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Most properties of liquid water are determined by its hydrogen-bond network.Because forming an aqueous interface requires termination of this network, onemight expect the molecular level properties of interfacial water to markedlydiffer from water in bulk. Intriguingly, much prior experimental andtheoretical work has found that, from the perspective of their time-averagedstructure and picosecond structural dynamics, hydrogen-bonded OH groups at anair/water interface behave the same as hydrogen-bonded OH groups in bulk liquidwater. Here we report the first experimental observation of interfacial water'slibration (i.e. frustrated rotation) using the laser-based techniquevibrational sum frequency spectroscopy. We find this mode has a frequency of834 cm$^{-1}$, $\approx 165$ cm$^{-1}$ higher than in bulk liquid water at thesame temperature and similar to bulk ice. Because libration frequency isproportional to the stiffness of water's rotational potential, this increasesuggests that one effect of terminating bulk water's hydrogen bonding networkat the air/water interface is retarding rotation of water around intacthydrogen bonds. Because in bulk liquid water the libration plays a key role instabilizing reaction intermediates and dissipating excess vibrational energy,we expect the ability to probe this mode in interfacial water to open newperspectives on the kinetics of heterogeneous reactions at aqueous interfaces.

physical chemistry chemical physics/pccp. physical chemistry chemical physics

Experimentally Probing the Libration of Interfacial Water: the  Rotational Potential of Water is Stiffer at the Air/Water Interface than in  Bulk Liquid