Water surfaces play an important role in many biological, chemical and physical processes. Many reactions occur at the water interface and some are even favored to happen there. Microscopically, many of the interfacial properties, such as the structure or dynamics of water molecules, are related to the termination of the dynamic hydrogen-bond network that is present in bulk water. Time resolved pump-probe vibrational sum-frequency generation spectroscopy at the water/hydrophobic interface in combination with total internal reflection geometry allows the observation of the surface dynamics with a large signal-to-noise ratio. Pump and probe beams are both resonant with the surface-specific dangling OH stretch vibration. The pump-induced orientational anisotropy is measured by pumping with polarizations parallel and perpendicular to the probe polarization. The measurements show that the decay of the dangling OH stretch excitation is dominated by a 1.61 \pm 0.10ps jump to a hydrogen-bonded configuration. The dangling OH jump time at hydrophobic interfaces is twice as fast as a jump between hydrogen-bonded configurations in bulk water and about 50% slower than what is reported for the water/air interface. Upon introduction of salt, changes in structure of water are observed in the bulk and at aqueous interfaces. This is reflected in macroscopic observables such as altered viscosity or surface tension. Moreover, research on bulk salt solutions shows significant changes in dynamics that, in some cases, extend beyond the first solvation shell and might be expected to alter the interfacial dynamics. However, our experiments suggest that the reorientation dynamics associated with the jump from a dangling to a bonded OH is not affected by the dissolved salts used in our research.
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