, TEM and CLSM provides systematic analysis of the porous structure and the silica distribution along the cellulose paper fiber length and cross-section. These results provide a deeper understanding of mesopore formation in paper and how the latter is influenced by paper fluidic properties
Single-fibre coating and additive manufacturing of multifunctional papers
Get PDFPaper-based materials with precisely designed wettabilities show great potential for fluid transport control, separation, and sensing. To tune the wettability of paper, paper sheets are usually modified after the paper manufacturing process. This limits the complexity of the local wettability design. We combined the wettability design of the individual fibres with subsequent paper sheet fabrication through either fibre deposition or fibre printing. Using silica-based cellulose fibre functionalization, the wettability of the paper sheets, containing only one specific fibre type, could be gradually tuned from highly hydrophilic to highly hydrophobic, resulting in water exclusion. The development of a silica-functionalized fibre library containing mesoporous or dense silica coatings, as well as silica with varying precursor compositions, further enabled the variation of the paper wettability and fluid flow. By combining this fibre library with the paper fabrication process by (i) fibre deposition or (ii) fibre printing, the paper wettability architecture and thus the local fibre composition were adjusted without any further processing steps. This enabled the fabrication of papers with wettability integration, such as a wettability pattern or a Janus paper design, containing wettability gradients along the paper sheet cross section. This asymmetric wettability along all three spatial dimensions enabled side-selective oil–water separation
Structural insights and biomedical potential of IgNAR scaffolds from sharks
Get PDFIn addition to antibodies with the classical composition of heavy and light chains, the adaptive immune repertoire of sharks also includes a heavy-chain only isotype, where antigen binding is mediated exclusively by a small and highly stable domain, referred to as vNAR. In recent years, due to their high affinity and specificity combined with their small size, high physicochemical stability and low-cost of production, vNAR fragments have evolved as promising target-binding scaffolds that can be tailor-made for applications in medicine and biotechnology. This review highlights the structural features of vNAR molecules, addresses aspects of their generation using immunization or in vitro high throughput screening methods and provides examples of therapeutic, diagnostic and other biotechnological applications
Cattle-derived knob paratopes grafted onto peripheral loops of the IgG1 Fc region enable the generation of a novel symmetric bispecific antibody format
Get PDFIn this work we present a novel symmetric bispecific antibody format based on engraftments of cattle-derived knob paratopes onto peripheral loops of the IgG1 Fc region. For this, knob architectures obtained from bovine ultralong CDR-H3 antibodies were inserted into the AB loop or EF loop of the CH3 domain, enabling the introduction of an artificial binding specificity into an IgG molecule. We demonstrate that inserted knob domains largely retain their binding affinities, resulting into bispecific antibody derivatives versatile for effector cell redirection. Essentially, generated bispecifics demonstrated adequate biophysical properties and were not compromised in their Fc mediated functionalities such as FcRn or FcγRIIIa binding
Improving Static and Dynamic Vulnerability Analysis using Values and Dataflows
Get PDFAn increasing amount of sensitive information is processed and stored in computer systems, particularly on mobile phones and cloud-based web services. While modern mobile operating systems such as Android employ techniques to protect the user data
on the system level, millions of Android applications have access to this sensitive information, such as photos, chats, e-mails, and calendars. Vulnerabilities in these applications can lead to a data breach or impact the integrity of the data. Due to the massive amount of applications available in modern application stores, performing manual security tests on all of them is impractical. While fully automatic vulnerability scanners exist, they are usually prone to false positives, which affects their usability. Furthermore, existing scanners often miss vulnerabilities, particularly when more advanced techniques such as reflective calls and intercomponent
communication are involved.
In this dissertation, we propose a static and dynamic framework to improve the precision and recall of our vulnerability scanner VUSC, which supports scanning Android and Java web applications. VUSC uses value analyses to resolve reflective and intercomponent communication calls to obtain a more complete call graph. Furthermore, VUSC requires value analyses to find concrete vulnerabilities
such as insecure URL protocols (e.g. HTTP), insecure cryptographic algorithms (e.g., MD5) as well as hardcoded cryptographic keys.
Therefore, we propose a novel approach called ValDroid to extract precise values statically. In order to compute values, ValDroid performs static program slicing to obtain paths and simulates the statements of this path using library models. Unlike existing static approaches, it handles loops as separate block entities, ensuring that the semantics of the original loop are preserved. This technique ensures that sliced paths are semantically equivalent to paths in the original program, which leads to more precise results.
We demonstrate that ValDroid performs better than comparable approaches on the JSA value benchmark suite. However, the
JSA value benchmark suite lacks some challenges regarding loops and arrays. As such, we propose a value benchmark suite called
ValBench. Then, we show that ValDroid performs better on ValBench than its competitors. Furthermore, we demonstrate that
ValDroid achieves higher precision and recall than other approaches on our dataset of real-world applications.
Finally, we show how ValDroid improves VUSC’s scanning ability on popular Java-based vulnerability benchmark suites and
real-world applications
Investigation of dynamic responses of high-speed railway bridges under consideration of moving load models and multi-body models
Get PDFThe investigation of dynamic responses of high-speed railway bridges is increasingly important due to the expanding stock of high-speed trains. Developments of new trains lead to insufficient coverage of dynamic effects by standards on bridge design. In this dissertation, the dynamic responses of railway bridges are studied based on extensive sets of moving load models as well as 2D multi-body models of high-speed passenger trains.
Moving load models allow a relatively fast evaluation of dynamic effects during bridge crossings and only require information of static axle loads and axle distances, which is available for a high number of trains. The moving load models of the currently operating high-speed passenger trains in the European network are used for the investigation of resonance excitations of bridges. The consideration of vehicle-bridge interaction effects requires a more detailed modeling of the trains using multi-body models with additional information about springs, dampers, and masses of the train. Multi-body models allow to take into account dynamic axle loads due to the movement of the train itself and therefore lead to calculation results closer to reality.
More than 3,000 moving load models for passenger trains are assembled and railway bridge responses are investigated using simplified calculation methods. Based on the dynamic train signature envelope of this train catalog, a new dynamic load model for high-speed trains is developed aiming for only 20 model trains to represent the dynamic effects of all operating passenger trains. Additionally, a preselection procedure for dynamically relevant trains is developed based on the simplified calculation methods of train signature and train spectra. The resulting 500 dynamically relevant trains can be used for more complex computations of dynamic bridge responses.
Vehicle-bridge interaction effects are recognized by comparing bridge responses of multi-body models, which cover interaction effects inherently, and moving load models, for which interaction effects can only be considered by an additional damping value which has to be calibrated. In calculations with moving loads the bridge damping ratio is increased by the additional damping value to account for the vehicle-bridge interaction. For the investigation of the vehicle-bridge interaction, multi-body models for more than 30 passenger trains (including conventional trains, trains with articulated bogies, and trains with equidistant wheelsets) are created. The algorithm for interaction calculations is implemented in MATLAB and the investigations are based on a parametric bridge set of about 100 single-span bridges. As only a small number of multi-body model parameters is publicly available and the correct representation of the dynamic train characteristics is uncertain, the eigenfrequencies of the theoretical multi-body models are validated based on measurements on stationary and moving trains. Beyond this, the influence of uncertainties in the data for stiffness and damping parameters in the multi-body models on the vehicle-bridge interaction is studied using a sensitivity analysis. The results of the comprehensive computational series of more than 1,000,000 bridge crossings are used to assess the validity of the additional damping method included in Eurocode 1 as well as further approaches for the additional damping given in literature.
This dissertation provides a new dynamic load model of 20 model trains based on the dynamic signature envelope of more than 3,000 operating passenger trains. The investigations of vehicle-bridge interaction effects lead to the conclusion that special attention should be paid to the primary spring parameter in the definition of multi-body models and an additional damping value needs to be defined based on detailed train characteristics
Grain Boundary Transport in the Argyrodite‐Type Li₆PS₅Br Solid Electrolyte: Influence of Misorientation and Anion Disorder on Li Ion Mobility
Get PDFTo realize efficient solid‐state batteries, many efforts are directed toward maximizing the bulk Li⁺ conductivity of sulfide superionic conductors, as demonstrated for the argyrodite‐type materials Li₆PS₅Cl and Li₆PS₅Br. Notably, in these archetype materials, the fast Li⁺ transport benefits from considerable anion disorder on the halide and sulfur sublattices. To further improve the Li⁺ conductivity, however, one must consider not only the bulk properties of the solid electrolyte (SE) but also microstructural aspects. It is, however, controversially discussed whether grain boundary (GB) transport is generally detrimental for the overall ion conductivity in agyrodite‐type SEs. Thus, by means of atomistic computer simulations, the Li⁺ ion transport is studied in twist and tilt GBs of Li₆PS₅Br, revealing that the Br/S site exchange determines whether the presence of GBs deteriorates the ionic conductivity: Whereas the material with 0% Br/S site exchange only shows locally limited bulk diffusion but enhanced GB conductivity, at higher degrees of site exchange, GBs deteriorate Li⁺ diffusion. These results show that the interplay of GB transport directly depends on the degree of site exchange in argyrodite‐type materials