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Spectroscopic Measurements (including SHG) of
Liquid/Liquid Interfaces

The formation of adsorbed monolayers at liquid/liquid interfaces frequently plays a central role in many natural and synthetic chemical systems that are relevant in the fields of separation science, electrochemistry, environmental science, and biology. To understand the function of these adsorbed monolayers completely, an intimate knowledge of the interfacial structure is required. For this reason, researchers from diverse scientific backgrounds have endeavored to understand the adsorption, orientation, organization, transport and reactivity of both solvent and solute molecules at various liquid-liquid interfaces. One particular type of liquid/liquid interface which has been of interest since the turn of the century is the interface between two immiscible electrolyte solutions (ITIES). In the past thirty years, a great deal of information on the structure of the ITIES has been obtained from a combination of interfacial tension and electrochemical measurements. However, to date there have been exceedingly few spectroscopic studies of molecular adsorption to the ITIES. A major obstacle in the spectroscopic study of the liquid-liquid interface is the separation of the optical response of the interface from that of the adjacent bulk media.

Optical second harmonic generation (SHG) is an inherently surface-selective and surface-sensitive process that overcomes this difficulty in signal separation. The second harmonic process converts two photons of frequency w to one photon of frequency 2w. In the electric dipole approximation, this process requires a non-centrosymmetric medium. For the case of an interface between two centrosymmetric media, such as the liquid-liquid interface, only the molecules which participate in the symmetry-breaking of the interface will contribute to the observed SHG. This symmetry requirement usually dictates that only the first few molecular monolayers at the interface are responsible for the entire surface second harmonic response. It is this extreme surface selectivity that makes the SHG technique so useful for spectroscopic measurements at liquid/liquid interfaces, and has led to its application in many areas of surface science.

At the ITIES, there are several possible molecular sources of surface SHG. First, all of the solvent molecules and electrolyte ions at the interface will have a weak nonlinear optical response. These "nonresonant" contributions to the surface SHG are typically small, but can be observed. A second possible source of SHG from the liquid-liquid electrochemical interface is the symmetry-breaking induced by the interfacial electrostatic fields. This "electric field-induced SHG" has been observed at semi-conductor, silica, and metal surfaces in contact with aqueous solutions, but again is typically weak at the ITIES. A third, larger contribution to the SHG from liquid-liquid electrochemical interfaces can be obtained when the laser frequencies are tuned into resonance with an electronic transition of an adsorbed molecule. This large molecular surface second harmonic response is sometimes referred to as "resonant SHG," and typically requires molecules which have been designed to provide a large nonlinear optical response. This molecular response is described by the second order molecular nonlinear polarizability tensor, beta, and can be calculated theoretically. In addition, the polarization dependence of the resonant SHG response from a monolayer of adsorbed molecules will depend upon the average molecular orientation at the surface. We have recently published a series of resonant molecular SHG studies of surfactant adsorption, orientation, and reaction at the liquid-liquid electrochemical interface.

Our most recent papers in this area are: