Molecular level descriptions of the structure and dynamics of water and the hydronium ion are essential to understand quantitatively the dispersion of contaminants in groundwater, the role of aqueous aerosols in atmospheric chemistry and the optimal design of biomaterials. The structure of water and the stabilization of the hydronium ion at interfaces is a function of a hydrogen bond network. Each hydrogen bond in the network breaks and reforms on picosecond timescales. Current work describes structure in these systems by measuring the interfacial water and hydronium OH stretch frequency range using vibrational sum frequency spectroscopy (VSFS). Generally VSFS is time averaged: each data point in a spectrum involves several seconds of data collection. This approach necessitates loss of molecular information (in reality water structure evolves on picosecond timescales) and makes comparison to simulation, where trajectories have a maximum length of tens of nanoseconds, challenging. The proposed experiments overcome this obstacle by examining the air/water and mineral/water systems using femtosecond time resolved two-dimensional VSFS (tr2D-VSFS). This method allows the quantification of hydrogen bond (as a function of frequency) and hydronium lifetime in interfacial water. The measurement of these quantities at a variety of interfaces will allow general insight into the structure of aqueous complexes at interfaces, directly connect with simulation and help supply an experimental molecular level picture of the air/water and mineral/water intefaces that has been lacking. tr2D-VSFS is a specialized technique (employed currently by 1-2 research groups in the world). The impact of this proposal rests on the combination of an Earth Scientist with a peculiar background (nonlinear optics and computational chemistry), a host research group at the forefront of chemical physics and a host institute well prepared to support such a multidisciplinary collaboration.
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