Here we report a detailed study on spectroscopy, structure and dynamics of water molecules at air/water interface, investigated with Sum Frequency Generation Vibrational Spectroscopy (SFG-VS). Quantitative polarization and experimental configuration analysis of the SFG data in different polarizations with four sets of experimental configurations can shed new lights on our present understanding of the air/water interface. Firstly, we concluded that the motion of the interfacial water molecules can only be in a limited angular range, instead rapidly varying over a broad angular range in the vibrational relaxation time suggested previously. Secondly, because different vibrational modes of different molecular species at the interface has different symmetry properties, polarization and symmetry analysis of the SFG-VS spectral features can help assignment of the SFG-VS spectra peaks to different interfacial species. These analysis concluded that the narrow 3693cm −1 and broad 3550cm −1 peaks belong to C∞v symmetry, while the broad 3250cm −1 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 single hydrogen bonded OH stretching mode, and the 3250cm −1 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 direct almost parallel to the interface, and is with a very narrow orientational distribution. The doubly hydrogen bond donor water molecules have their dipole vector point away from the liquid phase.

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In this report, we present an analysis of the interference effects in the SFG-VS spectra of the vapor/ethanol interfaces with normal and deuterated liquid ethanol. Crucial information on the ethanol molecular orientation can be obtained from analyzing the interference effects between the different SFG-VS spectral features. With the strength-coupled orientational functional dr(θ) of the different spectral features from the same or different molecular groups, and with the phase information contained in the SFG spectra measured under different polarization combinations, the detailed orientational conformation structure of the ethanol molecule at the vapor/ethanol interface can be obtained. The ethanol molecule is found to possess an orientational conformation with its CH 3 -group pointing away from the liquid phase at an orientational tilt angle of about 45°, its -CH 2 -group away from the liquid interface around 10°, and the C-C-O plane is near perpendicular to the plane of interface. Here, the conclusion that the -CH 2 -group has to tilt away from the liquid phase can only be reached with the knowledge of the spectral overlap and the analysis of the interference between the overlapping spectral features. The details of the molecular orientation angle and the orientational distribution width were discussed with novel approaches. These results suggested that studying coherent interference effects in the polarization analysis in SFG-VS can provide novel understanding of the interfacial molecular structure and interactions. The tools and the concepts of SFG-VS analysis developed in this study can be applied to more complex molecular interfaces and thin films.

Substantial progress has been made in the quantitative understanding and interpretation of the hydrogen bonding and ordering structure of the air/water interface since the first sum-frequency generation vibrational spectroscopy (SFG-VS) measurement by Q. Du et al. in 1993 (Phys. Rev. Lett. 1993, 70, 2312-2316). However, there are still disagreements and controversies on the consistency between the different experimental measurements, as well as in the theoretical and computational results. One critical problem lies in the lack of consistency between the SFG-VS intensity measurements and the recently developed SFG-VS phase spectra measurements of the neat air/water interface, which has inspired various theoretical efforts. In this report, the reliability of the SFG-VS intensity spectra of the air/neat water interface is to be quantitatively examined, and possible sources of inaccuracies in the SFG-VS phase spectral measurement are to be discussed based on the nonresonant SHG phase measurements. Solid evidence is shown indicating that the SFG-VS intensity spectra from different laboratories are now quantitatively converging and in agreement with each other. However, the possible inaccuracies and inconsistencies in the SFG-VS phase spectra measurements need to be carefully examined against a properly corrected phase standard to take full advantage of this powerful experimental tool.