| Abstract
| - In IR and Raman spectral studies, the congestion of the vibrational modes in the C−H stretching regionbetween 2800 and 3000 cm-1 has complicated spectral assignment, conformational analysis, and structuraland dynamics studies, even with quite a few of the simplest molecules. To resolve these issues, polarizedspectra measurement on a well aligned sample is generally required. Because the liquid interface is generallyordered and molecularly thin, and sum frequency generation vibrational spectroscopy (SFG-VS) is anintrinsically coherent polarization spectroscopy, SFG-VS can be used for discerning details in vibrationalspectra of the interfacial molecules. Here we show that, from systematic molecular symmetry and SFG-VSpolarization analysis, a set of polarization selection rules could be developed for explicit assignment of theSFG vibrational spectra of the C−H stretching modes. These polarization selection rules helped assignmentof the SFG-VS spectra of vapor/alcohol (n = 1−8) interfaces with unprecedented details. Previous approachon assignment of these spectra relied on IR and Raman spectral assignment, and they were not able to givesuch detailed assignment of the SFG vibrational spectra. Sometimes inappropriate assignment was made, andconsequently misleading conclusions on interfacial structure, conformation and even dynamics were reached.With these polarization rules in addition to knowledge from IR and Raman studies, new structural informationand understanding of the molecular interactions at these interfaces were obtained, and some new spectralfeatures for the C−H stretching modes were also identified. Generally speaking, these new features can beapplied to IR and Raman spectroscopic studies in the condensed phase. Therefore, the advancement onvibrational spectra assignment may find broad applications in the related fields using IR and Raman asvibrational spectroscopic tools.
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