Matsuzaki Korenobu, Nihonyanagi Satoshi, Yamaguchi Shoichi, Nagata Takashi, Tahara Tahei
†Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
‡Department of Chemistry, School of Science, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan.
J Phys Chem Lett. 2013 May 16;4(10):1654-8. doi: 10.1021/jz400829k. Epub 2013 May 1.
The interface selectivity of vibrational sum frequency generation (VSFG) spectroscopy is explained under the dipole approximation as resulting from the breakdown of inversion symmetry at the interface. From this viewpoint, VSFG is not expected to occur at the interface consisting of centrosymmetric molecules, because the inversion symmetry is preserved even at the interface. In reality, however, VSFG at the nonpolar benzene/air interface has been observed with traditional homodyne-detected VSFG. Here we report a heterodyne-detected VSFG study of the benzene/air interface. The result strongly indicates that VSFG at this interface cannot be explained within the framework of the dipole approximation. The selection rule and polarization dependence of the observed VSFG signal show that the quadrupole transition plays an essential role because of the field discontinuity at the interface. This finding implies the applicability of interface-selective VSFG to the nonpolar interfaces comprising centrosymmetric molecules, which opens a new possibility of VSFG spectroscopy.
在偶极近似下，振动和频产生（VSFG）光谱的界面选择性被解释为是由于界面处反演对称性的破坏所致。从这个观点来看，在由中心对称分子组成的界面处预计不会发生VSFG，因为即使在界面处反演对称性也得以保留。然而，实际上，传统零差检测的VSFG已观测到非极性苯/空气界面处的VSFG。在此我们报告一项关于苯/空气界面的外差检测VSFG研究。结果有力地表明，该界面处的VSFG无法在偶极近似框架内得到解释。所观测到的VSFG信号的选择规则和偏振依赖性表明，由于界面处的场不连续性，四极跃迁起着至关重要的作用。这一发现意味着界面选择性VSFG适用于由中心对称分子组成的非极性界面，这为VSFG光谱学开辟了新的可能性。
