Sun Shumei, Liang Rongda, Xu Xiaofan, Zhu Heyuan, Shen Y Ron, Tian Chuanshan
Department of Physics, State Key Laboratory of Surface Physics, and Key Laboratory of Micro- and Nano-Photonic Structures (MOE), Fudan University, Shanghai 200433, China.
Department of Optical Science and Engineering, Fudan University, Shanghai 200433, China.
J Chem Phys. 2016 Jun 28;144(24):244711. doi: 10.1063/1.4954824.
Phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS) has been established as a powerful technique for surface characterization, but for it to generate a reliable spectrum, accurate phase measurement with a well-defined phase reference is most important. Incorrect phase measurement can lead to significant distortion of a spectrum, as recently seen in the case for the air/water interface. In this work, we show theoretically and experimentally that a transparent, highly nonlinear crystal, such as quartz and barium borate, can be a good phase reference if the surface is clean and unstrained and the crystal is properly oriented to yield a strong SF output. In such cases, the reflected SF signal is dominated by the bulk electric dipole contribution and its phase is either +90° or -90°. On the other hand, materials with inversion symmetry, such as water, fused quartz, and CaF2 are not good phase references due to the quadrupole contribution and phase dispersion at the interface. Using a proper phase reference in PS-SFVS, we have found the most reliable OH stretching spectrum for the air/water interface. The positive band at low frequencies in the imaginary component of the spectrum, which has garnered much interest and been interpreted by many to be due to strongly hydrogen-bonded water species, is no longer present. A weak positive feature however still exists. Its magnitude approximately equals to that of air/D2O away from resonances, suggesting that this positive feature is unrelated to surface resonance of water.
相敏和频振动光谱（PS-SFVS）已成为一种用于表面表征的强大技术，但要使其产生可靠的光谱，使用定义明确的相位参考进行精确的相位测量最为重要。相位测量不正确会导致光谱出现显著失真，就像最近在空气/水界面的情况中所看到的那样。在这项工作中，我们通过理论和实验表明，如果表面清洁且无应变，并且晶体取向合适以产生强的和频输出，那么诸如石英和硼酸钡之类的透明、高度非线性晶体可以作为良好的相位参考。在这种情况下，反射的和频信号主要由体电偶极贡献主导，其相位为 +90° 或 -90°。另一方面，具有反演对称性的材料，如水、熔融石英和CaF2，由于界面处的四极贡献和相位色散，不是良好的相位参考。在PS-SFVS中使用合适的相位参考，我们找到了空气/水界面最可靠的OH伸缩光谱。光谱虚部低频处的正带不再存在，该正带引起了很多关注，许多人将其解释为归因于强氢键结合的水物种。然而，一个微弱的正特征仍然存在。其幅度大约等于远离共振时空气/D2O的幅度，这表明该正特征与水的表面共振无关。
