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

Area of Science:

• Physical Chemistry
• Surface Science
• Spectroscopy

Background:

• Nonlinear spectroscopy, specifically sum frequency generation (SFG), is vital for atomic-molecular level interface analysis.
• SFG response intensity is the square of a complex amplitude, complicating direct interpretation.
• Understanding buried interfaces is challenging but critical in materials science.

Purpose of the Study:

• To develop a nonlinear interferometric technique for directly measuring the real and imaginary components of SFG response.
• To apply this technique to study the interfacial behavior of poly(vinyl alcohol) (PVA) on CaF2.
• To elucidate the role of PVA configuration and water interaction in ice recrystallization inhibition.

Main Methods:

• Developed a nonlinear interferometric technique for optical projection of complex SFG response.
• Measured the real and imaginary components of the SFG response, which are linear with molecular constituents.
• Applied the technique to poly(vinyl alcohol) (PVA) adsorbed on a CaF2 substrate.

Main Results:

• The technique nearly directly yields real and imaginary components of the complex SFG response.
• Observed that PVA binds to the CaF2 interface similarly to simulated binding with ice.
• Identified distinct binding modes for PVA's alcohol groups with water: chemisorption and physisorption.

Conclusions:

• The developed nonlinear interferometric technique effectively probes interfacial molecular interactions.
• PVA's interfacial configuration and dual water-binding modes (strong and weak) are critical for its ice recrystallization inhibition activity.
• Polarization dependence and intensity modifications in SFG spectra aid in assigning interfacial resonances.