Characterization of Vibrational Resonances of Water-Vapor Interfaces by Phase-Sensitive Sum-Frequency Spectroscopy

Phase-sensitive sum-frequency spectroscopy provides correct characterization of vibrational resonances of water-vapor interfaces and allows better identification of interfacial water species contributing to different parts of the spectra. Iodine ions emerging at an interface create a surface field that tends to reorient the more loosely bonded water molecules below the topmost layer.

Figure 1

Spectra of $|{\chi }_S^{(2)}{|}^2$, $\mathrm{Im}{\chi }_S^{(2)}$, and $\mathrm{Re}{\chi }_S^{(2)}$ for neat water-vapor interface in the OH stretch range obtained with the phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS). The curves are for eye guiding.

Figure 2

Cartoons describing the structures of water-vapor interfaces of (a) neat water and (b) NaI solution. The interfacial water structure is modeled after a distorted hexagonal ice. The top layer of the topmost layer (oxygen marked red) is occupied by $DAA$ and $DDA$ molecules, and the second layer (oxygen marked yellow) is occupied by $DDAA$ molecules. Some $DDAA$ molecules connecting to the molecules in the top layer with two donor bonds have an overall ice-like tetrahedral bonding structure. They contribute to the ice-like part of the vibrational spectrum. Other $DDAA$ molecules are less symmetrically bonded to neighbors and contribute to the spectrum in the higher frequency region. The third and fourth layers (marked orange and red, respectively) have molecules with increasing randomness in position and orientation, closely resembling that of liquid water. In (b), an iodine ion straddled by water molecules at the lower side is shown to appear near the surface. It disrupts mainly the nearby water bonding structure beneath the topmost monolayer and reorients the more loosely bonded subphase molecules by the field it has created.

Figure 3

Spectra of $|{\chi }_S^{(2)}{|}^2$, $\mathrm{Im}{\chi }_S^{(2)}$, and $\mathrm{Re}{\chi }_S^{(2)}$ for water-vapor interface of a $0.036x$ NaI solution in the OH stretch range. For comparison, the eye-guiding spectra for the neat water-vapor interface from Fig. 1 are displayed. Solid squares in $\mathrm{Im}{\chi }_S^{(2)}$ are data points at discrete frequencies on the Im spectrum for the water-vapor interface of a $0.019x$ NaI solution.