• Ahsan, M.S.: Energy- and Time-Resolved Liquid-Microjet Photoelectron Spectroscopy of Paradigmatic Aqueous-Phase Polyatomic Molecules. , Freie Universität Berlin, Dissertation, 2023

Open Access Version

Energy and charge transfer processes in aqueous solution play important roles in biochemical processes, where the mechanisms of such phenomena are affected, and even driven, by aqueous-phase reactant-water interactions. Such processes and interactions have been probed using the Liquid-Jet (LJ)-based Photoelectron Spectroscopy (PES) technique. Energy-tunable, ionizing-radiation sources and the non-resonant, resonant, steady-state, and ultrafast-time-resolved PES techniques have been applied to probe the surface or bulk and valence or core-level electronic, and by extension molecular, structure of several archetypal, highly-soluble, aqueous-phase molecular systems: triiodide, pyrimidine, pyridazine, and pyrazine. The chemically-important, lowest vertical ionization energies and characteristic, atom- and isomer-specific core-level (I 3d/4d, C and N 1s) spectra of the four studied systems are presented here. Ultrafast time-resolution studies of the excited-state dynamics of aqueous pyrazine solutions are also presented. With the help of electronic structure theory and spectral simulations, the experimental triiodide X-ray PES results suggest a near-linear geometric structure and increased asymmetry of the anion in aqueous solution, compared to in ethanol and methanol. This asymmetry was found to be greater at the surface-vacuum interface than in the aqueous solution bulk. Additionally, non-resonant and C and N pre-K-edge resonant X-ray PES experiments were performed on the aqueous diazine molecules, revealing the vertical ionization potentials, bonding character and atomic parentage, intra- and intermolecular charge re-arrangement, and degree of localization of the valence band Molecular Orbitals (MOs). Having measured the ground-state ionization energetics, UV-pump (267 nm), EUV-probe (32.1 nm) femtosecond time-resolved LJ-PES measurements were performed on aqueous pyrazine solutions under Pump-Induced-Space-Charge (PISC) minimized conditions. Global fit analyses of the LJ-PES data revealed an initial 40 ± 20 fs internal conversion (IC) timescale, with subsequent population transfer to lower-lying states occurring in 35 ± 10 ps and 120 ± 30 ps. Slower relaxation behaviors were found in aqueous pyrazine solution in comparison to previously reported gas-phase studies. Associated solvent-induced relaxation mechanisms have been proposed. Furthermore, it is suggested that the relaxation dynamics of aqueous pyrazine are altered when similar experiments are performed under PISC conditions. In general, this thesis highlights the sensitivity of LJ-PES to solution-phase electronic structure, molecular geometries, and dynamic photophysicochemical processes. The experimental results provide the necessary information to enhance the implemented experimental techniques, model general polyatomic molecular behaviors in aqueous solutions, and better understand the photochemical processes that underly numerous real-world applications.