Molecular level elucidation of the structure and dynamics of water at hydrophobic interfaces is important in numerous chemical and biological processes such as “on-water” catalysis, protein folding, lipid membrane formation etc. Vibrational sum frequency generation (VSFG) spectroscopy is highly interface-selective and has been extensively utilized to study various interfaces [1]. Especially recently developed heterodyne-detected (HD-) VSFG [2] spectroscopy enables measure- ment of complex χ(2) (χ(2) is the second-order nonlinear susceptibility) spectrum of molecules at the interface. In particular, the imaginary part of the χ(2) (Imag χ(2)) spectrum represents the interface- selective vibrational spectrum of molecules and directly compared to the bulk infrared absorption spectrum which corresponds to imaginary part of the linear susceptibility (Imag χ(1)). Furthermore, the sign of the Imag χ(2) spectrum provides information about the net orientation of interfacial molecules. Very recently our group extended HD-VSFG to femtosecond time-resolved measure- ments by combining the unique capabilities of femtosecond IR excitation (pump) with HD-VSFG (probe) to explore the vibrational dynamics of interfacial molecules [3]. The time-resolved HD- VSFG (TR-HD-VSFG) enables us to measure ΔImag χ(2) (pump induced change in the Imag χ(2)), which can be interpreted similar to bulk transient absorption spectra. Furthermore, two dimensional (2D) HD-VSFG spectroscopy, which is an interface analogue of 2D IR, can be carried out by measuring the pump frequency dependence of the ΔImag χ(2) spectra [4].
In this talk, I will present our recent study on ultrafast vibrational dynamics of water at a model hydrophobic ion/water interface using time-resolved 2D HD-VSFG spectroscopy. The 2D HD-VSFG measurements surprisingly reveal the existence of two distinct sub-ensembles of water at hydrophobic ion/water interface, which are indistinguishable in the steady state spectrum.
[1] Y. R. Shen. Ann. Rev. Phys. Chem. 2013, 337, 129.
[2] S. Nihonyanagi, S. Yamaguchi, T. Tahara. J. Chem. Phys. 2009, 130, 204704.
[3] S. Nihonyanagi, P. C. Singh, S. Yamaguchi, T. Tahara. Bull. Chem. Soc. Jpn. 2012, 85, 758. [4] P. C. Singh, S. Nihonyanagi, S. Yamaguchi, T. Tahara. J. Chem. Phys. 2012, 137, 094706.