Ultra-broadband Ultrafast Vibrational Spectroscopy & Dynamics of Mineral/Aqueous Interfaces

The properties and behavior of water at interfaces is important in many fields. However, it is not clear what effects the presence of the surface, the charge that can develop on the surface, the solution ionic strength, and the interfacial electric field, have on how interfacial water molecules communicate with each other, e.g., how thermal (vibrational) energy flows. To address these issues we have investigated the ultrafast vibrational population and dephasing dynamics of the O-H stretch using IR pump-vibrational Sum Frequency Generation (vSFG) probe at the water/mineral interfaces.[1] Contrary to previous reports, the vibrational lifetime of the O-H stretch at the silica/water interface is ~ 600 fs, a factor 2-3 slower than bulk water, when the surface is neutral.[1] Charging the SiO2 interface appears to lead to a dramatic acceleration of vibrational relaxation. Experiments on the effect of ionic strength, suggest that the primary reason for accelerated dynamics at pH=6 is the sampling of water within the Debye length that has bulk-like solvation.[2] Acquisition of sum-frequency spectra of hydroxyl vibrations at silica water interfaces with a high signal-to-noise ratio over the frequency ranges of fundamental stretches and combination bands becomes possible with our newly developed broadband OPA – SFG setup.[4] A newly developed SFG spectrometer, based on a novel ultrabroadband optical parametric amplifie