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Femtosecond Dynamics in Liquids: Solvated Electrons and Small-Molecule Systems

  • Author(s): Farr, Erik Peter
  • Advisor(s): Schwartz, Benjamin J
  • et al.

This thesis is broadly concerned with understanding the structural and energetic details of condensed phase chemistry, primarily on ultrafast timescales. The first chapter focuses on novel contributions regarding the nature of the hydrated electron. It has been thought that this quasi-free solvent-supported electron resided in a cavity by its repulsive Coulombic interactions with nearby water molecules. Instead, a relatively modern but controversial simulation of the hydrated electron has shown that many observables are in fact better described by a non-cavity structure in which the hydrated electron's wave function resides in the interstitial spaces between water that is at, or slightly above, bulk density near and within the electron. The novel contributions have been understanding the effects of temperature on the structure and dynamics of the hydrated electron. This newly observed experimental temperature dependence of dynamics is highly consistent with the new non-cavity model of the hydrated electron. Secondarily, we show that previous methods of determining the hydrated electron's first excited-state lifetime from transient absorption were fraught with parameter correlation, making clean identification of the lifetime impossible. To resolve this we employ a more sophisticated model in combination with better signal to noise from broadband transient absorption measurements to show with certainty that the first excited-state lifetime of the hydrated electron at room temperature is on the order of 100 fs---in agreement with recent time-resolved photoelectron experiments. The second chapter brings these concepts of time-resolved spectroscopy to an advanced undergraduate level through a novel laboratory experiment. In order to provide access to undergraduates, I built a low-cost combined transient absorption and time-resolved fluorescence spectrometer. Simultaneously, I developed an experiment limited by the temporal and spectral resolution of the instrument in which undergraduates measure the fluorescent and phosphorescent lifetimes of the dye Eosin B. With these lifetimes in hand, the undergraduates then arrive at a complete photophysical picture for the molecule and quantitatively interpret their results with introductory quantum mechanics for electronic spectroscopy. Finally, the third chapter highlights time-resolved and steady-state spectroscopic investigations of singly linked di-perylenediimide, a key acceptor material used in competitive organic photovoltaics. We show that this molecule exists in a range geometrical configurations at room temperature, and that these conformations are spectrally distinct. Furthermore, the typical approximations used to describe this dimer as a Kasha H-/J-aggregate do not appear reasonable evidenced by detailed deconvolution of underlying spectral components with a high density of states---further confirmed with time-dependent density functional theory. The overarching theme of these chapters is to understand molecular photophysics in condensed phases on ultrafast timescales by using or refining modern principles of physical chemistry.

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