Interaction Geometry Causes Spectral Line-Shape Broadening at the Solid/Liquid Interface

Line shape broadness in vibrational spectra is usually associated with structural heterogeneity of the surrounding environment. At the solid/liquid interface, surface-sensitive sum frequency generation spectroscopy (SFG) has shown a variety of distributions of the vibrational frequency for sapphire surface hydroxyl groups in contact with several liquids. Even though the broadness in SFG spectra could be associated with the surface heterogeneity and diverse interfacial interactions, the origin remains elusive in experiments. To better understand the physical picture of interfacial interactions, and hence broadness, we perform SFG along with molecular dynamics (MD) simulations of liquid molecules in contact with sapphire. In the SFG spectra, line-shape broadness of the sapphire hydroxyl group vibrational frequency increases in the following order: chloroform, acetone, and dimethyl sulfoxide. The broadness in the interaction energy distributions calculated from the MD simulations for the molecules interacting with surface hydroxyl groups follows the same order. MD simulations show that liquid molecules are seen to interact locally with multiple sapphire hydroxyl groups. The number of interactions depends on the location of a molecule with respect to the surface hydroxyl groups, which relate to the packing of individual molecules on surface sites. Regardless of the number of hydroxyl groups that a molecule interacts with, the strength of the strongest interaction remains similar. However, neighboring hydroxyl groups interact with the same molecule with weaker energies, creating broadness in the interaction energy distribution for the strongly interacting (acetone and dimethyl sulfoxide) species. The energy distribution profiles correlate well with the experimental SFG spectra, highlighting the ability to interpret spectroscopic features with the physical insights gained from MD simulations.