Time-Resolved Dynamics of the OH Stretching Vibration in Aqueous NaCl Hydrate

Abstract

We report on the first time-resolved study of the OH stretching vibration in NaCl dihydrate with the use of two-color IR spectroscopy. The sample is characterized by conventional FTIR spectroscopy. The water molecules bound in the hydrate show two well separated absorption bands at 3426 cm(-1) and 3541 cm(-1). The transient data display an ultrafast heating of the polycrystalline ice-hydrate samples after excitation of the OH stretching vibration and its transient relaxation. The relaxation time of the low-frequency OH stretching band in the NaCl hydrate is measured to be 6.8 +/- 1 ps. The dynamics are significantly slower than those measured in neat water. This fact, together with the reproducible crystalline environment reveals the potential of aqueous hydrates for a systematic investigation of the OH stretching vibration in varying hydrogen bonding environments.