The structural properties of water play an essential role in many chemical and biophysical processes, but a detailed description of its molecular structure and how it influences the dynamic properties of other molecules remains unclear. To accurately address these research questions requires advanced spectroscopic techniques with a temporal resolution on the order of picoseconds, matching the time scale of the structural fluctuations of water. This thesis reports on a series of experiments in which the interactions between water molecules and ions are examined using GHz dielectric relaxation spectroscopy and femtosecond vibrational spectroscopy. GHz dielectric relaxation spectroscopy has been used to understand (i) the extent to which the water structure is modified by the strong local electric field generated by ions, (ii) the water-ion interactions that give rise to local molecular ordering in hydration shells, and (iii) how the structure of water facilitates or hinders the diffusion of ions in solution. Femtosecond vibrational spectroscopy allows us to investigate how solutes change the structure and dynamics of water. We have used this method to investigate aqueous solutions of two commonly occurring ions: hydroxide and phenolate, and to study vibrational energy transfer in alkaline solutions due to the broad absorption band of hydrogen-bonded hydroxide ions.

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