Dokument-ID Dokumenttyp Verfasser/Autoren Herausgeber Haupttitel Abstract Auflage Verlagsort Verlag Erscheinungsjahr Seitenzahl Schriftenreihe Titel Schriftenreihe Bandzahl ISBN Quelle der Hochschulschrift Konferenzname Quelle:Titel Quelle:Jahrgang Quelle:Heftnummer Quelle:Erste Seite Quelle:Letzte Seite URN DOI Abteilungen OPUS4-17848 unpublished Süß, Jasmin; Wehner, Johannes G.; Dostál, Jakub; Engel, Volker; Brixner, Tobias Mapping of exciton-exciton annihilation in a molecular dimer via fifth-order femtosecond two-dimensional spectroscopy We present a theoretical study on exciton-exciton annihilation (EEA) in a molecular dimer. This process is monitored using a fifth-order coherent two-dimensional (2D) spectroscopy as was recently proposed by Dostál et al. [Nat. Commun. 9, 2466 (2018)]. Using an electronic three-level system for each monomer, we analyze the different paths which contribute to the 2D spectrum. The spectrum is determined by two entangled relaxation processes, namely, the EEA and the direct relaxation of higher lying excited states. It is shown that the change of the spectrum as a function of a pulse delay can be linked directly to the presence of the EEA process. 2019 Journal of Physical Chemistry Letters urn:nbn:de:bvb:20-opus-178482 Institut für Physikalische und Theoretische Chemie OPUS4-13441 Dissertation Bieker, Steffen Time and Spatially Resolved Photoluminescence Spectroscopy of Hot Excitons in Gallium Arsenide The present thesis investigates the impact of hot exciton effects on the low-temperature time and spatially resolved photoluminescence (PL) response of free excitons in high-purity gallium arsenide (GaAs). The work at hand extends available studies of hot carrier effects, which in bulk GaAs have up to now focused on hot electron populations. In crucial distinction from previous work, we extensively study the free exciton second LO-phonon replica. The benefit of this approach is twofold. First, the two LO phonon-assisted radiative recombination allows to circumvent the inherent interpretation ambiguities of the previously investigated free exciton zero-phonon line. Second, the recombination line shape of the second LO-phonon replica provides direct experimental access to the exciton temperature, thereby enabling the quantitative assessment of hot exciton effects. In the first part of the thesis, we address the influence of transient cooling on the time evolution of an initially hot photocarrier ensemble. To this end, we investigate time-resolved photoluminescence (TRPL) signals detected on the free exciton second LO-phonon replica. Settling a long-standing question, we show by comparison with TRPL transients of the free exciton zero-phonon line that the slow free exciton photoluminescence rise following pulsed optical excitation is dominated by the slow buildup of a free exciton population and not by the relaxation of large K-vector excitons to the Brillouin zone center. To establish a quantitative picture of the delayed photoluminescence onset, we determine the cooling dynamics of the initially hot photocarrier cloud from a time-resolved line shape analysis of the second LO-phonon replica. We demonstrate that the Saha equation, which fundamentally describes the thermodynamic population balance between free excitons and the uncorrelated electron-hole plasma, directly translates the experimentally derived cooling curves into the time-dependent conversion of unbound electron-hole pairs into free excitons. In the second part of the thesis, we establish the impact of hot exciton effects on low-temperature spatially resolved photoluminescence (SRPL) studies. Such experiments are widely used to investigate charge carrier and free exciton diffusion in semiconductors and semiconductor nanostructures. By SRPL spectroscopy of the second LO-phonon replica, we show that above-band gap focused laser excitation inevitably causes local heating in the carrier system, which crucially affects the diffusive expansion of a locally excited exciton packet. Undistorted free exciton diffusion profiles, which are correctly described by the commonly used formulation of the photocarrier diffusion equation, are only observed in the absence of spatial temperature gradients. At low sample temperatures, the reliable determination of free exciton diffusion coefficients from both continuous-wave and time-resolved SRPL spectroscopy requires strictly resonant optical excitation. Using resonant laser excitation, we observe the dimensional crossover of free exciton diffusion in etched wire structures of a thin, effectively two-dimensional GaAs epilayer. When the lateral wire width falls below the diffusion length, the sample geometry becomes effectively one-dimensional. The exciton diffusion profile along the wire stripe is then consistently reproduced by the steady-state solution to the one-dimensional diffusion equation. Finally, we demonstrate the formation of macroscopic free and bound exciton photoluminescence rings in bulk GaAs around a focused laser excitation spot. Both ring formation effects are due to pump-induced local heating in the exciton system. For a quantitative assessment of the mechanism underlying the free exciton ring formation, we directly determine the exciton temperature gradient from a spatially resolved line shape analysis of the free exciton second LO-phonon replica. We demonstrate that a pump-induced hot spot locally modifies the thermodynamic population balance between free excitons and unbound electron-hole pairs described by the Saha equation, which naturally explains the emergence of macroscopic free exciton ring structures. In summary, we demonstrate that quantitative consideration of hot exciton effects provides a coherent picture both of the time-domain free exciton luminescence kinetics and of the distinct spatially resolved photoluminescence patterns developing under the influence of spatial photocarrier diffusion. 2015 urn:nbn:de:bvb:20-opus-134419 Physikalisches Institut OPUS4-4773 Dissertation Liu, Wenlan Exciton Coupling in Valence and Core Excited Aggregates of pi-Conjugated Molecules Im Rahmen dieser Arbeit werden theoretische Modelle zur Beschreibung von Valenz- und Rumpf-angeregten elektronischen Zuständen diskutiert. Im Fall der Valenz-Anregungen wurden time-dependend Hartree-Fock (TD-HF) und timedependent Dichtefunktionaltheorie (TD-DFT)Methoden mit verschiedenen Funktionalen für ein Perylenbisimid (PBI) System validiert. Eine einfache Analyse der Charaktäre der angeregten Zustände wurde vorgeschlagen, die auf den berechneten Übergangsdipolmomenten basiert. Dieser Ansatz ist allerdings auf Zustände beschränkt, die ein signifikantes Übergangsdipolmoment aufweisen. Deshalb wurde eine allgemeinere und fundiertere Methode entwickelt, die auf einer Analyse der berechneten CISWellenfunktion basiert. Darüberhinaus wurde ein literaturbekannter Model-Hamiltonoperator Ansatz von einem lokalisierten Molekülorbitalbild (MO) abgeleitet, das aus der generelleren Analyse-Methode resultiert. Auf diesem Weg ist ein Zugang zu diabatischen angeregten Zuständen und korrespondierenden Kopplungsparametern auf der Basis von ab initio Rechnungen gegeben. Für rumpfangeregte elektronische Zustände wurden drei Methoden für C 1s-angeregte und ionisierte Zustände verschiedener kleiner Moleküle validiert. Darüberhinaus wurde die Basissatzabhängigkeit dieser Zustände untersucht. Anhand der Resultate wurde die frozen core Näherung ausgewählt um rumpfangeregte Zustände von Naphthalintetracarbonsäuredianhydrid (NTCDA) zu berechnen. Um experimentelle Ergebnisse zu erklären, wurde ein Algorithmus entwicklet, der die Exzitonenkopplungsparameter im Fall von nicht-orthogonalen MOs berechnet. 2011 urn:nbn:de:bvb:20-opus-56169 Institut für Physikalische und Theoretische Chemie