@phdthesis{Knorr2015,
  author    = {Knorr, Johannes Walter},
  title     = {Femtosecond spectroscopy of photolysis reactions in the liquid phase},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-131362},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {Within the framework of this thesis, photolysis reactions in the liquid phase were investigated by means of ultrafast optical spectroscopy. Apart from molecular studies dealing with the highly spin-dependent reactivity of diphenylcarbene (DPC) in binary solvent mixtures and ligand dissociation reactions of so-called CO-releasing molecules (CORMs), special emphasis was put on the implementation and characterization of methods improving and extending the signal detection in conventional pump-probe transient absorption setups. The assumption of DPC being an archetypal triplet-ground-state arylcarbene was recently questioned by matrix-isolation studies at low temperatures. DPC embedded in argon matrices revealed a hitherto unknown reactivity when the carbene environment was modified by small amounts of methanol dopant molecules. To complement these findings with liquid-phase experiments at room temperature, femtosecond pump-probe transient absorption spectroscopy with probing in the visible and ultraviolet regime was employed to unravel primary reaction processes of DPC in solvent mixtures. Supported by quantum chemical simulations conducted by our collaborators, it was shown that a competition between the reaction pathways occurs that not only depends on the solvent molecule near-by but also on its interaction with other solvent molecules. In-depth analysis of the solvation dynamics and the amount of nascent intermediates corroborates the importance of a hydrogen-bonded complex with a protic solvent molecule, in striking analogy to complexes found at cryogenic temperatures. Probing the transient absorption of molecules in the mid-infrared spectral range benefits from the high chemical specificity of molecules' vibrational signatures. The technique of chirped-pulse upconversion (CPU) constitutes a promising alternative to standard direct multichannel MCT detection when accessing this spectral detection window. Hence, one chapter of this thesis is dedicated to a direct comparison between both detection methods. By conducting an exemplary pump-probe transient absorption experiment, it became evident, that the additional nonlinear interaction step is responsible for increased noise levels when using CPU. However, a correction procedure capable of removing these additional noise contributions—stemming from the fundamental laser radiation used for upconversion—was successfully tested. Perhaps most importantly for various spectroscopic applications, CPU scored with a significantly extended detection bandwidth owing to the high pixel numbers of modern CCD cameras. Transition-metal complexes capable of releasing small molecular messengers upon photoactivation are promising sources of gasotransmitters such as carbon monoxide (CO) or nitric oxide (NO) in biological applications. However, only little is known about the characteristic time scales of ligand dissociation in this class of molecules. For this purpose, two complexes were investigated with femtosecond time resolution: [Mn(CO)3(tpm)]Cl with tpm=tris(2-pyrazolyl)methane, a manganese tricarbonyl complex which has proven to be selective and cytotoxic to cancer cells, and [Mo(CO)2(NO)(iPr3tacn)]PF6 with iPr3tacn=1,4,7-triisopropyl-1,4,7-triazacyclononane, a molybdenum complex containing both carbonyl and nitrosyl ligands. By conducting pump-probe transient absorption measurements in different spectral probing windows supported by quantum chemical calculations and linear absorption spectroscopy, it was shown that both complexes are able to release one CO ligand within the first few picoseconds after UV excitation. The results complement existing studies which focused on the molecules' ligand-releasing properties upon long-term exposure. The additional information gained on an ultrafast time scale provides a comprehensive understanding of individual reaction steps connected with ligand release in this class of molecules. Hence, the studies might create new incentives to develop modified molecules for specific applications.},
  subject      = {Ultrakurzzeitspektroskopie},
  language  = {en}
}
@phdthesis{Kanal2015,
  author    = {Kanal, Florian},
  title     = {Femtosecond Transient Absorption Spectroscopy - Technical Improvements and Applications to Ultrafast Molecular Phenomena},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-118771},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {Photoinduced processes are nowadays studied with a huge variety of spectroscopic methods. In the liquid phase, transient absorption spectroscopy is probably the most versatile pump-probe technique used to study light-induced molecular phenomena. Optical time-resolved spectroscopy is established in a large number of laboratories and is still further being developed with respect to many technical aspects. Nevertheless, the full potential of shortening the data-acquisition time—necessary for the investigation of rapidly photodegrading samples and observation of macroscopically fast processes—achievable with high-repetition-rate laser systems and shot-to-shot detection was not fully exploited. Especially, shot-to-shot detection is highly beneficial due to the high correlation of subsequent laser pulses. The development and implementation of 100 kHz broadband shot-to-shot data acquisition was presented in Chapter 3. For an established laser dye as a benchmark system, ultrafast excited-state dynamics were measured for the first time with broadband shot-to-shot detection at 100 kHz. An analysis of both the noise characteristics of the employed laser and the correlation of subsequent pulses quantified the advantage of shot-to-shot data acquisition. In the utilized software environment, the time for measuring a complete data set could be sped up by a factor of three or even higher compared to a laser system working at 1 kHz. So far, the limiting factor is the data processing and the movement of the mechanical delay stage. Nevertheless, the new shot-to-shot detection has the potential to shorten the measurement time up to a factor of 100. The data quality is improved by a factor of three when the hitherto conventional averaging scheme is compared to shot-to-shot acquisition for the same number of laser pulses. The expansion of shot-to-shot data acquisition for high repetition rates will allow studies on sensitive samples as exposure times can strongly be reduced to achieve the same signal-to-noise ratio. In addition, multidimensional spectroscopy can also be extended to high-repetition shot-to-shot readout allowing an efficient recording of data. Therefore, in future experiments, dynamics and couplings in sensitive samples and kinetic processes could be studied in more detail. Complex photophysical and photochemical phenomena are subject of many fields of research. Many of these multifaceted processes are not yet fully understood. Therefore, a possible approach is the elucidation of single reaction steps with the combination of transient absorption spectroscopy and a suitable, less complex model system. The systematic variation of the model system's properties and environments, e.g., by chemical substitution or adequate choice of the solvent allows the determination of essential entities and reactivities thereof. Proper knowledge of an individual intermediate step and its determining factors can enhance the understanding of the complete photoreaction process. The application of transient absorption spectroscopy was shown for the optically-induced electron transfer in a series of donor-acceptor oligomers in Chapter 4. In general, the solvent relaxation times were isolated from the back-electron-transfer dynamics by a global lifetime analysis. For the smallest oligomeric structure where complete charge separation is possible, an ultrafast equilibration leads to charge recombination from the configuration showing the lowest barrier for recombination. The back-electron transfer strongly depends on the utilized solvent. Whereas in dichloromethane the back-electron transfer occurs with the maximum rate in the barrierless optimal region, the dynamics in toluene are governed by a Marcus inverted-region effect. The experimentally observed rates were also estimated by theoretical calculations of the respective barriers. The study did not only successfully unravel charge transfer in the oligomeric systems but also improved the understanding of the electron-transfer properties of larger polymers from an earlier study. Therefore, the combination of length variation and time-resolved spectroscopy is an important step towards the correct prediction of charge-carrier dynamics in macroscopic devices, e.g., for photovoltaics. The bond dissociation of a carbon-monoxide-releasing molecule in aqueous solution was studied in Chapter 5 as a prototype reaction for the photo-triggered breaking of a bond. It was shown that upon excitation only one carbon-monoxide ligand of the tricarbonyl complex is dissociated. A fraction of the photolyzed molecules restore the intact initial complex by geminate recombination within the temporal resolution of the experiment. However, the recombination could be detected by the hot ground-state infrared absorption of the complex. The detectable dicarbonyl formed upon CO release distributes excess energy from the absorbed photon into low-frequency modes which result in broadened absorption bands like for the recombined tricarbonyl. The free coordination site in the ligand sphere is filled with a solvent water molecule. Despite numerous studies of metal carbonyls studied in alkaneous solutions, the elucidation of the dynamics of a CORM in aqueous solution added another important detail to the photochemistry of this class of compounds. Experiments employing a second ultraviolet pump pulse did not trigger further CO dissociation and hence no formation of a monocarbonyl species; this might either be due to a different release mechanism without a further photochemical step or a strong spectral shift of the dicarbonyl's absorption. Both reasons could explain why degenerate pump-repump-probe spectroscopy is inefficient. However, further experiments with ultraviolet probe pulses could substantiate whether the intermediate dicarbonyl reacts further photochemically or not. Apart from the model-system character of the CORM for bond dissociation, the study could determine exactly how many CO ligands are initially photolyzed off. Detailed knowledge of the release mechanism will affect the previous use and application as well as the further development of CORMs as therapeutic prodrugs to deliver high local concentrations of CO in cancerous or pathological tissue. Hence, the study of two-photon absorption properties which are important for in vivo applications of CORMs should be the main focus in further spectroscopic experiments. In Chapter 6, both abovementioned molecular phenomena—electron transfer and bond dissociation—were studied in combination. The photochemistry of a tetrazolium salt was studied in detail in a variety of different solvents. Being a relatively small molecule, the studied tetrazolium cation shows a multifaceted photochemistry and is therefore a textbook example for the combination of ultrafast molecular phenomena studied in different environments. Within femtoseconds, the tetrazolium ring is opened. The biradicalic species is then reduced via uptake of an electron from the solvent. The formation of the ring-open formazan photoproduct from this point of the reaction sequence on was excluded by experiments with acidic pH value of the solution. The ring-open radical is stabilized by ring-closure. The resulting tetrazolinyl radical was already observed in experiments with microsecond time resolution. However, its formation was observed in real time for the first time in this study. Irradiation of a tetrazoliumsalt solution yields different photoproduct distributions depending on the solvent. However, it was shown that all photoproducts have a tetrazolinyl radical as a common precursor on an ultrafast time scale. In combination with studies from the literature, the complete photochemical conversion of a tetrazolium salt was clarified in this study. Apart from the prototype character of the reaction sequence, the reaction mechanism will have impact on research associated with life science where tetrazolium assays are used on a daily basis without taking into account of photochemical conversion of the indicating tetrazolium ion and its photochemically formed reactive intermediates. On the basis of the tetrazolium-ion photochemistry, the rich photochemistry of the formazan photoproduct, including structural rearrangements and subsequent reformation of the tetrazolium ion, might be the subject of future studies. This thesis shows a method advancement and application of transient absorption spectroscopy to exemplary molecular model systems. The insights into each respective field did not only enlighten singular aspects, but have to be seen in a much larger context. Understanding complex photoinduced processes bottom-up by learning about their constituting steps—microscopically and on an ultrafast time scale—is an ideal method to approach understanding and prediction of phenomena in large molecular systems like biological or artificial architectures as for example used in photosynthetic light-harvesting and photovoltaics.},
  subject      = {Ultrakurzzeitspektroskopie},
  language  = {en}
}
@phdthesis{Gerbich2015,
  author    = {Gerbich, Thiemo M. P.},
  title     = {Pikosekunden-zeitaufgel{\"o}ste Deaktivierungsprozesse in isolierten Molek{\"u}len - Fluorenon, NDCA, Me-NI und NTCDA},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-118654},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2015},
  abstract  = {Im Rahmen der vorliegenden Dissertation wurden die Dynamiken von strahlungslosen Deaktivierungsprozessen von vier verschiedenen Molek{\"u}len im elektronisch angeregten Zustand untersucht. Ein fundiertes Verst{\"a}ndnis der intramolekularen Energieumverteilung in isolierten pi-konjugierten Systemen ist neben dem Modellcharakter auch f{\"u}r Anwendungen in der organischen Elektronik von Interesse. Die Untersuchungen dienen zudem als optimaler Maßstab f{\"u}r theoretische Simulationen, die auf eine Nachbildung der molekularen Dynamik ausgerichtet sind. Die Inbetriebnahme des Pikosekunden-Lasersystems stellt in der Arbeitsgruppe ein großes Potential f{\"u}r die Untersuchung der Dynamik von isolierten pi-konjugierten Molek{\"u}len zur Verf{\"u}gung. Erste Experimente konnten an unterschiedlichen Heterocyclen mit interessantem zeitlichen Verhalten erfolgreich durchgef{\"u}hrt werden und lieferten bereits wichtige Erkenntnisse {\"u}ber die strahlungslose Deaktivierung auf der ps-Zeitskala. Selbst f{\"u}r große Molek{\"u}le mit geringem Dampfdruck, die nur mit hohem experimentellen Aufwand im isolierten Zustand charakterisierbar sind, konnten Relaxationszeiten der angeregten Zust{\"a}nde ermittelt werden. Der Fokus der einzelnen Studien lag in der Erforschung der isolierten Molek{\"u}le, welche durch Anwendung der Molekularstrahl-Technik mit zeitaufgel{\"o}ster REMPI-Spektroskopie anhand des ps-Systems untersucht werden sollten. Zur Kontrolle der experimentellen Ergebnisse wurden zudem Vergleichsmessungen der transienten Absorptionsspektroskopie (TA) in der Fl{\"u}ssigphase herangezogen, wodurch eine fundierte Interpretation der Dynamik m{\"o}glich wurde. Zu den wichtigen Zielen geh{\"o}rten jedoch die Vergleiche der experimentellen Ergebnisse von isolierten Molek{\"u}len mit Berechnungen der Zustandsenergien sowie Simulationen der Molek{\"u}ldynamik aus dem Theorie-Arbeitskreis von Prof. Mitric. Auf diese Weise konnten wichtige Erkenntnisse {\"u}ber die Dynamik der Deaktivierungsprozesse gewonnen werden. Die Kombination der Gasphasen-Experimente mit TA-Messungen in der Fl{\"u}ssigphase hat sich als besonders n{\"u}tzlich erwiesen, um bei mehrstufigen Deaktivierungsprozessen einen erweiterten Einblick in die Dynamik der Molek{\"u}le zu erhalten. - So konnte bei Fluorenon in Cyclohexan und Acetonitril durch Vergleich der Anregungen des S3- und S1-Zustands eine zus{\"a}tzliche Zeitkonstante von 8-16 ps beobachtet werden, welche die innere Umwandlung zum S1-Zustand dokumentiert und die Ergebnisse der Gasphasen-Messungen best{\"a}tigt. - Durch Verwendung von L{\"o}sungsmitteln unterschiedlicher Polarit{\"a}t und der damit verbundenen Verschiebung der elektronischen Zust{\"a}nde von Fluorenon konnte zudem der zweite Deaktivierungsprozess eindeutig einem ISC-Prozess mit Zeitkonstanten von 120-154 ps zugeordnet werden. In der Gasphase wurde dieser Prozess lediglich als langlebiger Offset wahrgenommen. - Unterschiedliche Anregungsenergien zeigten bei TA-Messungen von NDCA eine nahezu identische Molek{\"u}ldynamik mit ca. 200 ps, w{\"a}hrend f{\"u}r isoliertes NDCA ein starker Abfall der Lebensdauer mit zunehmender Schwingungsenergie beobachtet wurde. In der Gasphase wird somit von einer Deaktivierung {\"u}ber eine Energiebarriere ausgegangen, w{\"a}hrend in L{\"o}sung eine zu schnelle Abk{\"u}hlung durch Schwingungsrelaxation diesen Prozess verhindert. - Bei NTCDA konnten in den TA-Messungen nach Anregung des S1-Zustands eine Relaxation in die Triplett-Umgebung innerhalb von wenigen Pikosekunden beobachtet werden, was im Einklang mit der sehr schnellen Deaktivierung in der Gasphase betrachtet werden kann. Eine ausf{\"u}hrliche Vergleichsstudie von isolierten Molek{\"u}len mit computergest{\"u}tzten Rechnungen und Simulationen wurde f{\"u}r die Molek{\"u}le NDCA und Me-NI durchgef{\"u}hrt. Dabei wurde explizit auf den Einfluss von Spin-Bahn-Kopplungen und konischen Durchschneidungen eingegangen, welche zu konkurrierenden Deaktivierungsprozessen des S1-Zustands f{\"u}hren k{\"o}nnen. - Durch Simulationen der Surface-Hopping-Dynamik wurde deutlich, dass bei NDCA und Me-NI im ersten angeregten Zustand eine konische Durchschneidung (CI) zwischen dem S1- und S0-Zustand erreicht werden kann. - W{\"a}hrend die Dynamik von NDCA bei h{\"o}herer Schwingungsanregung stark durch die CI dominiert wird, spielt die direkte Relaxation in den elektronischen Grundzustand bei Me-NI offenbar keine Rolle. - In Abwesenheit der CI zeigen NDCA und Me-NI in einer mit Spin-Bahn-Kopplung erweiterten Simulation der Populationsdynamik einen signifikanten Populationstransfer in die Triplett-Umgebung (T1-T4). Eine innere Umwandlung in den Grundzustand konnte jedoch nur bei Erreichen der CI beobachtet werden. Eine weitere Verbesserung der ps-Experimente kann durch Aufbau eines Photoelektronen-Spektrometers erreicht werden, da durch diese Technik eine pr{\"a}zisere Aussage dar{\"u}ber getroffen werden kann, aus welchem elektronischen Zustand die Molek{\"u}le ionisiert wurden. Eine Unterscheidung von ISC- und IC-Prozessen k{\"o}nnte somit gew{\"a}hrleistet werden.},
  subject      = {REMPI},
  language  = {de}
}
@phdthesis{Bolze2018,
  author    = {Bolze, Tom},
  title     = {Photodynamics of a fluorescent tetrazolium salt and shaping of femtosecond Laguerre-Gaussian laser modes in time and space},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-160902},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2018},
  abstract  = {This thesis will outline studies performed on the fluorescence dynamics of phenyl-benzo- [c]-tetrazolo-cinnolium chloride (PTC) in alcoholic solutions with varying viscosity using time-resolved fluoro-spectroscopic methods. Furthermore, the properties of femtosecond Laguerre-Gaussian (LG) laser pulses will be investigated with respect to their temporal and spatial features and an approach will be developed to measure and control the spatial intensity distribution on the time scale of the pulse. Tetrazolium salts are widely used in biological assays for their low oxidation and reduction thresholds and spectroscopic properties. However, a neglected feature in these applications is the advantage that detection of emitted light has over the determination of the absorbance. To corroborate this, PTC as one of the few known fluorescent tetrazolium salts was investigated with regard to its luminescent features. Steady-state spectroscopy revealed how PTC can be formed by a photoreaction from 2,3,5-triphenyl-tetrazolium chloride (TTC) and how the fluorescence quantum yield behaved in alcoholic solvents with different viscosity. In the same array of solvents time correlated single photon counting (TCSPC) measurements were performed and the fluorescence decay was investigated. Global analysis of the results revealed different dynamics in the different solvents, but although the main emission constant did change with the solvent, taking the fluorescence quantum yield into consideration resulted in an independence of the radiative rate from the solvent. The non-radiative rate, however, was highly solvent dependent and responsible for the observed solvent-related changes in the fluorescence dynamics. Further studies with the increased time resolution of femtosecond fluorescence upconversion revealed an independence of the main emission constant from the excitation energy, however the dynamics of the cooling processes prior to emission were prolonged for higher excitation energy. This led to a conceivable photoreaction scheme with one emissive state with a competing non-radiative relaxation channel, that may involve an intermediate state. LG laser beams and their properties have seen a lot of scientific attention over the past two decades. Also in the context of new techniques pushing the limit of technology further to explore new phenomena, it is essential to understand the features of this beam class and check the consistency of the findings with theoretical knowledge. The mode conversion of a Hermite-Gaussian (HG) mode into a LG mode with the help of a spiral phase plate (SPP) was investigated with respect to its space-time characteristics. It was found that femtosecond LG and HG pulses of a given temporal duration share the same spectrum and can be characterized using the same well-established methods. The mode conversion proved to only produce the desired LG mode with its characteristic orbital angular momentum (OAM), that is conserved after frequency doubling the pulse. Furthermore, it was demonstrated that temporal shaping of the HG pulse does not alter the result of its mode-conversion, as three completely different temporal pulse shapes produced the same LG mode. Further attention was given to the sum frequency generation of fs LG beams and dynamics of the interference of a HG and a LG pulse. It was found that if both are chirped with inverse signs the spatial intensity distribution does rotate around the beam axis on the time scale of the pulse. A strategy was found that would enable a measurement of these dynamics by upconversion of the interference with a third gate pulse. The results of which are discussed theoretically and an approach of an experimental realization had been made. The simulated findings had only been reproduced to a limited extend due to experimental limitations, especially the interferometric stability of the setup.},
  subject      = {Tetrazoliumsalze},
  language  = {en}
}
@phdthesis{Schott2018,
  author    = {Schott, Sebastian},
  title     = {Identification of trihalide photodissociation patterns by global vibrational wavepacket analysis of broadband magic-angle transient absorption data},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-159677},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2018},
  abstract  = {The invention of laser pulse shapers allowed for various quantum control experiments, where a chemical reaction is guided by specifically tailored laser pulses. However, despite of the prominent role of the liquid phase in chemistry, no successful attempt for controlling the selectivity of a bond-fission reaction has yet been reported in this state of matter. Promising candidates for such an experiment are C\$_{\infty\mathrm{v}}\$-symmetric trihalide anions with two different chemical bonds like \$\ce{I2Cl-}\$, because these molecules notionally offer the most simplest selectivity-control scenario of breaking either the one or the other bond and they are expected to dissociate under ultraviolet (UV) irradiation like it is known for the most-studied trihalide \$\ce{I3-}\$. In order to investigate in this thesis the possibility that the dissociation reaction of such trihalides branches into two different photofragments, the ultrafast photodissociation dynamics of \$\ce{I3-}\$, \$\ce{Br3-}\$, \$\ce{IBr2-}\$ and \$\ce{ICl2-}\$ (point group D\$_{\infty\mathrm{h}}\$) as well as of \$\ce{I2Br-}\$ and \$\ce{I2Cl-}\$ (point group C\$_{\infty\mathrm{v}}\$) in dichloromethane solution were measured with broadband transient absorption spectroscopy in magic-angle configuration. The identification of the reaction pathway(s) relies on vibrational wavepacket oscillations, which survive the dissociation process and therefore carry not only informations about the reactant trihalides but also about the fragment dihalides. These characteristic vibrational wavenumbers were extracted from the measured transient absorption spectra by globally fitting the population dynamics together with the wavepacket dynamics. Until recently, such a combined model function was not available in the well-established fitting tool Glotaran. This made it inevitable to develop a custom implementation of the underlying variable-projection fitting algorithm, for which the computer-algebra software Mathematica was chosen. Mathematica's sophisticated built-in functions allow not only for a high flexibility in constructing arbitrary model functions, but also offer the possibility to automatically calculate the derivative(s) of a model function. This allows the fitting procedure to use the exact Jacobian matrix instead of approximating it with the finite difference method. Against the expectation, only one of the two thinkable photodissociation channels was found for each of the investigated C\$_{\infty\mathrm{v}}\$ trihalides. Since the photofragments recombine, their absorption signal as well as the reactant ground state bleach recover. This happens in a biexponential manner, which in the case of \$\ce{I3-}\$ was interpreted by Ruhman and coworkers with the direct formation of a neutral dihalogen fragment \$\ce{I2}\$ beside the negatively charged dihalide fragment \$\ce{I2-}\$. In this thesis, such a direct reaction channel was not found and instead the fast component of the biexponential decay is explained with vibrational excess energy mediating the recombination-preceding electron transfer process \$\ce{I2- + I -> I2 + I-}\$, while the slow component is attributed to cooled-down fragments. In addition to the trihalide experiments, the possibility of a magic-angle configuration for polarization-shaping control experiments was theoretically investigated in this thesis by deriving magic-angle conditions for the third-order electric-dipole response signal of arbitrarily polarized laser pulses. Furthermore, the subtleties of anisotropy signals violating the well-known range of \numrange{-0.2}{0.4} were studied.},
  subject      = {Femtosekundenspektroskopie},
  language  = {en}
}