Ahmed Mohammed, Nihonyanagi Satoshi, Kundu Achintya, Yamaguchi Shoichi, Tahara Tahei
Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
Ultrafast Spectroscopy Research Team, RIKEN Center for Advanced Photonics (RAP), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
J Phys Chem Lett. 2020 Nov 5;11(21):9123-9130. doi: 10.1021/acs.jpclett.0c02644. Epub 2020 Oct 14.
Recently, there has been controversy over whether the HOH bend signal of water in the vibrational sum frequency generation (VSFG) spectrum arises from the conventional dipole mechanism or the quadrupole mechanism. Here, we show that the Im χ (the imaginary part of the second-order nonlinear susceptibility) spectra of the HOH bend mode of water at oppositely charged monolayer/water interfaces all exhibit positive bands, irrespective of the difference in the sign of the charge at the interface. Furthermore, it is found that the peak frequency of the HOH bend band substantially changes depending on the chemical structure of the charged headgroup located at the interface. These results demonstrate that the VSFG signal of the HOH bend vibration is generated from interfacial water with the interfacial quadrupole mechanism that is associated with the large field gradient of incident lights localized in a very thin region at the interface.
最近，关于振动和频产生（VSFG）光谱中水分子的HOH弯曲信号是源于传统偶极子机制还是四极子机制存在争议。在此，我们表明，在带相反电荷的单层/水界面处，水分子HOH弯曲模式的Im χ（二阶非线性极化率的虚部）光谱均呈现正峰，而与界面电荷符号的差异无关。此外，还发现HOH弯曲带的峰值频率会根据位于界面处的带电头基的化学结构而发生显著变化。这些结果表明，HOH弯曲振动的VSFG信号是由界面水通过界面四极子机制产生的，该机制与局域在界面极薄区域内的入射光的大场梯度相关。
