Ultrafast vibrational dynamics of interfacial water
Avishek Ghosh a, Marc Smits a, Maria Sovago a, Jens Bredenbeck a,1, Michiel Mu¨ ller b, Mischa Bonn a,*
Chemical Physics 350 (2008) 23–30

摘要
We report investigations of the vibrational dynamics of water molecules at the water–air and at the water–lipid interface. Following vibrational excitation with an intense femtosecond infrared pulse resonant with the O–H stretch vibration of water, we follow the subsequent relaxation processes using the surface-specific spectroscopic technique of sum frequency generation. This allows us to selectively follow the vibrational relaxation of the approximately one monolayer of water molecules at the interface. Although the surface vibrational spectra of water at the interface with air and lipids are very similar, we find dramatic variations in both the rates and mechanisms of vibrational relaxation. For water at the water–air interface, very rapid exchange of vibrational energy occurs with water molecules in the bulk, and this intermolecular energy transfer process dominates the response. For membrane-bound water at the lipid interface, intermolecular energy transfer is suppressed, and intramolecular relaxation dominates. The difference in relaxation mechanism can be understood from differences in the local environments experienced by the interfacial water molecules in the two different systems.

正文
本文作者利用IR pump VSFG Probe的方法研究了water–air和water–lipid界面水分子的OH伸缩振动的弛豫动力学。实验技术是通过add to the VSFG scheme an additional 'pump', or excitation, pulse, which excites a significant fraction of O–H groups to their first vibrationally excited state [22–24]。

The femtosecond time-resolved SFG study presented here allows to selectively probe the lifetime dynamics (T1) of the O–H stretch vibration and provides new insights in the structure and dynamics of interfacial water. Comparing the water–air [23] interface
and the water–lipid interface [24], pronounced differences of the mechanism and timescale of interfacial vibrational energy flow are found.

实验结果发现，water–air界面的水分子振动弛豫中有两个timescales而water–lipid界面水分子只有一个timescales。所以water–air界面的水分子的弛豫动力学中包含了一个中间态，而water–lipid界面水分子则是不包含中间态的一个单指数的过程。

For the water–air interface (Fig. 3A), two timescales seem apparent: a fast (200 fs) relaxation time corresponding to the recovery of the pump-induced bleach signal (most clearly evident at m = 3500 cm1, and a slower (500 fs) timescale by which the final SFG level is reached (most apparent at m = 3200 cm1). Such dynamics are reminiscent of previous observations for bulk water [29,36] and observations for water at the water–quartz interface [22]. Hence, it is clear that the hydrogen-bonded network vibrational dynamics at the water–air interface are dominated by ultrafast vibrational energy transfer processes. This also explains the similarity between the dynamics at the water–air interface and those at the hydrophilic and hydrophobic silica/water interface recently reported using
SFG in total internal reflection (TIR) geometry [22]. For the latter interface, McGuire and Shen [22] reported somewhat slower dynamics, with T1 = 300 fs and Tthermalization = 700 fs. The rigid silica surface (required for TIR-SFG) has been shown to induce order in the interfacial water compared to water at the water–air interface [38]. The effect of the increased order on the vibrational dynamics is limited due to the ultrafast energy transfer processes that dominate the observed relaxation behavior.

Remarkably, Fig. 3B reveals that the behavior of water at the water–lipid interface is very different from that at the water–air interface. Unlike the water–air interface, the data can be described very well by a single exponential decay, with distinct time constants at different probe frequencies within the hydrogen-bonded regime.

Reference:
[22] J.A. McGuire, Y.R. Shen, Science 313 (2006) 1945.
[23] M. Smits, A. Ghosh, M. Sterrer, M. Muller, M. Bonn, Phys. Rev. Lett. 98 (2007) 4.
[24] A. Ghosh, M. Smits, J. Bredenbeck, M. Muller, M. Bonn, J. Am. Chem. Soc. 129 (2007) 9608.

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