Comparing Vibrational Sum Frequency Generation Responses at Fused Silica and Fluorite/Liquid Ethanol Interfaces

Abstract We investigate the vibrational sum frequency generation response of liquid CH3CD2OH molecules at fused silica (SiO2) and fluorite (calcium fluoride, CaF2) surfaces. The Fermi resonance doublet between the symmetric stretching and deformation overtone vibrational modes of the methyl group is reassigned according to a two-level model. It is discovered that the Fermi resonance coupling in methyl group is stronger at the CaF2/liquid CH3CD2OH interface than that at the SiO2 surface. The polarization-dependent VSFG measurements further reveal that the orientations of interfacial ethanol molecules are sensitive to the changes of local chemical environments at different solid surfaces. The possible mechanisms are discussed.