Second-Order Vibrational Lineshapes from the Air/Water Interface

Abstract

We explore by means of modeling how absorptive-dispersive mixing between the second- and third-order terms modifies the imaginary χ responses from air/water interfaces under conditions of varying charge densities and ionic strength. To do so, we use published Im(χ) and χ spectra of the neat air/water interface that were obtained either from computations or experiments. We find that the χ spectral lineshapes corresponding to experimentally measured spectra contain significant contributions from both interfacial χ and bulk χ terms at interfacial charge densities equivalent to less than 0.005% of a monolayer of water molecules, especially in the 3100 to 3300 cm 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 cm frequency range.

Specifics about Specific Ion Adsorption from Heterodyne-Detected Second Harmonic Generation

“…4 shows that the DC phase angle, , is almost zero at 100 mM ionic strength. 45 Our previous work found a reversible phase shift ( ∆ ) of 19.1° ± 0.4° between 100 mM NaCl and deionized water (CO2 equilibrated and at a measured pH of 5.8). 42 We note that this value was obtained from experiments that initially cycled between deionized water and 100 mM NaCl solution at least three times before performing NaCl screening (going from low to high ionic strength) so as to overcome hysteresis effects 41,81 resulting from screening from low to high ionic strength.…”

Beyond the Gouy-Chapman Model with Heterodyne-Detected Second Harmonic Generation

“…!! , the interference between signal generated at different depths away from the interface results in the now firmly established model (2): [6][7][8]17,[22][23][24][25][26] ! !…”

“…! term taking the exact solution (derivation of this form can be found in the SI, Section 1): 17,[22][23][24]26…”

Beyond the Gouy-Chapman Model with Heterodyne-Detected Second Harmonic Generation

“…! term taking the exact solution (derivation of this form can be found in the SI, Section 1): 17,[22][23][24]26 ! = arctan !!…”