Aqueous Heterogeneity at the Air/Water Interface Revealed by 2D‐HD‐SFG Spectroscopy |
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| Authors: | Dr Cho‐Shuen Hsieh Dr Masanari Okuno Dr Johannes Hunger Dr Ellen H G Backus Dr Yuki Nagata Prof Mischa Bonn |
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| Institution: | 1. Department for Molecular Spectroscopy, Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz (Germany);2. FOM Institute AMOLF, Science Park 104, 1098 XG Amsterdam (The Netherlands);3. Physical Chemistry Department, Science II, University of Geneva, 30, Quai Ernest Ansermet, 1211 Geneva (Switzerland);4. Division of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tennodai, Tsukuba, 305‐8571 (Japan) |
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| Abstract: | Water molecules interact strongly with each other through hydrogen bonds. This efficient intermolecular coupling causes strong delocalization of molecular vibrations in bulk water. We study intermolecular coupling at the air/water interface and find intermolecular coupling 1) to be significantly reduced and 2) to vary strongly for different water molecules at the interface—whereas in bulk water the coupling is homogeneous. For strongly hydrogen‐bonded OH groups, coupling is roughly half of that of bulk water, due to the lower density in the near‐surface region. For weakly hydrogen‐bonded OH groups that absorb around 3500 cm?1, which are assigned to the outermost, yet hydrogen‐bonded OH groups pointing towards the liquid, coupling is further reduced by an additional factor of 2. Remarkably, despite the reduced structural constraints imposed by the interfacial hydrogen‐bond environment, the structural relaxation is slow and the intermolecular coupling of these water molecules is weak. |
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| Keywords: | interfacial water spectral diffusion sum‐frequency generation two‐dimensional spectroscopy vibrational dynamics |
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