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Baudisch, Bastian (2018): Time resolved broadband spectroscopy from UV to NIR: beneficial use of the coherent artifact and pyrene dynamics. Dissertation, LMU München: Fakultät für Physik
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Abstract

Transient absorption (TA) spectroscopy utilizes short laser pulses to probe photo induced molecular dynamics. Commonly samples are excited by a short ‘pump’ pulse and a delayed ‘probe’ pulse monitors the sample’s absorption change at selected pump probe delays. Transient spectra are recorded from the femtosecond (fs) to the millisecond regime, where intramolecular relaxation processes and fast chemical reactions take place. The first part of this thesis focuses on the shortest delays, where pump and probe pulses overlap in time. In the visible (VIS) and ultraviolet (UV) spectral range the interaction of pump and probe in the sample often leads to the observation of the ‘coherent artifact’ (CA). Depending on the probe chirp it can obscure molecular dynamics up to a few hundred femtoseconds. Only accurate modeling facilitates extraction of molecular information in this range. While in the UV two-photon absorption is the dominant process, cross phase modulation (XPM) is determined to be most relevant in the VIS and near infrared (NIR). Simulations of XPM induced CAs show that in the most common scenario of a chirped probe, the shortest pump pulse does not yield the shortest CA. Instead, for increasingly chirped probe pulses longer pump pulses are preferable. As a series of simulations and experiments yields strongly varying shapes and widths, a novel para-meterization is developed, allowing accurate fit and subtraction of the CA in all observed cases. The CA can be used to characterize the group delay (GD) of dielectric mirror coatings from the UV to the NIR. With recent advances in broadband coating technology, performance testing will become increasingly important. This novel technique will likely not replace established interferometric techniques, but provides comparable results and overcomes technical limitations in the UV. The method is tested on custom, dispersion compensated CMUV08 UV and PC5-L VIS/NIR mirrors and applied to novel Balzers DIFLEX® utrabroadband mirrors. An overall GD accuracy of better than ±1 fs was achieved. In the second part of this thesis the overlapping excited state signatures of pyrene exemplify that spectrally resolved broadband detection is often necessary to acquire a ‘full picture’ of the mole-cular dynamics. Recently pyrene has been considered as a building block for functionalized mate-rials in organic electronics. Despite many applications as a fluorescence marker and multiple use as a textbook example, the oft proposed reversibility of the excimer formation as well as the role of the triplet state are still debated. The dynamics of pyrene and selected derivatives are syste-matically reevaluated from the fs to the µs timescale using state-of-the-art broadband detection. Taking advantage of spectral domain fitting and global analysis techniques, ambiguities in the current models are resolved. In contrast to earlier claims, the pyrene excimer formation is not re-versible at room temperature. Double exponential decays of the ‘monomer’ fluorescence are due to overlapping monomer and excimer fluorescence bands. Commonly neglected transient effects of diffusion increasingly impact the fitted rate constants at concentrations > 1 mM. A simplified rate model is proposed, describing the dynamics of pyrene as well as linked pyrene dimers. TA studies yield transient spectra of the S1-, excimer and triplet states. Despite efforts to minimize the oxygen concentration in the solution, oxygen quenching of the singlet state proved to be the source of most of the observed triplets. Correcting for oxygen quenching leads to 98% quantum yield for the excimer at 10 mM, of which 3% form triplet states whose signatures closely resemble that of the triplet formed from the monomer. In pyrene dimers a direct link at the 1-position drastically accelerates the dynamics. Units linked at the 4-position are candidates for advanced materials, featuring a long-lived fluorescence even after polymerization.