A Proposed Method to Obtain Surface Specificity with Pump-probe and 2D Spectroscopies.

Surfaces and interfaces are ubiquitous in nature. From cell membranes, to photovoltaic thin films, surfaces have important function in both biological and materials systems. Spectroscopic techniques have been developed to probe systems like these, such as sum frequency generation (SFG) spectroscopies. The advantage of SFG, a second-order spectroscopy, is that it can distinguish between signals produced from molecules in the bulk versus on the surface. We propose a polarization scheme for third-order spectroscopy experiments, like pump-probe and 2D spectroscopy, to select for surface signals and not bulk signals. This proposed polarization condition uses one pulse perpendicular compared to the other three to isolate cross-peaks arising from molecules with polar and uniaxial (i.e. biaxial) order at a surface while removing the signal from bulk isotropic molecules. In this work, we focus on two of these cases: XXXY and YYYX, which differ by the sign of the cross-peak they create. We compare this technique to SFG and VCD to provide insight as to the behavior of the cross-peak signal. We propose that these singularly cross-polarized schemes provide odd-ordered spectroscopies the surface-specificity typically associated with even-ordered techniques.