title: Reconsideration of second-harmonic generation from isotropic liquid interface: broken Kleinman symmetry of neat air/water interface from dipolar contribution.
abstract: It has been generally accepted that there are significant quadrupolar and bulk contributions to the second-harmonic generation (SHG) reflected from the neat air/water interface, as well as common liquid interfaces. Because there has been no general methodology to determine the quadrupolar and bulk contributions to the SHG signal from a liquid interface, this conclusion was reached based on the following two experimental phenomena: the breaking of the macroscopic Kleinman symmetry and the significant temperature dependence of the SHG signal from the neat air/water interface. However, because the sum frequency generation vibrational spectroscopy (SFG-VS) measurement of the neat air/water interface observed no apparent temperature dependence, the temperature dependence in the SHG measurement has been reexamined and proven to be an experimental artifact. Here we present a complete microscopic analysis of the susceptibility tensors of the air/water interface, and show that dipolar contribution alone can be used to address the issue of the breaking of the macroscopic Kleinman symmetry at the neat air/water interface. Using this analysis, the orientation of the water molecules at the interface can be obtained, and it is consistent with the measurement from SFG-VS. Therefore, the key rationales to conclude significantly quadrupolar and bulk contributions to the SHG signal of the neat air/water interface can no longer be considered as valid as before. This new understanding of the air/water interface can shed light on our understanding of the nonlinear optical responses from other molecular interfaces as well.
authors.affiliation: State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China.
authors.forename: Wen-kai
authors.initials: WK
authors.lastname: Zhang
authors.forename: De-sheng
authors.initials: DS
authors.lastname: Zheng
authors.forename: Yan-yan
authors.initials: YY
authors.lastname: Xu
authors.forename: Hong-tao
authors.initials: HT
authors.lastname: Bian
authors.forename: Yuan
authors.initials: Y
authors.lastname: Guo
authors.forename: Hong-fei
authors.initials: HF
authors.lastname: Wang