title: Aqueous heterogeneity at the air/water interface revealed by 2D-HD-SFG spectroscopy.
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.
authors: Department for Molecular Spectroscopy, Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz (Germany); FOM Institute AMOLF, Science Park 104, 1098 XG Amsterdam (The Netherlands); Physical Chemistry Department, Science II, University of Geneva, 30, Quai Ernest Ansermet, 1211 Geneva (Switzerland).
authors: Cho-Shuen
authors: CS
authors: Hsieh
authors: Masanari
authors: M
authors: Okuno
authors: Johannes
authors: J
authors: Hunger
authors: Ellen H G
authors: EH
authors: Backus
authors: Yuki
authors: Y
authors: Nagata
authors: Mischa
authors: M
authors: Bonn
keywords: interfacial water; spectral diffusion; sum-frequency generation; two-dimensional spectroscopy; vibrational dynamics