Messung des Landé-gJ-Faktors im Grundzustand von Ca + und Untersuchung des Speicherverhaltens von Elektronen in einem Penningkäfig

Ionenkäfige und speziell Penningfallen stellen sich in der Atomphysik als außergewöhnliche Werkzeuge heraus. Zum einen bieten diese 'Teilchencontainer' die Möglichkeit atomphysikalische Präzisionsmessungen durchzuführen und zum anderen stellen Penningfallen schwingungsfähige Systeme dar, in welchen nichtlineare dynamische Prozesse an gespeicherten Teilchen untersucht werden können. In einem ersten Teil der Arbeit wurde mit der in der Atomphysik bekannten Methode der optischen Mikrowellen-Doppelresonanz Spektroskopie der elektronische g-Faktor von Ca+ mit einer Genauigkeit von 4*10^{-8} zu gJ=2,00225664(9) bestimmt. g-Faktoren von Elektronen in gebundenen ionischen Systemen sind fundamentale Größen der Atomphysik, die Informationen über die atomare Wellenfunktion des zu untersuchenden Zustandes liefern. In einem zweiten Teil der Arbeit wurde hinsichtlich der Untersuchungen zur nichtlinearen Dynamik von parametrisch angeregten gespeicherten Elektronen beobachtet, dass ab bestimmten kritischen Teilchendichten in der Penningfalle die gespeicherten Elektronen kollektive Eigenschaften manifestieren. Weiterhin wurde bei der Anregung der axialen Eigenbewegung ein Schwellenverhalten der gemessenen Subharmonischen zur 2*omega_z-Resonanz beobachtet. Dieser Schwelleneffekt lässt sich mit der Existenz eines Dämpfungsmechanismus erklären, der auf die Elektronenwolke einwirkt, so dass eine Mindestamplitude der Anregung erforderlich ist, um diese Dämpfung zu überwinden. Durch Bestimmung der charakteristischen Kurven der gedämpften Mathieuschen Differentialgleichung konnte das beobachtete Phänomen theoretisch verstanden werden.

Magnetization dynamics in spin valves

Key technology applications like magnetoresistive sensors or the Magnetic Random Access Memory (MRAM) require reproducible magnetic switching mechanisms. i.e. predefined remanent states. At the same time advanced magnetic recording schemes push the magnetic switching time into the gyromagnetic regime. According to the Landau-Lifschitz-Gilbert formalism, relevant questions herein are associated with magnetic excitations (eigenmodes) and damping processes in confined magnetic thin film structures.rnObjects of study in this thesis are antiparallel pinned synthetic spin valves as they are extensively used as read heads in today’s magnetic storage devices. In such devices a ferromagnetic layer of high coercivity is stabilized via an exchange bias field by an antiferromagnet. A second hard magnetic layer, separated by a non-magnetic spacer of defined thickness, aligns antiparallel to the first. The orientation of the magnetization vector in the third ferromagnetic NiFe layer of low coercivity - the freelayer - is then sensed by the Giant MagnetoResistance (GMR) effect. This thesis reports results of element specific Time Resolved Photo-Emission Electron Microscopy (TR-PEEM) to image the magnetization dynamics of the free layer alone via X-ray Circular Dichroism (XMCD) at the Ni-L3 X-ray absorption edge.rnThe ferromagnetic systems, i.e. micron-sized spin valve stacks of typically deltaR/R = 15% and Permalloy single layers, were deposited onto the pulse leading centre stripe of coplanar wave guides, built in thin film wafer technology. The ferromagnetic platelets have been applied with varying geometry (rectangles, ellipses and squares), lateral dimension (in the range of several micrometers) and orientation to the magnetic field pulse to study the magnetization behaviour in dependence of these magnitudes. The observation of magnetic switching processes in the gigahertz range became only possible due to the joined effort of producing ultra-short X-ray pulses at the synchrotron source BESSY II (operated in the so-called low-alpha mode) and optimizing the wave guide design of the samples for high frequency electromagnetic excitation (FWHM typically several 100 ps). Space and time resolution of the experiment could be reduced to d = 100 nm and deltat = 15 ps, respectively.rnIn conclusion, it could be shown that the magnetization dynamics of the free layer of a synthetic GMR spin valve stack deviates significantly from a simple phase coherent rotation. In fact, the dynamic response of the free layer is a superposition of an averaged critically damped precessional motion and localized higher order spin wave modes. In a square platelet a standing spin wave with a period of 600 ps (1.7 GHz) was observed. At a first glance, the damping coefficient was found to be independent of the shape of the spin-valve element, thus favouring the model of homogeneous rotation and damping. Only by building the difference in the magnetic rotation between the central region and the outer rim of the platelet, the spin wave becomes visible. As they provide an additional efficient channel for energy dissipation, spin waves contribute to a higher effective damping coefficient (alpha = 0.01). Damping and magnetic switching behaviour in spin valves thus depend on the geometry of the element. Micromagnetic simulations reproduce the observed higher-order spin wave mode.rnBesides the short-run behaviour of the magnetization of spin valves Permalloy single layers with thicknesses ranging from 3 to 40 nm have been studied. The phase velocity of a spin wave in a 3 nm thick ellipse could be determined to 8.100 m/s. In a rectangular structure exhibiting a Landau-Lifschitz like domain pattern, the speed of the field pulse induced displacement of a 90°-Néel wall has been determined to 15.000 m/s.rn

New biosensor applications of surface plasmon and hydrogel optical waveguide spectroscopy

Rapid and sensitive detection of chemical and biological analytes becomes increasingly important in areas such as medical diagnostics, food control and environmental monitoring. Optical biosensors based on surface plasmon resonance (SPR) and optical waveguide spectroscopy have been extensively pushed forward in these fields. In this study, we combine SPR, surface plasmon-enhanced fluorescence spectroscopy (SPFS) and optical waveguide spectroscopy with hydrogel thin film for highly sensitive detection of molecular analytes.rnrnA novel biosensor based on SPFS which was advanced through the excitation of long range surface plasmons (LRSPs) is reported in this study. LRSPs are special surface plasmon waves propagating along thin metal films with orders of magnitude higher electromagnetic field intensity and lower damping than conventional SPs. Therefore, their excitation on the sensor surface provides further increased fluorescence signal. An inhibition immunoassay based on LRSP-enhanced fluorescence spectroscopy (LRSP-FS) was developed for the detection of aflatoxin M1 (AFM1) in milk. The biosensor allowed for the detection of AFM1 in milk at concentrations as low as 0.6 pg mL-1, which is about two orders of magnitude lower than the maximum AFM1 residue level in milk stipulated by the European Commission legislation.rnrnIn addition, LRSPs probe the medium adjacent to the metallic surface with more extended evanescent field than regular SPs. Therefore, three-dimensional binding matrices with up to micrometer thickness have been proposed for the immobilization of biomolecular recognition elements with large surface density that allows to exploit the whole evanescent field of LRSP. A photocrosslinkable carboxymethyl dextran (PCDM) hydrogel thin film is used as a binding matrix, and it is applied for the detection of free prostate specific antigen (f-PSA) based on the LRSP-FS and sandwich immunoassay. We show that this approach allows for the detection of f-PSA at low femto-molar range, which is approximately four orders of magnitude lower than that for direct detection of f-PSA based on the monitoring of binding-induced refractive index changes.rnrnHowever, a three dimensional hydrogel binding matrix with micrometer thickness can also serve as an optical waveguide. Based on the measurement of binding-induced refractive index changes, a hydrogel optical waveguide spectroscopy (HOWS) is reported for a label-free biosensor. This biosensor is implemented by using a SPR optical setup in which a carboxylated poly(N-isoproprylacrylamide) (PNIPAAm) hydrogel film is attached on a metallic surface and modified by protein catcher molecules. Compared to regular SPR biosensor with thiol self-assembled monolayer (SAM), HOWS provides an order of magnitude improved resolution in the refractive index measurements and enlarged binding capacity owing to its low damping and large swelling ratio, respectively. A model immunoassay experiment revealed that HOWS allowed detection of IgG molecules with a 10 pM limit of detection (LOD) that was five-fold lower than that achieved for SPR with thiol SAM. For the high capacity hydrogel matrix, the affinity binding was mass transport limited.rnrnThe mass transport of target molecules to the sensor surface can play as critical a role as the chemical reaction itself. In order to overcome the diffusion-limited mass transfer, magnetic iron oxide nanoparticles were employed. The magnetic nanoparticles (MNPs) can serve both as labels providing enhancement of the refractive index changes, and “vehicles” for rapidly delivering the analytes from sample solution to an SPR sensor surface with a gradient magnetic field. A model sandwich assay for the detection of β human chorionic gonadotropin (βhCG) has been utilized on a gold sensor surface with metallic diffraction grating structure supporting the excitation of SPs. Various detection formats including a) direct detection, b) sandwich assay, c) MNPs immunoassay without and d) with applied magnetic field were compared. The results show that the highly-sensitive MNPs immunoassay improves the LOD on the detection of βhCG by a factor of 5 orders of magnitude with respect to the direct detection.rn

Advanced schemes for surface plasmon resonance and plasmon-enhanced fluorescence biosensors

Advanced optical biosensor platforms exploiting long range surface plasmons (LRSPs) and responsive N-isopropylacrylamide (NIPAAm) hydrogel binding matrix for the detection of protein and bacterial pathogen analytes were carried out. LRSPs are optical waves that originate from coupling of surface plasmons on the opposite sites of a thin metallic film embedded between two dielectrics with similar refractive indices. LRSPs exhibit orders of magnitude lower damping and more extended profile of field compared to regular surface plasmons (SPs). Their excitation is accompanied with narrow resonance and provides stronger enhancement of electromagnetic field intensity that can advance the sensitivity of surface plasmon resonance (SPR) and surface plasmon-enhanced fluorescence spectroscopy (SPFS) biosensors. Firstly, we investigated thin gold layers deposited on fluoropolymer surface for the excitation of LRSPs. The study indicates that the morphological, optical and electrical properties of gold film can be changed by the surface energy of fluoropolymer and affect the performance of a SPFS biosensor. A photo-crosslinkable NIPAAm hydrogel was grafted to the sensor surface in order to serve as a binding matrix. It was modified with bio-recognition elements (BREs) via amine coupling chemistry and offered the advantage of large binding capacity, stimuli responsive properties and good biocompatibility. Through experimental observations supported by numerical simulations describing diffusion mass transfer and affinity binding of target molecules in the hydrogel, the hydrogel binding matrix thickness, concentration of BREs and the profile of the probing evanescent field was optimized. Hydrogel with a up to micrometer thickness was shown to support additional hydrogel optical waveguide (HOW) mode which was employed for probing affinity binding events in the gel by means of refractometric and fluorescence measurements. These schemes allow to reach limits of detection (LODs) at picomolar and femtomolar levels, respectively. Besides hydrogel based experiments for detection of molecular analytes, long range surface plasmon-enhanced fluorescence spectroscopy (LRSP-FS) was employed for detection of bacterial pathogens. The influence of capture efficiency of bacteria on surfaces and the profile of the probing field on sensor response were investigated. The potential of LRSP-FS with extended evanescent field is demonstrated for detection of pathogenic E. coli O157:H7 on sandwich immunoassays . LOD as low as 6 cfu mL-1 with a detection time of 40 minutes was achieved.rn

Gespeicherte Ionen in der Paulfalle: Voruntersuchungen zur Lebensdauermessung des metastabilen 6D-3/2-Zustands des 226 Ra +, Speichereigenschaften von 138 Ba + und 28 N 2 + bei Variation des Puffergasdrucks

Auf Paulfallen basierende Experimente spielen eine wichtige Rolle in verschiedenen Bereichen der Physik, z.B. der Atomphysik zum Test theoretischer Modelle und der Massenspektroskopie. Die vorliegende Arbeit widmet sich beiden Themengebieten und gliedert sich entsprechend in zwei Teilbereiche: 1) Erdalkali-Ionen sind aufgrund ihrer Energieniveaus optimale Kandidaten für Laserspektroskopie-Experimente mit Ionenfallen und bestens geeignet, um mittels der spektroskopischen Daten die theoretischen Modelle zu testen. Lediglich für Ra+ fehlen bislang als einzigem Erdalkali-Ion diese Daten wie z.B. die Lebensdauern der metastabilen Niveaus. Diese wären auch von Interesse für bereits geplante Radium-Experimente zur Paritätsverletzung. Im ersten Teil dieser Arbeit wird der Aufbau eines Laser-Paulfallenexperiments zur Messung der Lebensdauer des 6D3/2 Zustands von 226Ra+ dokumentiert und es werden Testmessungen mit 138Ba+ vorgestellt. 2) Für die Verwendung der Paulfalle in der Massenspektroskopie und zur Analyse von Reaktionsprodukten ist die Kenntnis der Lage der im Speicherbereich auftretenden nichtlinearen Resonanzen wesentlich, ebenso wie deren Veränderung durch Dämpfung und Raumladung. Im zweiten Teil dieser Arbeit werden detaillierte Untersuchungen der Speicherung großer puffergasgekühlter Ionenwolken an zwei unterschiedlichen Paulfallen-Experimenten vorgestellt. Am ersten wurden 138Ba+-Ionenwolken kontinuierlich durch Laserspektroskopie bzw. über einen elektronischen Nachweis beobachtet, während das zweite N2+-Molekülionen automatisiert destruktiv nachwies. Am N2+-Experiment wurden zwei hochaufgelöste Messungen des ersten Speicherbereichs durchgeführt, die erstmals eine direkte Überprüfung der theoretisch berechneten Verläufe der Resonanzen mit experimentellen Daten erlauben. Die Nachweiseichung ermöglichte dabei zum ersten Mal die Angabe absoluter Ionenzahlen. Im Gegensatz zu vergleichbaren früheren Messungen wurden hierbei die sich überlagernden Speicherbereiche von 4 simultan gespeicherten Ionensorten beobachtet und zur Analyse der Resonanzen herangezogen. Die nichtlinearen Resonanzen wurden untersucht bei Variation von Puffergasdruck und Ionenzahl, wobei kollektive Resonanzen ohne zusätzliche externe Anregung beobachtet wurden. Die gemessenen Raumladungsverschiebungen wurden mit theoretischen Modellen verglichen. Bei Variation des Puffergasdrucks wurde mit Bariumionen die räumliche Ausdehnung der Ionenwolke gemessen und mit Stickstoffionen die Verschiebung des Punktes optimaler Speicherung bestimmt. Dabei wurde festgestellt, daß der zum Ioneneinfang optimale Puffergasdruck kleiner ist als der, bei dem die längsten Speicherdauern erzielt werden. Mit gespeicherten N2+-Ionen wurde die Position extern angeregter kollektiver und individueller Resonanzen im Frequenzspektrum bei Änderung der Parameter Ionenzahl, Puffergasdruck und Dauer der Anregung untersucht, ebenso wie die Resonanzform, die mit theoretischen Linienformen verglichen wurde. Bei Änderung der Fallenparameter wurden verstärkende Effekte zwischen nahen kollektiven Resonanzen festgestellt. Die Beobachtung, welche der im Frequenzspektrum vorher identifizierten Bewegungs-Resonanzen sich bei Variation der Fallenparameter a bzw. q überlagern, ermöglicht eine bislang nicht beschriebene einfache Methode der Bestimmung von nichtlinearen Resonanzen im Stabilitätsdiagramm.

Synthesis and application of stimuli responsive tunable systems based on physical interactions

Novel single step synthetic procedure for hydrophobically modified alkali soluble latexes (HASE) via a miniemulsion-analogous method is presented. This facile method simplifies the copolymerization of the monomers with basically “opposite” character in terms of their hydrophilic/hydrophobic nature, which represent one of the main challenges in water based systems. Considered systems do not represent classical miniemulsions due to a high content of water soluble monomers. However, the polymerization mechanism was found to be rather similar to miniemulsion polymerization process.rnThe influence of the different factors on the system stability has been investigated. The copolymerization behavior studies typically showed strong composition drifts during copolymerization. It was found that the copolymer composition drift can be suppressed via changing the initial monomer ratio.rnThe neutralization behavior of the obtained HASE systems was investigated via potentiometric titration. The rheological behavior of the obtained systems as a function of the different parameters, such as pH, composition (ultrahydrophobe content) and additive type and content has also been investigated.rnDetailed investigation of the storage and loss moduli, damping factor and the crossover frequencies of the samples showed that at the initial stages of the neutralization the systems show microgel-like behavior.rnThe dependence of the rheological properties on the content and the type of the ultrahydrophobe showed that the tuning of the mechanical properties can be easily achieved via minor (few percent) but significant changes in the content of the latter. Besides, changing the hydrophobicity of the ultrahydrophobe via increasing the carbon chain length represents another simple method for achieving the same results.rnThe influence of amphiphilic additives (especially alcohols) on the rheological behavior of the obtained systems has been studied. An analogy was made between micellation of surfactants and the formation of hydrophobic domains between hydrophobic groups of the polymer side chain.rnDilution induced viscosity reduction was investigated in different systems, without or with different amounts or types of the amphiphilic additive. Possibility of the controlled response to dilution was explored. It was concluded that the sensitivity towards dilution can be reduced, and in extreme cases even the increase of the dynamic modulus can be observed, which is of high importance for the setting behavior of the adhesive material.rnIn the last part of this work, the adhesive behavior of the obtained HASE systems was investigated on different substrates (polypropylene and glass) for the standard labeling paper. Wet tack and setting behavior was studied and the trends for possible applications have been evaluated.rnThe novel synthetic procedure, investigation of rheological properties and the possibility of the tuning via additives, investigated in this work create a firm background for the development of the HASE based adhesives as well as rheology modifiers with vast variety of possible applications due to ease of tuning the mechanical and rheological properties of the systems.