Local environment and interactions of liquid and solid interfaces revealed by spectral line shape of surface selective nonlinear vibrational probe

Vibrational spectral line shape contains important detailed information on molecular vibration and reports its specific interactions and couplings to its local environment. In this work, recently developed sub-1 cm^-1 high-resolution broadband sum frequency generation vibrational spectroscopy (HR-BB-SFG-VS) was used to measure the-C≡N stretch vibration in the 4-n-octyl-4′-cyanobiphenyl (8CB) Langmuir or Langmuir-Blodgett (LB) monolayer as a unique vibrational probe, and the spectral line shape analysis revealed the local environment and interactions at the air/water, air/glass, air/calcium fluoride, and air/α-quartz interfaces for the first time. The 8CB Langmuir or LB film is uniform, and the vibrational spectral line shape of its-C≡N group has been well characterized, making it a good choice as the surface vibrational probe. Line shape analysis of the 8CB-C≡N stretch SFG vibrational spectra suggests the coherent vibrational dynamics and the structural and dynamic inhomogeneity of the-C≡N group at each interface are uniquely different. In addition, it is also found that there are significantly different roles for water molecules in the LB films on different substrate surfaces. These results demonstrated the novel capabilities of the surface nonlinear spectroscopy in characterization and in understanding the specific structures and chemical interactions at the liquid and solid interfaces in general.