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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Investigating Origins of Anomalous Behavior in Single Molecule Translational Measurements of Polystyrene Near its Glass Transition Temperature

Yang, Han January 2024 (has links)
Rotational-translational decoupling, a phenomenon commonly observed in supercooled liquids, has been a topic of great interest. Despite its prevalence, the underlying cause of this phenomenon, often attributed to dynamic heterogeneity, has not been conclusively elucidated. This thesis investigates and evaluates how dynamic heterogeneity may lead to this decoupling using simultaneous single-molecule rotational and translational measurements. In the experimental study, single molecule fluorescence imaging experiments are performed on the ideal probe N,N’-dipentyl-3,4,9,10-perylenedicarboximide in high molecular weight polystyrene near its glass transition temperature. A novel trajectory linking method based on hierarchical clustering is developed to facilitate single molecule tracking even in imaging data where specific molecules cannot be observed visually for a substantial number of frames. This linking algorithm then allows molecules to be localized over full movies, such that rotational and translational measurements can be compared over comparable timespans. The investigation of translational dynamics using such long trajectories, which was not previously achieved, reveals that both rotational-translational decoupling and translational enhancement persist on the single molecule level, supporting the hypothesis that temporally heterogeneous dynamics experienced by the probe molecules is a contributing factor in observed rotational-translational breakdown in both ensemble and single molecule studies. A tendency towards dynamical convergence between subgroups with fast and slow dynamics is observed, demonstrating temporal heterogeneity at the single molecule level. In comparison to rotational dynamics, translational dynamics was discovered to have a longer lifetime. Other key observations facilitated by the linked trajectory analysis include that apparent diffusion coefficient of probe molecules decreases with longer observation time, a finding inconsistent with normal diffusive behavior. To investigate the origin of this anomalous slowing in single molecule studies existing alongside the observed overall enhancement in translational motion, temporally heterogeneous models with multiple types of correlation were studied via simulations. The results emphasize the critical role that bias in translational and rotational measurements can play when investigating and observing dynamic heterogeneity, as nearly all models including dynamic heterogeneity show increasing diffusion coefficient with increasing number of dynamic environments explored. Strikingly, translational enhancement is evident in single molecule translational simulations even when slow dynamics are reinforced via positive correlation in the models. A comparison of the diffusion coefficient evolution between simulations and experiments reveals that the sub-diffusive continuous time random walk model is the most plausible candidate to account for the set of observations seen in experiment.
2

Ultraschnelle Dynamik in Flüssigkeiten

Laurent, Thomas 23 October 2000 (has links)
Die vorliegende Arbeit untersucht ultraschnelle Rotations- und Translationsbewegungen in molekularen Flüssigkeiten. Dazu wurde deren Optischer Kerr-Effekt/Raman-Induzierter Kerr-Effekt (OKE/RIKE) zeitaufgelöst mithilfe der Pump/Probetechnik gemessen. Die erzielte Zeitauflösung betrug 30 fs. Langzeitschwankungen des Signals konnten zusätzlich mit einer Echtzeit-Meßtechnik eliminiert werden. In der transienten Doppelbrechung wurde so ein Signal/Rausch Verhältnis von 10^7 erreicht. Das Ziel war eine möglichst genaue Beschreibung der Responsfunktion für den Optischen Kerr-Effekt der Flüssigkeiten Chloroform, Acetonitril, Trichloracetonitril, Tetrachlorkohlenstoff, Methylchloroform und Fluoroform. Die Fourier-Transformation dieser Responsfunktion entspricht dem depolarisierten Raman Spektrum der Flüssigkeit. Die Responsfunktion wurde auf zwei Wegen aus dem gemessenen OKE Signal erhalten: a) in der Zeitdomäne durch Anpassung empirisch gewählter Terme und b) in der Frequenzdomäne durch Anpassung von Brownschen Oszillatormoden. Die Analyse bezieht den gesamten Datenumfang ein, ohne numerisch problematische Entfaltungsverfahren nutzen zu müssen. In den untersuchten Systemen erfolgt die elektronische Antwort instantan auf den anregenden Laserimpuls. Innerhalb der ersten halben Pikosekunde beobachtet man eine intermolekulare Dynamik, die auf Librationen und kollisionsinduzierte Translationsbewegungen zurückgeht. Diese nur schwer unterscheidbare Dynamik verschwindet typischerweise mit Zeitkonstanten bis 250 fs. Die Langzeitrelaxation in den isotropen Zustand wird der diffusiven Reorientierung zugeordnet. Es werden hierfür Zeitkonstanten tau zwischen 1.2 (Methylchloroform) und 3 ps (Chloroform) beobachtet. Durch die hohe Bandbreite des Laserimpulses werden außerdem niederfrequente Raman-Linien (bis ca. 750 cm^-1) angeregt. Diese äußern sich durch untergedämpfte Signaloszillationen (RIKE). Erstmals wird der OKE von flüssigem Fluoroform untersucht. Aufgrund seiner Eigenschaften kommt CHF3 der Modellvorstellung einer polaren harten Kugel sehr nahe. Das beobachtete Signal ist ca. 110 mal schwächer als in Chloroform. Die diffusive Reorientierung verläuft auß erdem deutlich schneller (tau = 0.8 ps). Der (intermolekulare) OKE wächst nun mit steigender Polarisierbarkeitsanisotropie, während der (intramolekulare) RIKE von der änderung der Polarisierbarkeitsanisotropie mit der Schwingungskoordinate des Moleküls abhängt. Durch vergleichende Messungen mit isotropen Tetrachlorkohlenstoff kann auch prinzipiell der Einfluß der kollisionsinduzierten Effekte (CI) in den untersuchten Flüssigkeiten abgeschätzt werden. Rotations- und Translationsbewegungen korrelieren allerdings miteinander. Die daraus resultierenden Kreuzterme sind dann aus den Responsfunktionen allein nicht mehr zu ermitteln. Sie sind nur aus weiterführenden moleküldynamischen Simulationen erhältlich. Für Acetonitril wird eine übereinstimmung zwischen experimenteller und theoretisch vorhergesagter OKE Responsfunktion gefunden. Die OKE Responsfunktionen sind mit den Relaxationsfunktionen der dielektrischen Relaxation und der Solvatation von polaren Sondenmolekülen verknüpft. Die Messungen des zeitaufgelösten Optischen Kerr-Effektes sollen hier künftig dazu dienen, die reinen Flüssigkeitsbeiträge zur polaren und nichtpolaren Solvatation zu erkennen. / Within this work the ultrafast rotational and translational motions of molecular liquids are investigated. Therefore their Optical Kerr effect/Raman induced Kerr effect (OKE/RIKE) was measured time-resolved with the pump/probe technique. Achieved time resolution was 30 fs. Long-time fluctuations of the signal might be eliminated by an additional realtime measurement technique. Hence a signal/noise ratio of ca. 10^7 in the transient birefringence was obtained. A detailed description of the response function derived from Optical Kerr effect was targeted for the liquids chloroform, acetonitrile, trichloroacetonitrile, carbon tetrachloride, 1,1,1-trichloroethane and fluoroform. The Fourier-Transformation of the response functions of the liquids is eqivalent to their depolarized Raman spectra. The response function is obtained from measured signals in two ways: a) by fitting empirical terms in time domain and b) fitting Brownian oscillator modes in frequency domain. Analysis includes complete data range without using numerical deconvolution techniques. In all investigated systems an electronical response follows to the stimulating laser pulse instantaneously. Within the first half pico second one observes intermolecular dynamics due to librational and collision-induced translational motions. These (hard to distinguish) dynamics disappear with typical time-constants of up to 250 fs. The long-time relaxation into an isotropic distribution of molecules is termed diffusive reorientation. Here time-constants tau between 1.2 (CCl3CH3) and 3 ps (CHCl3) are observed. Additionally low-frequent Raman lines (up to 750 cm^-1) may be stimulated due to the high pulse bandwidth, resulting in underdamped signal oscillations (RIKE). The OKE of liquid fluoroform is investigated for the first time. It comes closest to the Mean Spherical Approximation model for a fluid composed of polar, nonpolarizable hard spheres. The observed signal is ca. 110 times weaker than in chloroform. The diffusional reorientation occurs also faster (tau = 0.8 ps). Generally (intermolecular) OKE rises with growing polarizability anisotropy, while (intramolecular) RIKE depends from the change of polarizability anisotropy with the vibrational coordinate. Influence of collision-induced effects (CI) is derived in principle from compared measurements in isotropic carbon tetrachloride. However a correlation exists between rotational and translational motion. Resulting cross-terms cannot be obtained from response functions alone. These are only available in molecular-dynamic simulations in literature. In acetonitril one finds similar response functions derived from OKE measurements and predicted theoretical simulation. The OKE respons functions are related to relaxation functions of the dielectrical relaxation and the solvation of polar molecules. Time-resolved measurements of the OKE reveal here contributions of the pure liquid in polar and nonpolar solvation.

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