Phase-Sensitive SFG of Poly(vinyl alcohol) on Calcium Fluoride: Interaction with Water Using Backside Geometry.

One effective tool for probing interfaces at the atomic-molecular level, particularly buried interfaces, is nonlinear spectroscopy: sum frequency generation (SFG). SFG produces a complex response; the observed intensity is the square of this complex response. Removing complications due to the nonlinearity requires determining the complex amplitude. This contribution reports a nonlinear interferometric technique that nearly directly yields the real and imaginary components of the complex response via optical projection. Both the real and imaginary components are linear in the constituents generating the response, enabling monitoring interactions between individual moieties and added molecules. The technique is applied to poly(vinyl alcohol) (PVA) on a CaF₂ substrate. PVA is of interest since it is one of the most effective synthetic ice recrystallization inhibitors (IRIs); both its configuration at the interface and its interaction with water are thought to play critical roles in its IRI activity. Results show that PVA binds in a manner reminiscent of reported simulations of PVA binding to ice; some alcohol groups strongly bind water, chemisorbing it, while other groups weakly bind water, physisorbing it. Both the polarization dependence of the SFG response and modified intensity due to the interaction with water are leveraged to assign resonances.