Intramolecular vibrational coupling in water molecules revealed by compatible multiple nonlinear vibrational spectroscopic measurements

Abstract

The complex structure of water and its interactions with solid surfaces require the development of multiple vibrational spectroscopic measurements to study the molecular structure of interfacial water and bulk water near the solid surface. In this study, a newly developed compatible multiple nonlinear vibrational spectroscopy system has been applied to investigate the molecular structure of water in the interfacial region of an ionic solid (CaF₂ substrate) and bulk isotopic D₂O–HOD–H₂O mixtures. Using this compatible system, the sum frequency generation (SFG) vibrational spectra and infrared-infrared-visible three-pump-field four-wave-mixing (IIV-TPF-FWM) spectra of the same water molecules can be collected at the same experimental geometry. It is found that SFG and IIV-TPF-FWM can be used to characterize the molecular structures of interfacial water and bulk water molecules at an interfacial distance below 42 nm, respectively. SFG and IIV-TPF-FWM results both suggest an intramolecular vibrational coupling dominates the spectra. The results achieved by this method are helpful to clarify the origination of the vibrational coupling of the interfacial water as well as the bulk water near the solid surface.