Strong frequency dependence of vibrational relaxation in bulk and surface water reveals sub-picosecond structural heterogeneity.

Sietse T Van Der Post,Yuki Nagata,Masanari Okuno,Huib J Bakker,Mischa Bonn,Johannes Hunger,Cho-Shuen Hsieh

doi:10.1038/ncomms9384

Abstract

Because of strong hydrogen bonding in liquid water, intermolecular interactions between water molecules are highly delocalized. Previous two-dimensional infrared spectroscopy experiments have indicated that this delocalization smears out the structural heterogeneity of neat H2O. Here we report on a systematic investigation of the ultrafast vibrational relaxation of bulk and interfacial water using time-resolved infrared and sum-frequency generation spectroscopies. These experiments reveal a remarkably strong dependence of the vibrational relaxation time on the frequency of the OH stretching vibration of liquid water in the bulk and at the air/water interface. For bulk water, the vibrational relaxation time increases continuously from 250 to 550 fs when the frequency is increased from 3,100 to 3,700 cm−1. For hydrogen-bonded water at the air/water interface, the frequency dependence is even stronger. These results directly demonstrate that liquid water possesses substantial structural heterogeneity, both in the bulk and at the surface.

Highlights

Because of strong hydrogen bonding in liquid water, intermolecular interactions between water molecules are highly delocalized
There is evidence from scattering experiments that liquid water exhibits substantial structural heterogeneity[3], the quantum nature of the hydrogen bond (H-bond), in particular for light water, results in significant delocalization of the intermolecular interaction[4,5], which may smear out the structural heterogeneity
It has remained elusive whether the extremely broad bandwidth of the OH stretching band is a sole result of the delocalized nature[11] and originates from strong intra- and intermolecular coupling[12] or that the delocalization occurs on a smaller length scale than the structural heterogeneity, which implies that the OH stretching band is inhomogeneously broadened

Summary

Introduction

Because of strong hydrogen bonding in liquid water, intermolecular interactions between water molecules are highly delocalized. The signals recorded at oprobe 1⁄4 3,100 cm À 1 (Fig. 2b) directly reveal, at the interface, a pronounced frequency dependence of the vibrational relaxation dynamics: the transient signals decay with a time constant at 3,300 and o3f,t11001⁄4c(m35À0±1, 2r0e)spfsecatnivdel(y1.60F±or30t)hfes for excitation excitation at

Results

Conclusion