Polarization-Dependent Sum-Frequency Generation Spectroscopy for Angstrom-Scale Depth Profiling of Molecules at Interfaces

The three-dimensional spatial distribution of molecules at soft matter interfaces is crucial for processes ranging from membrane biophysics to atmospheric chemistry. While several techniques can access surface composition, obtaining information on the depth distribution is challenging. We develop a noninvasive, polarization-resolved, surface-specific sum-frequency generation spectroscopy providing quantitative depth information. We demonstrate the technique on formic acid molecules at the air-water interface. With increasing molar fraction from 2.5% to 10%, the formic acid molecules shift, on average, ???0.9 ?? into the bulk. The consistency with the simulation data manifests that the technique allows for probing the ??ngstrom-scale depth profile.

Automated summary

This article introduces a noninvasive, polarization-resolved, surface-specific sum-frequency generation spectroscopy that provides quantitative depth information for studying the three-dimensional spatial distribution of molecules at soft matter interfaces. Through demonstrations on formic acid molecules at the air-water interface, the technique's credibility and effectiveness in probing Ångstrom-scale depth profiles are showcased.