Observation and Identification of a New OH Stretch Vibrational Band at the Surface of Ice.

We study the signatures of the OH stretch vibrations at the basal surface of ice using heterodyne-detected sum-frequency generation and molecular dynamics simulations. At 150 K, we observe seven distinct modes in the sum-frequency response, five of which have an analogue in the bulk, and two pure surface-specific modes at higher frequencies (∼3530 and ∼3700 cm-1). The band at ∼3530 cm-1 has not been reported previously. Using molecular dynamics simulations, we find that the ∼3530 cm-1 band contains contributions from OH stretch vibrations of both fully coordinated interfacial water molecules and water molecules with two donor and one acceptor hydrogen bond.