Communication: Spectroscopic phase and lineshapes in high-resolution broadband sum frequency vibrational spectroscopy: Resolving interfacial inhomogeneities of “identical” molecular groups

The ability to achieve sub-wavenumber resolution (0.6 cm⁻¹) and a large signal-to-noise ratio in high-resolution broadband sum-frequency generation vibrational spectroscopy (HR-BB-SFG-VS) allows for the detailed SFG spectral lineshapes to be used in the unambiguous determination of fine spectral features. Changes in the structural spectroscopic phase in SFG-VS as a function of beam polarization and experimental geometry proved to be instrumental in the identification of an unexpected 2.78 ± 0.07 cm⁻¹ spectral splitting for the two methyl groups at the vapor/dimethyl sulfoxide (DMSO, (CH₃)₂SO) liquid interface as well as in the determination of their orientational angles.