Observation and Identifcation 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 7 distinct modes in the sum-frequency response, five of which have an analogue in the bulk, and two pure surface-specific modes can be identified 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 of water molecules with two donor and one acceptor hydrogen-bond.