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- Institut für Integrierte Naturwissenschaften, Abt. Physik (6) (remove)
Spektroskopie zweiatomiger Moleküle bei Einstrahlung ultrakurzer Laserpulse und ihre Anwendung
(2020)
Even with moderate pulse energies and average powers, ultrashort pulse lasers achieve very high peak powers, whose effect on matter is fundamentally different from that of other light sources. The high electric field strength does not only cause an increase of optically nonlinear effects such as second harmonic generation, but it is also responsible for the “cold“ ablation, which leads to colder plasmas. An investigation of these two circumstances in terms of a simplification of the pulse duration measurement and an improvement of the molecular formation in cooling plasmas is the topic of this work. In this context, it is shown that when selecting suitable process parameters, especially when purposefully defocusing the medium, the use of ultrashort pulse lasers improves the spectroscopy of several emitting molecules such as aluminum oxide. Therefore, their detection is possible even without the time-resolving spectrometers required in literature. In addition, ultrashort pulses enable spatially resolved crystallization of zinc oxide on zinc surfaces prepared by basic means. The resulting wurtzites usually align their c-axis approximately perpendicular to the underlying surface and can be used to generate scattered second harmonics. Fiber-based femtosecond lasers with pulse energies in the microjoule range, pulse durations of a few 100fs and very low maintenance requirements have proven to be a powerful instrument for these purposes. For measuring the pulse duration, the high pulse energy also enables the usage of frequency doublers with much lower conversion eciencies. Despite nonuniform crystal axes, the scattering second harmonic generating aluminum nitride has proven to be particularly suitable for optical autocorrelation. Compared to the commonly used monocrystalline beta-barium borate, the sintered aluminum nitride ceramic plates facilitate the adjustment, simplify the material handling and reduce the expenses by two to three orders of magnitude. The method developed in this work is therefore also suitable for confirmatory measurements of the pulse duration during the production process of such systems – especially when the occurring pulse energies are high or rather too high for beta-barium borate.
This dissertation presents the application of the molecular LIBS method, a novelapproach of Laser-Induced Breakdown Spectroscopy (LIBS), to optimize the detection of pitting chlorides in concrete structures, which are e.g. contaminated bydeicing salt in winter. Potentiometric titration as the standard method for chloride determination in building material analysis is costly and time-consuming. Ithas the decisive disadvantage that the determination of chloride concentrationis based on the total mass of the concrete and not on the cement content asrequired by the European standard EN 206. The imaging capabilities of LIBS forphase separation of the concrete meet this requirement. LIBS was already usedby BAM in 1998 in building material analysis, but the detection of chlorides withLIBS requires expensive helium purging and spectrometers outside the visiblespectral range to detect emissions of atomic chlorine. The approach of molecular LIBS is to quantify the emission of chloride-containing molecular radicalsformed during the cooling phase of the laser-induced plasma. The advantagescompared to conventional LIBS method are the emission in the visible spectralrange and the applicability without noble gas purging. In this thesis the influenceof the experimental components on the time behaviour of the relevant molecularemission bands is investigated, signal deviations due to plasma fluctuations aresignificantly reduced and for plasma analysis the molecular formation is simulated on atomistic scales and compared with standard methods. In simultaneousmeasurements, atomic and molecular Cl emission are directly compared and the quantification is optimized by data combination. Molecular LIBS will be extended to a quantifying and imaging method that can detect chlorides withoutnoble gas purging.
The analysis of three-dimensional and complex motion sequences
of human gait and therefore the most important question
of kenesiology ”why are we falling?” is the essence of this
paper. The gerontology and its science of movement is currently
limited to simple and one-dimensional methods and models. An
extensive literature research shows the latest state of research of
the three common methods to determine the stability of human
gait. To assess the margin of stability, local stability and orbital
stability it is shown, how those methods are applicable to evaluate
the subject’s ability to recover from any disturbences of
the subject’s gait. Based on this assessment and the method’s
advantages and disadvantages a new method will be derived
that allows spatial analysis of dynamic instability of linear and
non-linear human gait. A motion capture system and the timed
up and go test serve as a basis for this new method and will be
explained. A numerical approximation to optimise the number
of markers within a marker-set of a motion capture analysis
and its maximum correlation with the full-body-marker-set will
be given. This simplification is very helpful for further clinical
or scientific research. To validate the new method a trial
with subjects will be shown and discussed. New appreciable
variables and snapshots of specific situations during the gait
offer new and different interpretations of the human gait. The
new method is the most applicable and appropriate assessment
of human gait and the individual development of the human
gait while aging, as well as to detect and prevent falling and
associated injuries. Especially directional change of a non-linear
gait become assessable with the new method.
An assistance system has been developed for the purpuse of supporting the surgeon during the repositioning phase of intramedullary nailing of femural shaft fractures. As a basic principle the high forces generated by femural muscles and ligaments are taken by a linear sledge and a threaded rod. In order to move bone fragments directly Schanz screws are used as bone-machine interface. Two more Schanz screws are used for fine tuning orientation and position of the fragments according to the well known Joystick technique. The screws are fixed to two articulated arms, one passive and one fully robotic with manipulator. Thanks to the serial kinematic configuration of the system only minmal space of the surgeons working area gets occupied. Running a realtime operating system, the central control unit consits could be implemented as an embedded system comprising of a ARM Cortex-M0 microcontroller at it’s heart. This enables realtime computation and motor control of each joints value of the robotic arm using inverse kinematics. As inverse kinematics solver the iterative FABRIK algorithm was chosen. Serving as innovative and single user interface for the surgeon an optical force-torque sensor is used. The robotic arm always follows the surgeons motion when interacting with the sensor. Using the proposed demonstrator system a positioning resulution of <0,1mm could be accomplished. Thus by using the proposed solution during intramedullary nailing of femural shaft fractures a tremendous gain in positioning precision of bone fragments can be achieved. Furthermore a massive reduction of x-ray exposition of the surgeon is possible when applying the proposed approach. Also this approach enables the chance of cost reduction of femural fracture therapy due to reduction of needed time and staff.
Die laserinduzierte Plasmaspektroskopie (”Laser Induced Breakdown Spectroscopy”, im Folgenden auch ”LIBS” genannt) stellt eine schnelle und berührungslose Messmethode zur Elementanalyse von festen, flüssigen oder gasförmigen Stoffen unter normalen Umgebungsbedingungen ohne besondere Probenvorbereitung dar. Dazu wird ein gepulster Laser, dessen Intensität einen bestimmten Grenzwert überschreiten muss, auf eine Probe fokussiert. Das dort bestrahlte Material verdampft schlagartig und es bildet sich bei einer Temperatur von rund 10000 K ein Plasma aus. Die angeregten Atome und Ionen im Plasma strahlen bei der Rückkehr in energetisch niedrigere Zustände ein charakteristisches optisches Emissionsspektrum ab, welches über eine schnelle spektroskopische Analyse die Elementzusammensetzung des untersuchten Materials liefert. LIBS bietet in diesem Fall auch die Möglichkeit, ein unkompliziertes und bildgebendes Messverfahren aufzubauen, indem Elementverteilungen auf einer topographischen Oberfläche analysiert werden, um beispielsweise Materialübergänge, Einschlüsse oder Verschmutzungen sicher zu detektieren. Bei unebenen Oberflächen wird eine ständige Anpassung des Laserfokus an die Probenkontur benötigt, da die notwendige Intensität zur Erzeugung des Plasmas nur im Fokus aufgebracht werden kann. Als Grundlage dafür dient ein neu entwickelter Fokussieralgorithmus, der ohne jegliche Zusatzgeräte auskommt, und die Reproduzierbarkeit von LIBS-Messungen deutlich steigern kann, da die Messungen kontrolliert im Fokus stattfinden. Durch ihn ergeben sich neue Möglichkeiten des sogenannten „Element-Mappings", dem Erzeugen von Elementlandkarten, welche die Elementverteilungen in Falschfarben grafisch darstellen. Dabei ist das System nun nicht mehr auf eine ebene Oberfläche angewiesen, sondern kann beliebige Strukturen, auch mit scharfen Kanten und Löchern, sicher vermessen. Als Ergebnis erhält man ein flächiges Höhenprofil, welches zusätzlich die Elementinformationen für jeden Messpunkt beinhaltet. Dies erleichtert es dem Benutzer, gezielt Punkte von Interesse schnell wiederzufinden und zu analysieren. Die vorliegende Arbeit beschreibt die Entwicklung eines bildgebenden Low-Power-LIBSSystems mit niedriger Pulsenergie und hoher Pulsrate, welches sich mit dem dazugehörigen
Fokussieralgorithmus automatisiert an unebene Probenoberflächen anpassen kann. Als Ergebnisse werden die Analysen von ausgewählten metallhaltigen, geologischen, organischen und archäologischen Proben bzw. Fundstücken gezeigt.