Second-Order Vibrational Lineshapes from the Air/Water Interface

Area of Science:

• Physical Chemistry
• Surface Science
• Nonlinear Spectroscopy

Background:

• Understanding the nonlinear optical responses of interfaces is crucial for surface characterization.
• The interplay between different orders of nonlinear susceptibility (e.g., χ(2) and χ(3)) at interfaces is complex.
• Experimental and computational data for neat air/water interfaces provide a basis for theoretical exploration.

Purpose of the Study:

• To model the absorptive-dispersive mixing effects between second- and third-order nonlinear optical terms.
• To investigate how these mixing effects modify the imaginary χ(2) total (χtotal(2)) responses at air/water interfaces.
• To examine the influence of varying charge densities, ionic strength, and surface potentials on interfacial spectroscopy.

Main Methods:

• Utilized modeling approaches to simulate nonlinear optical responses.
• Employed published imaginary χ(2) (Im(χ(2))) and χ(3) spectra from computational and experimental studies of neat air/water interfaces.
• Analyzed spectral lineshapes under varying conditions, including those mimicking brine solutions.

Main Results:

• Significant contributions from both interfacial χ(2) and bulk χ(3) terms were observed in χtotal(2) spectral lineshapes for experimentally measured spectra.
• These contributions are particularly pronounced at low interfacial charge densities (<0.005% of a monolayer) and in the 3100-3300 cm-1 frequency range.
• Static dipole potentials, if present, spectroscopically manifest in the hydrogen-bond network around 3200 cm-1, especially under brine-like conditions.

Conclusions:

• Absorptive-dispersive mixing between second- and third-order nonlinearities significantly impacts the observed spectral responses at air/water interfaces.
• The bulk χ(3) response plays a non-negligible role in the overall interfacial nonlinear optical signal, even at low charge densities.
• Spectroscopic signatures of surface potentials are detectable within the hydrogen-bond network region, providing insights into interfacial structure.