@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{Rudolf2014,
  author    = {Rudolf, Philipp Benjamin},
  title     = {Uncovering photoinduced chemical reaction pathways in the liquid phase with ultrafast vibrational spectroscopy},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-96200},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2014},
  abstract  = {The experimental technique predominantly employed within the scope of this Thesis constitutes one subarea of femtochemistry: the time-resolved spectroscopy of photoin- duced chemical reactions in the liquid phase by means of molecular signatures in the mid-infrared (MIR) spectral range. Probing transient vibrational states, i.e., dynamic changes in the vibrational motion of speci� c molecular subunits or functional Groups allows for a distinct separation and assignment of measured signals to emerging molecular species. For this purpose, one key building block is indispensable, which most of the investigations carried out within the � eld of femtochemistry have in common: a coherent light source delivering ultrashort laser pulses with a temporal duration that matches the femtosecond time scale on which molecular motions typically occur. This instrumentation enables the observation of photoinduced chemical reactions from the starting point|the excitation event to the appearance of intermediates to the nal formation of stable photoproducts after several pico- or nanoseconds. This work comprises the acquisition and presentation of time-resolved spectroscopic data related to promising molecular systems upon photoexcitation as well as the im- plementation and testing of experimental optical techniques both for the presented experiments but as well for experiments conceivable in the future. In addition, linear spectroscopy measurements and quantum-chemical simulations on the emerging chemical species have been carried out. In so doing, the primary processes and subse- quently emerging reaction products of two compounds on a timescale of several nanoseconds after photoexcitation have been elucidated in great detail. Both compounds, the [Mn(CO)3(tpm)]+ (tpm = tris(2-pyrazolyl)methane) CO-releasing molecule (CORM) and the 5-diazo Meldrum's acid (DMA), are of academic interest but in addition belong to molecular classes that might be utilized in the near future as dark-stable prodrugs under physiological conditions or that are already utilized in industrial chemistry procedures, respectively. The � ndings of both studies gave rise to implement and examine two techniques for prospective transient absorption experiments, namely the shaping and characterization of ultraviolet (UV) laser pulses and the recording of two-photon excitation spectra. Beyond that, since each of the depicted experiments is based on the detection of weak transient absorption signals in the MIR spectral region, two dif- ferent detection schemes, via chirped-pulse upconversion (CPU) on the one hand and via direct multichannel MCT detection on the other hand, have been juxtaposed at the conclusion of this work. Since both techniques are suitable in femtosecond pump-probe measurements but thereby exhibit individual strengths and weaknesses, a comparative study provides clari� cation of the respective pros and cons. The � first study introduced within this work investigates the complex photochemistry of DMA, a photoactive compound used in lithography and industrial chemistry. By femtosecond MIR transient absorption spectroscopy covering several nanoseconds, the light-induced dynamics and ultrafast formation of several photoproducts from the manifold of reaction pathways have been disclosed to form a coherent picture of the overall reaction scheme. After UV excitation of DMA dissolved in methanol to the second excited state S2, 70\% of excited molecules relax back to the S0 ground state. In compet- ing processes, they can either undergo an intramolecular Wolff rearrangement to form ketene, which reacts with a solvent molecule to an enol intermediate and further to carboxylate ester, or they � rst relax to the DMA S1 state, from where they can isomerize to a diazirine. The third competing reaction channel, having the lowest quantum efficiency with respect to the � rst two channels, is the formation of a singlet carbene out of the S1 state. From there an ylide can arise or, via an intersystem crossing, the triplet form of the carbene follows. Whereas the primary reaction steps occur on a picosecond timescale, the subsequently arising intermediates and stable photoproducts are formed within a few hundreds to thousands of picoseconds. For a reliable identi� cation of the involved compounds, density functional theory calculations on the normal modes and Fourier-transform infrared spectroscopy of the reactant and the photoproducts in the chemical equilibrium accompany the analysis of the transient spectra. Additional experiments in ethanol and isopropanol led to slight spectral shifts as well as elongated time constants due to steric hindrance in transient spectra connected with the ester Formation channel, further substantiating the assignment of the occurring reaction pathways and photoproducts. The study demonstrated that the combination of linear and time-resolved spectroscopic measurements in conjunction with quantum-chemical calculations constitutes a powerful tool to unravel even highly complex photoreactions exhibiting multiple consecutive intermediate states within parallel reaction pathways. Although some of the individual reaction steps, for example the ketene formation via Wolff rearrangement, have been observed on ultrashort time scales before, this work encompassed the Observation of the whole set of appearing photoproducts of DMA in different alcohol solutions within several nanoseconds. In this sense, the ultrafast photochemistry of DMA represents a prototype example for a multisequential reaction scheme, elucidated by the capabilities of femtosecond MIR spectroscopy. With a modi� fied instrumentation concerning amongst others the system delivering the fundamental laser pulses or the generation of the UV pump pulses, the next ob- jective within this work was to elucidate the primary processes upon UV Irradiation of a manganese tricarbonyl CORM in aqueous environment. The time-resolved experiment was performed with two different pump wavelengths and furthermore supported by linear spectroscopy methods and time-dependent density functional theory (TDDFT) calculations on the excited states as well as DFT calculations on the ground states. The measurements revealed that irradiating the compound with UV excitation pulses primarily leads to ultrafast photolysis of one CO ligand. Geminate recombination may occur within one picosecond but it remains a minor process as the photolyzed CO group is liberated and the unoccupied coordination site is predominantly fi� lled by an incoming solvent molecule. There was no evidence for hot CO bands, i.e., the remaining CO ligands|in the dicarbonyl photoproduct as well as in the intact CORM are not vibrationally excited through the UV excitation of the CORM. According to this, the excess energy merges into low-frequency vibrational modes associated with the molecule as a whole. Since studies on a macroscopic scale at irradiation times of several minutes prove that UV irradiation eventually leads to the release of two or even all three CO ligands, further loss of CO most likely necessitates manganese oxidation or another interaction with light. To clarify the latter, a consecutive UV pulse was employed in order to excite the photoproducts subsequent to the initial pump interaction. However, the data obtained was not instructive enough to de� nitely exclude the manganese oxidation being responsible for the loss of further CO groups. Besides the exchange of a CO Group by a solvent molecule or the geminate recombination, the employment of two different excitation wavelengths in combination with � ndings derived from the TDDFT calculations suggested another reaction process, namely the possibility that the excitation does not lead to any bond cleavage at all. As the CORM under investigation is tissue-selective and cytotoxic against cancer cells, knowledge of these � rst photoinduced reaction steps is essential for a full understanding of its biological activity. Inspired by these two studies, experimental techniques for prospective transient absorption measurements have been implemented and tested within preparative measure- ments. First, in the course of a UV-pump-MIR-probe experiment with speci� cally tailored excitation pulses, one could pursue the aim of coherently controlling the outcome of a photoreaction in the liquid phase. Out of the rich photochemistry of DMA the vibrational signature of a particular molecular species might thereby serve as a feedback signal, which is a central part of a learning loop that adaptively determines the pulse shape that steers the quantum mechanical system upon photoexcitation into a desired direction. This motivated the installation and testing of devices by means of which the shaping and characterization of ultrashort laser pulses in the UV could be performed. Second, motivated by the biological applications of CORMs, one can imagine a scenario where a certain amount of CORMs is deposited inside cancerous tissue. Since the activation of CO loss by means of UV pulses is not possible due to the absorption characteristics of biological tissue, the simultaneous excitation via two photons from the visible spectral regime seems appealing. However, success or failure of such an application depends on whether the deposited compound efficiently absorbs two photons simultaneously, i.e., whether the two-photon absorption cross section is large enough. Therefore, a setup to record two-photon excitation spectra under full consideration of the crucial laser pulse parameters like the pulse duration, energy and central wavelength was arranged and tested. The � rst results were obtained with a commercially available reference system (Mn2CO10) but the setup as well as the described measurement and data analysis procedure can easily be applied to record the two-photon absorption cross section of more promising molecular systems. Third, as the detection of probe pulses in the MIR spectral region is part of each time-resolved measurement throughout this thesis, a comparison between the newly established technique of CPU and direct multi- channel MCT detection is presented by means of pump{probe experiments on Mn2CO10 and Co4CO12 with a 1 kHz shot-to-shot data acquisition. It was shown that the CPU detection technique scores with its high spectral resolution and coverage of the easy-to-handle and more cost-effective CCD detectors. On the other hand, in the course of the additional nonlinear upconversion process intensity fluctuations of the chirped fundamental pulses are transferred to the probe spectrum in the visible regime. This entails a lower signal-to-noise ratio than the direct MCT detection, which can be compensated by an additional normalization procedure applied to the CPU probe pulses. As a consequence, the CPU detection scheme offers more flexibility for future investigations employing MIR probe pulses. This is of great importance for many applications within the presented � eld of femtochemistry as a huge variety of time-resolved investigations on a multitude of systems in the liquid phase is based on the detection of weak transient absorption signals in the MIR spectral region.},
  subject      = {Ultrakurzzeitspektroskopie},
  language  = {en}
}