空气/水界面和频产生(SFG)振动光谱的偏振特性和实验装置分析
Here we report a detailed study on spectroscopy, structure and dynamics of water molecules atair/water interface, investigated with Sum Frequency Generation Vibrational Spectroscopy (SFGVS).Quantitative polarization and experimental configuration analysis of the SFG data in differentpolarizations with four sets of experimental configurations can shed new lights on our presentunderstanding of the air/water interface. Firstly, we concluded that the motion of the interfacialwater molecules can only be in a limited angular range, instead rapidly varying over a broad angularrange in the vibrational relaxation time suggested previously. Secondly, because different vibrationalmodes of different molecular species at the interface has different symmetry properties, polarizationand symmetry analysis of the SFG-VS spectral features can help assignment of the SFG-VS spectrapeaks to different interfacial species. These analysis concluded that the narrow 3693cm− 1 and broad3550cm− 1 peaks belong to C∞v symmetry, while the broad 3250cm− 1 and 3450cm− 1 peaks belongto the symmetric stretching modes with C2v symmetry. Thus, the 3693cm− 1 peak is assigned tothe 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 hydrogendonors for hydrogen bonding (with C2v symmetry), respectively. Thirdly, analysis of the SFG-VSspectra concluded that the singly hydrogen bonded water molecules at the air/water interface havetheir dipole vector direct almost parallel to the interface, and is with a very narrow orientationaldistribution. The doubly hydrogen bond donor water molecules have their dipole vector point awayfrom the liquid phase.