Development of Electronic Sum Frequency Generation Spectroscopies and Their Application to Liquid Interfaces

Abstract

This feature article describes our recent work on the development of interfaceselective second-order nonlinear electronic spectroscopies and their application to molecular structure and dynamics at liquid interfaces. We describe three methods that measure electronically resonant second-order nonlinear optical susceptibility (χ (2) ) of interfacial molecules without contribution from the bulk: electronic sum-frequency generation (ESFG) provides high-quality interfacial electronic spectra by employing the combination of narrowband and broad-band visible/near-infrared femtosecond pulses. Time-resolved ESFG (TR-ESFG) is the extension of ESFG to time-resolved measurements and provides femtosecond time-resolved electronic spectra at interfaces. Heterodyne-detected ESFG (HD-ESFG) is the ultimate second-order nonlinear electronic spectroscopy that directly provides the real and imaginary parts of χ (2) . In particular, the imaginary χ (2) spectra obtained with HD-ESFG can be directly compared to the absorption spectra of bulk solution, and their signs contain information about the absolute orientation of the interfacial molecules. These interface-selective electronic spectroscopies enable rigorous comparison between the electronic spectra of liquid interfaces and those of bulk liquids, and the application to the following topics is discussed in this article: effective polarity at the air/water interface, interfacial ultrafast dynamics, absolute alignment of interfacial molecules, pH of the air/water interface, and the origin of nonresonant background. These studies reveal unique properties of molecules at air/liquid interfaces, which is markedly different from that in the bulk from a microscopic viewpoint.

Mobility of nanometer-size solutes in water driven by electric field

“…Overall, the main result of the general formalism summarized by Eq. (11) and simulations performed here is that the charge of free carriers and the effective electrokinetic charge incorporating the interfacial dipolar order can be significantly different.…”

“…Therefore, the actual concentration of adsorbed ions can be significantly lower than estimated from mobility. This observation might help to explain the disagreement between the electrokinetic measurements [5,10,12] and surface-sensitive spectroscopies [9,10,11,12] regarding the concentration of the surface adsorbed ions. The actual extent of overpolarization requires more extensive simulations in realistic electrolytes.…”

“…Recent calculations [7,8] and measurements by surface-sensitive second-harmonic generation techniques [9,10,11,12] do not support excessive adsorption of hydroxide to the oil-water [10] and air-water [9,11] interfaces. In addition, the total X-ray reflection fluorescence spectroscopy [13] provides the upper estimate for the free surface charges at the air-water interface at the level of 0.002 (e/nm 2 ).…”

“…Since the microscopic susceptibility of the nanometer interface can significantly deviate from the rules of macroscopic continuum electrostatics [38,42,40], σ q = P n = −P a is left unspecified in Eq. (11). The simple message delivered by Eqs.…”

“…To make our assignment clear, the water-oil interface with water's hydrogen pointing toward the oil phase [46,10,47] will, according to Eqs. (11) and (12), produce a negative charge of the water drop.…”