| Summary: | Vibrational spectroscopy is an ideal tool to probe the complex structure of ice, water and other aqueous systems. However, the interpretation of experimental spectra is usually not straightforward, due to complex spectral features associated with different bonding configurations present in these systems. Therefore, accurate theoretical predictions are required to assign spectral signatures to specific structural properties and hence to fully exploit the potential of vibrational spectroscopies. My dissertation focused on the development and applications of first-principles electronic structure methods for the simulation of vibrational spectra of water and aqueous systems, as well as of their basic electronic properties.
In particular, I focused on the calculation of response properties of aqueous systems in the presence of external electric fields, including the computation of dipole and quadrupole moments and polarizabilities, which were then used to simulate vibrational spectra, e.g. Raman and sum frequency generation (SFG) spectra. I developed linear response and finite field methods based on electronic structure calculations within density functional theory (DFT), which were applied to accurate and efficient evaluations of the electric field response, and coupled to large-scale first-principles electronic structure and molecular dynamics (MD) simulations. Our implementation enabled on-the-fly calculations of polarizabilities in first-principles MD (FPMD) simulations, which are necessary to compute vibrational spectra using time correlation functions (TCF) formulations. In addition, in this dissertation I provide the first ab initio implementation of SFG calculations, inclusive of quadrupole contributions as well as of electric field gradients at the interface.
I present the first calculation of the Raman spectra of liquid water using FPMD simulations. Interesting signatures were found in the low frequency region of the spectra, indicating intermolecular charge fluctuations that accompany hydrogen bond stretching vibrations. Furthermore I applied the newly developed method for the calculation of SFG spectra, a surface specific spectroscopic probe, to the investigation of the ice Ih basal surfaces. Note that all the methods developed here, although only applied to aqueous systems, are of general applicability to semiconductors and insulators.
Finally I present investigations of the vibrational and electronic properties of aqueous sulfuric acid systems, including sulfate-water clusters and sulfuric acid solutions. Our results on the energy alignment between sulfate and water states bear important implications on the relative reactivities of ions and water in electrochemical environment used in water splitting reactions.
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