Inoue Ken-ichi, Nihonyanagi Satoshi, Singh Prashant C, Yamaguchi Shoichi, Tahara Tahei
Molecular Spectroscopy Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
J Chem Phys. 2015 Jun 7;142(21):212431. doi: 10.1063/1.4918644.
Two-dimensional heterodyne-detected vibrational sum-frequency generation (2D HD-VSFG) spectroscopy is applied to study the ultrafast vibrational dynamics of water at positively charged aqueous interfaces, and 2D HD-VSFG spectra of cetyltrimethylammonium bromide (CTAB)/water interfaces in the whole hydrogen-bonded OH stretch region (3000 cm(-1) ≤ ωpump ≤ 3600 cm(-1)) are measured. 2D HD-VSFG spectrum of the CTAB/isotopically diluted water (HOD-D2O) interface exhibits a diagonally elongated bleaching lobe immediately after excitation, which becomes round with a time constant of ∼0.3 ps due to spectral diffusion. In contrast, 2D HD-VSFG spectrum of the CTAB/H2O interface at 0.0 ps clearly shows two diagonal peaks and their cross peaks in the bleaching region, corresponding to the double peaks observed at 3230 cm(-1) and 3420 cm(-1) in the steady-state HD-VSFG spectrum. Horizontal slices of the 2D spectrum show that the relative intensity of the two peaks of the bleaching at the CTAB/H2O interface gradually change with the change of the pump frequency. We simulate the pump-frequency dependence of the bleaching feature using a model that takes account of the Fermi resonance and inhomogeneity of the OH stretch vibration, and the simulated spectra reproduce the essential features of the 2D HD-VSFG spectra of the CTAB/H2O interface. The present study demonstrates that heterodyne detection of the time-resolved VSFG is critically important for studying the ultrafast dynamics of water interfaces and for unveiling the underlying mechanism.
二维外差检测振动和频产生（2D HD-VSFG）光谱被用于研究带正电的水相界面处水的超快振动动力学，并测量了十六烷基三甲基溴化铵（CTAB）/水界面在整个氢键OH伸缩区域（3000 cm⁻¹ ≤ ω泵浦 ≤ 3600 cm⁻¹）的2D HD-VSFG光谱。CTAB/同位素稀释水（HOD-D₂O）界面的2D HD-VSFG光谱在激发后立即呈现出对角拉长的漂白峰，由于光谱扩散，该峰在约0.3 ps的时间常数下变为圆形。相比之下，CTAB/H₂O界面在0.0 ps时的2D HD-VSFG光谱在漂白区域清晰地显示出两个对角峰及其交叉峰，对应于稳态HD-VSFG光谱中在3230 cm⁻¹和3420 cm⁻¹处观察到的双峰。二维光谱的水平切片表明，CTAB/H₂O界面处漂白的两个峰的相对强度随泵浦频率的变化而逐渐改变。我们使用一个考虑了OH伸缩振动的费米共振和不均匀性的模型来模拟漂白特征的泵浦频率依赖性，模拟光谱再现了CTAB/H₂O界面的2D HD-VSFG光谱的基本特征。本研究表明，时间分辨VSFG的外差检测对于研究水界面的超快动力学和揭示潜在机制至关重要。
