  Most properties of liquid water are determined by its hydrogen-bond network.
Because forming an aqueous interface requires termination of this network, one
might expect the molecular level properties of interfacial water to markedly
differ from water in bulk. Intriguingly, much prior experimental and
theoretical work has found that, from the perspective of their time-averaged
structure and picosecond structural dynamics, hydrogen-bonded OH groups at an
air/water interface behave the same as hydrogen-bonded OH groups in bulk liquid
water. Here we report the first experimental observation of interfacial water's
libration (i.e. frustrated rotation) using the laser-based technique
vibrational sum frequency spectroscopy. We find this mode has a frequency of
834 cm$^{-1}$, $\approx 165$ cm$^{-1}$ higher than in bulk liquid water at the
same temperature and similar to bulk ice. Because libration frequency is
proportional to the stiffness of water's rotational potential, this increase
suggests that one effect of terminating bulk water's hydrogen bonding network
at the air/water interface is retarding rotation of water around intact
hydrogen bonds. Because in bulk liquid water the libration plays a key role in
stabilizing reaction intermediates and dissipating excess vibrational energy,
we expect the ability to probe this mode in interfacial water to open new
perspectives on the kinetics of heterogeneous reactions at aqueous interfaces.
