Direct Evidence for a Surface and Bulk Specific Response in the Sum-Frequency Generation Spectrum of the Water Bend Vibration.

C J Moll,H J Bakker,J Versluis

doi:10.1103/physrevlett.127.116001

Abstract

We study the bending mode of pure water and charged aqueous surfaces using heterodyne-detected vibrational sum-frequency generation spectroscopy. We observe a low (1626 cm^{-1}) and a high (1656 cm^{-1}) frequency component that can be unambiguously assigned to an interfacial dipole and a bulk quadrupolar response, respectively. We thus demonstrate that probing the bending mode provides structural and quantitative information on both the surface and the bulk.

Highlights

The structural properties of the neat water/air interface[16,17] and charged aqueous interfaces have been extensively studied with vibrational sum-frequency generation spectroscopy (VSFG) in the frequency region of the water OH stretch vibrations.[11,18,19,20]
An interesting observation is that the dipolar contribution has a significantly lower frequency than the quadrupolar contribution
The lower frequency of the dipolar contribution indicates that the hydrogen bonds of the contributing water molecules are weaker than those of the water molecules giving rise to the quadrupolar contribution

Summary

Introduction

The structure and dynamics of aqueous interfaces is of high relevance for many different scientific fields.[1,2,3,4,5,6,7,8,9] Many important chemical and biological processes take place at aqueous interfaces, like for instance molecular recognition at bio-membranes, protein folding, and energy conversion and storage.[10,11,12,13,14,15] The structural properties of the neat water/air interface[16,17] and charged aqueous interfaces have been extensively studied with vibrational sum-frequency generation spectroscopy (VSFG) in the frequency region of the water OH stretch vibrations.[11,18,19,20] The frequency region of the water bending vibrations has been much less investigated, even though this region can provide unique information on the hydrogen-bond structure, because, in contrast to the stretching mode, the water bending mode is hardly influenced by intermolecular coupling due to its small transition dipole moment.[21,22,23] there is only one water bending mode per water molecule, strongly decreasing the effects of intramolecular coupling on the spectrum.[21]. These findings have been confirmed by recent computational studies by Nagata et al.[23], and Ni and Skinner[21], that showed that the different hydrogen-bonded water species correlate with different net orientations of the bending mode transition dipole moment

Methods

Results

Conclusion