On Second-Order Vibrational Lineshapes of the Air/Water Interface

TL;DR

This study models how second- and third-order nonlinear optical effects influence vibrational lineshapes at the air/water interface, revealing significant bulk contributions and potential surface potential signatures in the spectra.

Contribution

It introduces a modeling approach to analyze how absorptive-dispersive mixing affects vibrational spectra at the air/water interface, highlighting the importance of bulk chi(3) effects and surface potentials.

Findings

Bulk chi(3) contributions significantly affect the spectral lineshapes.

Surface potentials may manifest in the 3200 wavenumber region.

Interfacial charge densities as low as 0.005% of a monolayer influence spectra.

Abstract

We explore by means of modeling how absorptive-dispersive mixing between the second- and third-order terms modify the imaginary chi(2)total responses from air/water interfaces under conditions of varying charge densities and ionic strength. To do so, we use published Im(chi(2)) and chi(3) spectra of the neat air/water interface that were obtained either from computations or experiments. We find that the chi(2)total spectral lineshapes corresponding to experimentally measured spectra contain significant contributions from both interfacial chi(2) and bulk chi(3) terms at interfacial charge densities equivalent to less than 0.005% of a monolayer of water molecules, especially in the 3100 wavenumber to 3300 wavenumber frequency region. Additionally, the role of short-range static dipole potentials is examined under conditions mimicking brine. Our results indicate that surface potentials, if indeed present at the air/water interface, manifest themselves spectroscopically in the tightly bonded H-bond network observable in the 3200 wavenumber frequency range.