Second Harmonic and Sum-Frequency Generation from Aqueous Interfaces Is Modulated by Interference

Abstract

The interfacial region of aqueous systems also known as the electrical double layer can be characterized on the molecular level with second harmonic and sum-frequency generation (SHG/SFG). SHG and SFG are surface specific methods for isotropic liquids. Here, we model the SHG/SFG intensity in reflection, transmission, and scattering geometry taking into account the spatial variation of all fields. We show that, in the presence of a surface electrostatic field, interference effects, which originate from oriented water molecules on a length scale over which the potential decays, can strongly modify the probing depth as well as the expected intensity at ionic strengths <10 −3 M. For reflection experiments this interference phenomenon leads to a significant reduction of the SHG/SFG intensity. Transmission mode experiments from aqueous interfaces are hardly influenced. For SHG/SFG scattering experiments the same interference leads to an increase in intensity and to modified scattering patterns. The predicted scattering patterns are verified experimentally.

Characterization of the interface of binary mixed DOPC:DOPS liposomes in water: The impact of charge condensation

“…Figure 3(c) shows the maximum scattered intensity (at θ = 50 • ) as a function of DOPS content (w/w %) in the liposome. The SH intensity that is directly linked to the orientational order of interfacial water molecules 13 can be seen to increase with increasing amount of DOPS molecules in the membrane and saturates at ∼10% (w/w) DOPS content.…”

“…Surface and particle Susceptibility elements 13,24 Form factor functions and scattering vector 13,21…”

“…These assumptions were validated in previous studies. 11,13,25 Although we employ the RGD assumption for the linear interaction, the nonlinear interaction contains a correction to account for the change in field amplitude when the optical electromagnetic fields cross the particle/water interface. Dadap et al showed that a linear correction term to the second-order susceptibility is sufficient to correct for the change in the electromagnetic field when it crosses the interface.…”

“…12 Polarization resolved AR-SHS can be described by a set of exact analytical expressions 13 that rely on the surface potential and one non-vanishing surface susceptibility tensor element that describes the orientational order of water at the interface. Combined AR-SHS and sum-frequency scattering (SFS) studies 10 were recently used to probe the transmembrane asymmetry in single component anionic DOPS and zwitterionic DOPC liposomes, as well as liposomes composed of a 1:1 mixture of DOPC and DOPS.…”

Surface-specific vibrational spectroscopy of the water/silica interface: screening and interference

“…As a consequence, the generated V-SFG signal contains a w (2) contribution from directly bonded interfacial molecules and a w (3) contribution from up to B1 mm away from the surface for low ion concentrations. 15 In contrast to w (2) , not only oriented but also isotropic water may contribute to w (3) signal, i.e. w (3) aniso and w (3) iso , where the anisotropic term reflects the DC-field induced orientation of near-surface water, and the isotropic term the contribution from bulk-like, randomly oriented water.…”