Vibrational Polarization Spectroscopy of CH Stretching Modes of the Methylene Group at the Vapor/Liquid Interfaces with Sum Frequency Generation

Abstract

In this paper we report detailed examples of the surface sum frequency generation vibrational spectroscopy (SFG-VS) as a polarization spectroscopic technique for vibrational spectral band assignment and orientational analysis for molecular groups at the interfaces. Surface sum frequency generation vibrational spectroscopy (SFG-VS) has been widely used as an important spectroscopy probe for chemical bonding, structural conformation and molecular interactions of both fundamentally and technologically important interfaces. However, the potential for SFG-VS as a polarization spectroscopic technique is yet to be fully explored. In IR and Raman studies, polarized spectroscopy (PS) can provide information on molecular symmetries, which is necessary for accurate vibrational band assignment in complex chemical environments. We shall show that SFG-vibrational polarization spectroscopy (VPS) is the polarization spectroscopic tool on this purpose for the interface, along with its advantage of submonolayer sensitivity. This ability of SFG-VPS comes from the fact that vibrational bands from different symmetry types do not have strongest peak intensities in the same polarization configuration of the SFG-VPS spectra, because molecular groups are aligned or partially aligned at the interface. We chose to study the SFG-VPS of three diols, namely, ethylene glycol, 1,3-propanediol, and 1,5-pentanediol, at the vapor/liquid interfaces as model systems for the methylene-only molecules. The polarization analysis of the SFG spectra resulted in few explicit polarization selection rules or guidelines for the assignment of the CH stretching modes of the methylene groups. These results could be used to provide clarifications for some of the disagreements and controversies still exist in the literatures. These results can also shed light on the IR and Raman studies on methylene group in the bulk condensed phases.