Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako 351-0198, Japan.
J Chem Phys. 2012 Sep 7;137(9):094706. doi: 10.1063/1.4747828.
Two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) spectroscopy is performed for an aqueous interface for the first time. The 2D HD-VSFG spectra in the OH stretch region are obtained from a positively charged surfactant/water interface with isotopically diluted water (HOD/D(2)O) to reveal the femtosecond vibrational dynamics of water at the charged interface. The 2D HD-VSFG spectrum is diagonally elongated immediately after photoexcitation, clearly demonstrating inhomogeneity in the interfacial water. This elongation almost disappears at 300 fs owing to the spectral diffusion. Interestingly, the 2D HD-VSFG spectrum at the 0 fs shows an oppositely asymmetric shape to the corresponding 2D IR spectrum in bulk water: The bandwidth of the bleach signal gets narrower when the pump wavenumber becomes higher. This suggests that the dynamics and mechanism of the hydrogen bond rearrangement at the charged interface are significantly different from those in bulk water.
首次在水界面上进行二维差频探测振动和频光谱（2D HD-VSFG）实验。通过使用氘代水（HOD/D2O）对带正电荷的表面活性剂/水界面进行二维 HD-VSFG 实验，得到了在 OH 伸缩区域的 2D HD-VSFG 光谱，以揭示带电荷界面上水的飞秒振动动力学。在光激发后，2D HD-VSFG 光谱立即呈现出对角线拉长的形状，清楚地表明了界面水的不均匀性。这种拉长在 300fs 时由于光谱扩散几乎消失。有趣的是，0fs 时的 2D HD-VSFG 光谱与体相水中的相应 2D IR 光谱具有相反的非对称形状：当泵浦波数增加时，漂白信号的带宽变窄。这表明在带电荷界面上氢键重排的动力学和机制与体相水中显著不同。
