Orientation and Motion of Water Molecules at Air/Water Interface

TL;DR

This study uses vibrational spectroscopy to reveal that water molecules at the air/water interface exhibit libratory motion within a limited angular range, challenging previous beliefs of broad, ultrafast orientational variation.

Contribution

It provides new experimental evidence showing limited libratory motion of interfacial water molecules, contrasting prior conclusions of broad orientational dynamics.

Findings

Water molecules exhibit libratory motion at the interface.

Orientational motion is confined within a limited angular range.

Contradicts previous evidence of ultrafast broad orientational variation.

Abstract

Analysis of SFG vibrational spectra of OH stretching bands in four experimental configurations shows that orientational motion of water molecule at air/water interface is libratory within a limited angular range. This picture is significantly different from the previous conclusion that the interfacial water molecule orientation varies over a broad range within the vibrational relaxation time, the only direct experimental evidence for ultrafast and broad orientational motion of a liquid interface by Wei et al. [Phys. Rev. Lett. 86, 4799, (2001)] using single SFG experimental configuration.