Second-order nonlinear optics can be used to quantitatively determine the orientation of chemical bonds or submoieties of a fairly complicated molecule at an interface, and therefore completely map out its orientation and conformation. As a specific example, we have studied pentyl-cyanoterphenyl molecules at the air-water interface. We have measured the orientation of all three parts of the molecule (cyano head group, terphenyl ring, and pentyl chain) by optical second-harmonic generation and infrared-visible sum-frequency generation. A quantitatively consistent picture of the molecular configuration has been obtained. The technique can be applied to situations where other methods would fail (e.g., the surface of neat liquids or buried interfaces).

The structure of a water film formed on mica at room temperature, in equilibrium with water vapor at various relative humidities (RH), was studied using sum-frequency-generation (SFG) vibrational spectroscopy and scanning polarization force microscopy (SPFM). Analysis of the O-D stretch modes in the SFG spectra of ${\mathrm{D}}_2\mathrm{O}$ on mica indicates that as RH increases, the submonolayer water structure evolves into a more ordered hydrogen-bonding network. At full monolayer coverage $\left(\sim 90\%\mathrm{RH}\right)$, the SFG spectrum suggests an icelike film with no dangling O-D groups, in agreement with a recent molecular dynamics simulation.

Surface melting on the (0001) face of hexagonal ice ( ${\mathrm{I}}_h$) was studied by sum-frequency vibrational spectroscopy in the OH stretch frequency range. The degree of orientational order of the dangling OH bonds at the surface was measured as a function of temperature. Disordering sets in around 200 K and increases dramatically with temperature. The results show that the disordered (quasiliquid) layer on ice is structurally different from normal liquid water.

Sum-frequency vibrational spectroscopy was used to study the (0001) surface of hexagonal ice ${(I}_h)$ in contact with: air, a hydrophobic substrate, and a hydrophilic substrate. The spectra in the O-H stretch frequency range were obtained at various temperatures. For the air/ice interface, the degree of orientational order of the dangling OH bonds at the surface was measured as a function of temperature. Surface disordering appears to set in around 200 K and increases dramatically with temperature, which is a strong indication of the presence of surface melting of ice. For the hydrophobic and hydrophilic ice interfaces, a similar temperature dependence of the hydrogen-bonded OH stretch peak was observed. The free OH stretch mode, however, appears to be different from that of the air/ice interface due to the interactions with substrates at the interfaces.