@phdthesis{Margraf2010,
  author    = {Margraf, Markus Johann},
  title     = {Spektroskopie an π-konjugierten Molek{\"u}len},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-54032},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2010},
  abstract  = {Femtosekunden-zeitaufgel{\"o}ste transiente Absorptionsspektroskopie einer neutralen organischen gemischtvalenten Verbindung In einem Femtosekunden-zeitaufgel{\"o}sten Anrege-Abfrage-Experiment wurde die Dynamik des Elektronentransfers einer neutralen organischen gemischtvalenten Verbindung untersucht. Neben der Abh{\"a}ngigkeit des R{\"u}ckelektronentransfers von der Solvenspolarit{\"a}t wurde auch die Rotationsdiffusion in n-Hexan, Toluol, Dibutylether, tert-Butylmethylether und Benzonitril studiert. Die transiente Dynamik l{\"a}sst sich mit einer Lebensdauer beschreiben, verursacht durch einen R{\"u}ckelektronentransfer. W{\"a}hrend dieser in unpolaren L{\"o}semitteln relativ langsam verl{\"a}uft, beobachtet man deutlich schnellere ET-Raten mit steigender Polarit{\"a}t des L{\"o}semittels. Die Lebensdauer variiert von 1.2 ps f{\"u}r Benzonitril bis 260 ps f{\"u}r n-Hexan. Rotationsdiffusion konnte nicht beobachtet werden. Die gemessenen Raten wurden mit theoretischen Raten verglichen. F{\"u}r unpolare L{\"o}semittel konnte eine gute {\"U}bereinstimmung gefunden werden. In polaren L{\"o}semitteln bewirkt eine Korrektur, die die Solvensrelaxationszeit ber{\"u}cksichtigt, eine sehr gute {\"U}bereinstimmung von berechneten und gemessenen R{\"u}ckelektronentransferraten. Zeit- und frequenzaufgel{\"o}ste Photoionisation des C 2A2-Zustandes des Benzylradikals Die Lebensdauer des C 2A2-Zustandes des Benzylradikals wurde in Abh{\"a}ngigkeit der {\"U}berschussenergie bestimmt. Die zeitabh{\"a}ngigen Ionensignale konnten dabei mit einer biexponentiellen Dynamik beschrieben werden. Bei einer Anregung am Ursprung (305nm) betragen die Lebensdauern τ1= 400 fs und τ2 = 4.5 ps. Die k{\"u}rzere Lebensdauer τ1 beschreibt die interne Konversion vom C-Zustand zu den stark koppelnden A/B-Zust{\"a}nden, die l{\"a}ngere Lebensdauer τ2 die interne Konversion von den A/B-Zust{\"a}nden in den elektronischen Grundzustand. Mit steigender Anregungsenergie beobachtet man eine stete Abnahme beider Lebensdauern. Bei einer Anregung mit einem Puls der Wellenl{\"a}nge von 301 nm beobachtet man deutlich k{\"u}rzere Lebensdauern mit τ1 = 350 fs und τ2 = 2.8 ps. Erfolgt die Anregung mit einem Puls der Wellenl{\"a}nge von 298 nm, betragen die Zeitkonstanten τ1 = 180 fs und τ2 = 2.1 ps. Desweiteren konnte ein zeitabh{\"a}ngiges Ionensignal f{\"u}r eine Spezies mit der Zusammensetzung C7H5 beobachtet werden. Der Tr{\"a}ger des Signals ist das Fulvenallenylradikal.},
  subject      = {Valenzgemischte Verbindungen},
  language  = {de}
}
@phdthesis{Eckstein2019,
  author    = {Eckstein, Klaus},
  title     = {Linear and Nonlinear Spectroscopy of Doped Carbon Nanotubes},
  doi       = {10.25972/OPUS-18897},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-188975},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {Doping plays a decisive role for the functionality of semiconductor-based (opto-)electronic devices. Hence, the technological utilization of semiconductors necessitates control and a fundamental understanding of the doping process. However, for low-dimensional systems like carbon nanotubes, neither concentration nor distribution of charge carriers is currently well known. The research presented in this thesis investigated the doping of semiconducting carbon nanotubes by spectroscopic methods. Samples of highly purified, intrinsic (6,5) single-wall carbon nanotubes were fabricated using polymer stabilization. Chapter 4 showed that both electro- and redox chemical \$p\$-doping lead to identical bleaching, blueshift, broadening and asymmetry of the S\$_1\$ exciton absorption band. The similar spectral changes induced by both doping schemes suggest that optical spectra can not be used to infer what process was used for doping. Perhaps more importantly, it also indicates that the distribution of charges and the character of the charge transfer states does not depend on the method by which doping was achieved. The detailed analysis of the doping-induced spectral changes in chapter 5 suggests that surplus charges are distributed inhomogeneously. The hypothesis of carrier localization is consistent with the high sensitivity of the S\$_1\$ exciton photoluminescence to additional charge carriers and with the stretched-exponential decay of the exciton population following ultrafast excitation. Both aspects are in good agreement with diffusion-limited contact quenching of excitons at localized charges. Moreover, localized charges act - similar to structural defects - as perturbations to the bandstructure as evidenced by a doping-induced increase of the D-band antiresonance in the mid-infrared spectrum. Quantum mechanical model calculations also suggest that counterions play a crucial role in carrier localization. Counterion adsorption at the nanotube surface is thus believed to induce charge traps of more than 100 meV depth with a carrier localization length on the order of 3 - 4 nm. The doping-induced bleach of interband absorption is accompanied by an absorption increase in the IR region below 600 meV. The observed shift of the IR peak position indicates a continuous transition from localized to rather delocalized charge carriers. This transition is caused by the increase of the overlap of charge carrier wavefunctions at higher charge densities and was modeled by classical Monte-Carlo simulations of intraband absorption. Chapter 6 discussed the spectroscopy of heavily (degenerately) doped nanotubes, which are characterized by a Drude-response of free-carrier intraband absorption in the optical conductivity spectrum. In the NIR spectral region, the S\$_1\$ exciton and X\$+^_1\$ trion absorption is replaced by a nearly 1 eV broad and constant absorption signal, the so-called H-band. The linear and transient absorption spectra of heavily doped nanotubes suggest that the H-band can be attributed to free-carrier interband transitions. Chapter 7 dealt with the quantification of charge carrier densities by linear absorption spectroscopy. A particularly good measure of the carrier density is the S\$_1\$ exciton bleach. For a bleach below about 50 \%, the carrier density is proportional to the bleach. At higher doping levels, deviations from the linear behavior were observed. For doping levels exceeding a fully bleached S\$_1\$ band, the determination of the normalized oscillator strength f\$\text{1st}\$ over the whole first subband region (trion, exciton, free e-h pairs) is recommended for quantification of carrier densities. Based on the nanotube density of states, the carrier density \$n\$ can be estimated using \$n = 0.74\,\text{nm}^{-1} \cdot (1 - f_\text{1st})\$. In the last part of this thesis (chapter 8), the time-resolved spectroelectrochemistry was extended to systems beyond photostable carbon nanotube films. The integration of a flowelectrolysis cell into the transient absorption spectrometer allows the investigation of in-situ electrochemically generated but photounstable molecules due to a continuous exchange of sample volume. First time-resolved experiments were successfully performed using the dye methylene blue and its electrochemically reduced form leucomethylene blue.},
  subject      = {Dotierung},
  language  = {en}
}
@article{MalyBrixner2021,
  author    = {Mal{\´y}, Pavel and Brixner, Tobias},
  title     = {Fluorescence-Detected Pump-Probe Spectroscopy},
  series = {Angewandte Chemie International Edition},
  volume    = {60},
  journal   = {Angewandte Chemie International Edition},
  number    = {34},
  doi       = {10.1002/anie.202102901},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-244811},
  pages     = {18867 -- 18875},
  year      = {2021},
  abstract  = {We introduce a new approach to transient spectroscopy, fluorescence-detected pump-probe (F-PP) spectroscopy, that overcomes several limitations of traditional PP. F-PP suppresses excited-state absorption, provides background-free detection, removes artifacts resulting from pump-pulse scattering, from non-resonant solvent response, or from coherent pulse overlap, and allows unique extraction of excited-state dynamics under certain conditions. Despite incoherent detection, time resolution of F-PP is given by the duration of the laser pulses, independent of the fluorescence lifetime. We describe the working principle of F-PP and provide its theoretical description. Then we illustrate specific features of F-PP by direct comparison with PP, theoretically and experimentally. For this purpose, we investigate, with both techniques, a molecular squaraine heterodimer, core-shell CdSe/ZnS quantum dots, and fluorescent protein mCherry. F-PP is broadly applicable to chemical systems in various environments and in different spectral regimes.},
  language  = {en}
}
@phdthesis{Roeder2017,
  author    = {R{\"o}der, Anja M.},
  title     = {Excited-State Dynamics in Open-Shell Molecules},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-151738},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2017},
  abstract  = {In this thesis the excited-state dynamics of radicals and biradicals were characterized with femtosecond pump-probe spectroscopy. These open-shell molecules play important roles as combustion intermediates, in the formation of soot and polycyclic aromatic hydrocarbons, in atmospheric chemistry and in the formation of complex molecules in the interstellar medium and galactic clouds. In these processes molecules frequently occur in some excited state, excited either by thermal energy or radiation. Knowledge of the reactivity and dynamics of these excited states completes our understanding of these complex processes. These highly reactive molecules were produced via pyrolysis from suitable precursors and examined in a molecular beam under collision-free conditions. A first laser now excites the molecule, and a second laser ionizes it. Time-of-flight mass spectrometry allowed a first identification of the molecule, photoelectron spectroscopy a complete characterization of the molecule - under the condition that the mass spectrum was dominated by only one mass. The photoelectron spectrum was obtained via velocity-map imaging, providing an insight in the electronic states involved. Ion velocity map imaging allowed separation of signal from direct ionization of the radical in the molecular beam and dissociative photoionization of the precursor. During this thesis a modified pBasex algorithm was developed and implemented in python, providing an image inversion tool without interpolation of data points. Especially for noisy photoelectron images this new algorithm delivers better results. Some highlighted results: • The 2-methylallyl radical was excited in the ππ*-state with different internal energies using three different pump wavelengths (240.6 , 238.0 and 236.0 nm). Ionized with 800 nm multi-photon probe, the photoelectron spectra shows a s-Rydberg fingerprint spectrum, a highly positive photoelectron anisotropy of 1.5 and a bi-exponential decay ( τ1= 141\pm43 fs, τ2= 4.0\pm0.2 ps for 240.6 nm pump), where the second time-constant shortens for lower wavelengths. Field-induced surface hopping dynamics calculations confirm that the initially excited ππ*-state relaxes very fast to an s-Rydberg state (first experimentally observed time-constant), and then more slowly to the first excited state/ground state (second time-constant). With higher excitation energies the conical intersection between the s-Rydberg-state and the first excited state is reached faster, resulting in shorter life-times. • The benzyl radical was excited yith 265 nm and probed with two wavelengths, 798 nm and 398 nm. Probed with 798 nm it shows a bi-exponential decay (\tau_{1}=84\pm5 fs, \tau_{2}=1.55\pm0.12 ps), whereas with 398 nm probe only the first time-constant is observed (\tau_{1}=89\pm5 fs). The photoelectron spectra with 798 nm probe is comparable to the spectrum with 398 nm probe during the first 60 fs, at longer times an additional band appears. This band is due to a [1+3']-process, whereas with 398 nm only signal from a [1+1']-process can be observed. Non-adiabatic dynamic on the fly calculations show that the initially excited, nearly degenerate ππ/p-Rydberg-states relax very fast (first time-constant) to an s-Rydberg state. This s-Rydberg state can no longer be ionized with 398 nm, but with 798 nm ionization via intermediate resonances is still possible. The s-Rydberg state then decays to the first excited state (second time-constant), which is long-lived. • Para-xylylene, excited with 266 nm into the S2-state and probed with 800 nm, shows a bi-exponential decay (\tau_{1}=38\pm7 fs, \tau_{2}=407\pm9 fs). The initially excited S2-state decays quickly to S1-state, which shows dissociative photoionization. The population of the S1-state is directly visible in the masses of the dissociative photoionization products, benzene and the para-xylylene -H. • Ortho-benzyne, produced via pyrolysis from benzocyclobutendione, was excited with 266 nm in the S2 state and probed with 800 nm. In its time-resolved mass spectra the dynamic of the ortho-benzyne signal was superposed with the dynamics from dissociative photoionization of the precursor and of the ortho-benzyne-dimer. With time-resolved ion imaging gated on the ortho-benzyne these processes could be seperated, showing that the S2-state of ortho-benzyne relaxes within 50 fs to the S1-state.},
  subject      = {Radikal <Chemie>},
  language  = {en}
}
@phdthesis{Schmitt2017,
  author    = {Schmitt, Hans-Christian},
  title     = {Deaktivierungsprozesse in isolierten aromatischen Heterocyclen und Pyrenen},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-155445},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2017},
  abstract  = {In der vorliegenden Arbeit wurde erfolgreich eine neue Gasphasen-Apparatur f{\"u}r Photoelektronen-Imaging-Experimente simuliert, aufgebaut und in Verbindung mit einem ps-Lasersystem in Betrieb genommen. Neben dem Aufbau der Apparatur stand die Aufkl{\"a}rung der Dynamik angeregter Zust{\"a}nde von aromatischen Heterocyclen und Pyrenen im Fokus dieser Arbeit. Die untersuchten Molek{\"u}le wurden durch Resonanzverst{\"a}rkte Mehrphotonenionisation in einem Molekularstrahlexperiment sowohl zeit-, als auch frequenzaufgel{\"o}st untersucht.},
  subject      = {Laserspektroskopie},
  language  = {de}
}
@article{KanalKeiberEcketal.2014,
  author    = {Kanal, Florian and Keiber, Sabine and Eck, Reiner and Brixner, Tobias},
  title     = {100-kHz shot-to-shot broadband data acquisition for high-repetition-rate pump-probe spectroscopy},
  doi       = {10.1364/OE.22.016965},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-112853},
  year      = {2014},
  abstract  = {Shot-to-shot broadband detection is common in ultrafast pump-probe spectroscopy. Taking advantage of the intensity correlation of subsequent laser pulses improves the signal-to-noise ratio. Finite data readout times of CCD chips in the employed spectrometer and the maximum available speed of mechanical pump-beam choppers typically limit this approach to lasers with repetition rates of a few kHz. For high-repetition (≥ 100 kHz) systems, one typically averages over a larger number of laser shots leading to inferior signal-to-noise ratios or longer measurement times. Here we demonstrate broadband shot-to-shot detection in transient absorption spectroscopy with a 100-kHz femtosecond laser system. This is made possible using a home-built high-speed chopper with external laser synchronization and a fast CCD line camera. Shot-to-shot detection can reduce the data acquisition time by two orders of magnitude compared to few-kHz lasers while keeping the same signal-to-noise ratio.},
  language  = {en}
}