Vibrational spectra and molecular orientation with experimental configuration analysis in surface sum frequency generation (SFG)

Recent developments in sum frequency generation-vibrational spectroscopy (SFG-VS) have shown that SFG-VS is not only a spectroscopic probe of the molecular interfaces but also an important tool for studying molecular spectroscopy in general. It has been demonstrated that through polarization and symmetry analysis a few sets of polarization selection rules can explicitly help the vibrational spectral assignment. This worked because of the coherent nature and the strong orientational dependence of the SFG process from the molecular interfaces or films. In this work, we further discuss the dependence on the experimental configurations in the SFG-VS polarization analysis. Such experiment and analysis can further increase the ability of the SFG-VS as the tool for discerning spectral details and overlapping spectral features, as well as the ability to obtain detailed molecular orientational information. The experimental configuration dependence of the SFG spectra is most significant in the ppp polarization combination, less significant in the sps and ssp polarization combinations, due to the fact that ppp is the result of the combination of four different susceptibility tensor elements, while both ssp and sps are of a single tensor element. Such complexity of the ppp spectrum, which is very useful as demonstrated in this work, used to be avoided in the SFG-VS studies. The spectral details of the SFG-VS from the vapor/methanol, vapor/ethanol, and vapor/ethylene glycol interfaces are studied in different experimental configurations. The strong experimental configuration dependence observed for these liquid interfaces also indicated that they possess well-ordered interfacial structures. The concepts and conclusions in this report may find future applications in studying more complex molecular interfaces and films.