Orientation and motion of water molecules at air/water interface

Here we report a quantitative study of the orientational structure and motion of water molecule at the air/water interface. Analysis of Sum Frequency Generation (SFG) vibrational peak of the free O-H stretching band at 3700 cm(-1) in four experimental configurations showed that orientational motion of water molecule at air/water interface is libratory within a limited angular range. The free OH bond of the interfacial water molecule is tilted around 33 degrees from the interface normal and the orientational distribution or motion width is less than 15 degrees. This picture is significantly different from the previous conclusion that the interfacial water molecule orientation varies over a broad range within the ultrafast vibrational relaxation time, the only direct experimental study concluded for ultrafast and broad orientational motion of a liquid interface by Wei et al. (Phys. Rev. Lett. 86, 4799, (2001)) using single SFG experimental configuration.