Phase-referenced nonlinear spectroscopy of the α-quartz/water interface

Abstract

Probing the polarization of water molecules at charged interfaces by second harmonic generation spectroscopy has been heretofore limited to isotropic materials. Here we report non-resonant nonlinear optical measurements at the interface of anisotropic z-cut α-quartz and water under conditions of dynamically changing ionic strength and bulk solution pH. We find that the product of the third-order susceptibility and the interfacial potential, χ(3) × Φ(0), is given by (χ1(3)−iχ2(3)) × Φ(0), and that the interference between this product and the second-order susceptibility of bulk quartz depends on the rotation angle of α-quartz around the z axis. Our experiments show that this newly identified term, iχ(3) × Φ(0), which is out of phase from the surface terms, is of bulk origin. The possibility of internally phase referencing the interfacial response for the interfacial orientation analysis of species or materials in contact with α-quartz is discussed along with the implications for conditions of resonance enhancement.

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

“…Here, use of any combination of cations and anions produces SHG response decreases by as much as 50% when compared to the condition of zero salt added, following wellcharacterized charge-screening behavior. 13,50,66 The extent of the SHG response decay with increasing salt concentration appears to trend with cation and anion softness. Fitting eqn.…”

“…The increase in response Regime I is observed for ionic strengths below about 0.1 mM to 1 mM and results from interference effects of the changing χ (3) phase angle and its coefficient. 13,[49][50][51][52]57 Fig. 1A and Fig.…”

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

“…3,30 Second harmonic generation (SHG) spectroscopy has been widely employed for studying the silica/water interface. 6,[11][12][13][14]16,17 In contrast to V-SFG, SHG probes a non-linear signal that is not resonantly enhanced and therefore not sensitive to a particular molecule or moiety but may result from any molecular and atomic species present which is in particular relevant for high salt concentrations. Generally speaking, any added substance could cause an unintentional increase of the SHG response.…”

Second-Order Vibrational Lineshapes from the Air/Water Interface

“…Such mixing has been shown to have important consequences for the spectral lineshapes observed in second-order vibrational responses from charged interfaces. [22][23][24][25][26] Here, we explore through modeling how χ (2) and χ (3) mixing modifies the imaginary ! !…”