Reorientation-induced relaxation of free OH at the air/water interface revealed by ultrafast heterodyne-detected nonlinear spectroscopy

Ken-Ichi Inoue,Satoshi Nihonyanagi,Tahei Tahara,Mohammed Ahmed

doi:10.1038/s41467-020-19143-8

Ken-Ichi Inoue, Satoshi Nihonyanagi + Show 2 more

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https://doi.org/10.1038/s41467-020-19143-8

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Abstract

The uniqueness of water originates from its three-dimensional hydrogen-bond network, but this hydrogen-bond network is suddenly truncated at the interface and non-hydrogen-bonded OH (free OH) appears. Although this free OH is the most characteristic feature of interfacial water, the molecular-level understanding of its dynamic property is still limited due to the technical difficulty. We study ultrafast vibrational relaxation dynamics of the free OH at the air/water interface using time-resolved heterodyne-detected vibrational sum frequency generation (TR-HD-VSFG) spectroscopy. With the use of singular value decomposition (SVD) analysis, the vibrational relaxation (T1) times of the free OH at the neat H2O and isotopically-diluted water interfaces are determined to be 0.87 ± 0.06 ps (neat H2O), 0.84 ± 0.09 ps (H2O/HOD/D2O = 1/2/1), and 0.88 ± 0.16 ps (H2O/HOD/D2O = 1/8/16). The absence of the isotope effect on the T1 time indicates that the main mechanism of the vibrational relaxation of the free OH is reorientation of the topmost water molecules. The determined sub-picosecond T1 time also suggests that the free OH reorients diffusively without the switching of the hydrogen-bond partner by the topmost water molecule.

Highlights

The uniqueness of water originates from its three-dimensional hydrogen-bond network, but this hydrogen-bond network is suddenly truncated at the interface and non-hydrogen-bonded OH appears
Contrary to the previous homodyne TR-Vibrational sum-frequency generation (VSFG) studies, we found that the relaxation T1 time of the free OH determined by TR-HDVSFG does not change with isotopic dilution
We adopted this isotopic dilution ratio for direct comparison with the previous homodyne VSFG study[28]. Both spectra exhibit a broad negative OH stretch band around 3150–3550 cm−1 and a sharp positive OH stretch band at 3700 cm−1. The former low-frequency broad band is assigned to the hydrogen-bonded OH (HB OH) of interfacial water, and its frequency and bandwidth are very similar to those of the HB OH band in the bulk IR spectrum

Summary

Introduction

The uniqueness of water originates from its three-dimensional hydrogen-bond network, but this hydrogen-bond network is suddenly truncated at the interface and non-hydrogen-bonded OH (free OH) appears This free OH is the most characteristic feature of interfacial water, the molecular-level understanding of its dynamic property is still limited due to the technical difficulty. One of the most notable observations with VSFG is a sharp OH stretch band at ∼3700 cm−1 at the air/water interface, which appears with a broad hydrogen-bonded OH stretch band peaked at ∼3450 cm−115. Such a sharp highfrequency band is absent in either IR or Raman spectrum of liquid water, and it is characteristic of the air/water interface. The ultrafast vibrational dynamics of water at interfaces was initially studied by conventional homodyne-detected timeresolved (TR−) VSFG23–25, and the free OH dynamics at the air/ water interface was investigated by Bonn and coworkers[26,27,28]

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