Polarization and experimental configuration analyses of sum frequency generation vibrational spectra, structure, and orientational motion of the air/water interface

Here we report a detailed study on spectroscopy, structure, and orientational distribution, as well as orientational motion, of water molecules at the air/water interface, investigated with sum frequency generation vibrational spectroscopy (SFG-VS). Quantitative polarization and experimental configuration analyses of the SFG data in different polarizations with four sets of experimental configurations can shed new light on our present understanding of the air/water interface. Firstly, we concluded that the orientational motion of the interfacial water molecules can only be in a limited angular range, instead of rapidly varying over a broad angular range in the vibrational relaxation time as suggested previously. Secondly, because different vibrational modes of different molecular species at the interface has different symmetry properties, polarization and symmetry analyses of the SFG-VS spectral features can help the assignment of the SFG-VS spectra peaks to different interfacial species. These analyses concluded that the narrow $3693cm−1$ and broad $3550cm−1$ peaks belong to $C∞v$ symmetry, while the broad 3250 and $3450cm−1$ peaks belong to the symmetric stretching modes with $C2v$ symmetry. Thus, the $3693cm−1$ peak is assigned to the free OH, the $3550cm−1$ peak is assigned to the singly hydrogen-bonded OH stretching mode, and the 3250 and $3450cm−1$ peaks are assigned to interfacial water molecules as two hydrogen donors for hydrogen bonding (with $C2v$ symmetry), respectively. Thirdly, analysis of the SFG-VS spectra concluded that the singly hydrogen-bonded water molecules at the air/water interface have their dipole vector directed almost parallel to the interface and is with a very narrow orientational distribution. The doubly hydrogen-bonded donor water molecules have their dipole vector pointing away from the liquid phase.