Vibrational spectroscopy of crystalline multilayer ice: Surface modes in the intermolecular-vibration region

Vibrational studies have been made of single-crystalline ice by the use of electron energy loss spectroscopy (EELS). The EELS spectra exhibit vibrational features due to the surface as well as the bulk molecules. To differentiate between the surface and bulk modes, we examine primary energy dependence of the spectra and the adsorption of additional water molecules on the "clean" ice surface to selectively perturb the surface vibrations. Surface optical modes are detected in the intermolecular vibrational-energy region, and we observe a loss at 100 (95) cm(-1) which is ascribed to the hindered-translational vibration of the outermost H2O (D2O) along the surface normal direction. The surface hindered-rotational modes are observed at 470, 665, and 825 (355, 500, and 620) cm(-1). We suggest that the water molecules initially adsorbed on the ice surface are isolated and not clustered at 85 K. The admolecules are metastable at 85 K and restructure to form a stable bilayer-terminated ice surface at 128 K.