Vibrational Coupling and Hydrogen Bonding at the Air/Water Interface.

The ‘Free OH’ stretch mode observed by sum-frequency generation (SFG) vibrational spectroscopy presents a unique probe of the H-bonding at the air/water interface. This relatively narrow transition is well separated from the much broader and red-shifted part of the water OH-stretch band, thus permitting frequency-domain spectroscopic analysis. Furthermore, water molecules with the ‘free OH’ are only found in the top-most monolayer at the interface, which eliminates the ambiguity of the probe location (depth) in the interface. [ABSTRACT FROM AUTHOR]