Abstract

IR-Visible sum frequency generation (SFG) is a surface specific, nonlinear optical process that can provide information on structure of molecular adsorbates at both liquid-solid and gas-solid interfaces [1,2]. The nonlinear susceptibility of the surface region is resonantly enhanced when the input IR field is tuned across a vibrational transition in a molecular adsorbate, provided this transition is both Raman and IR-active [3]. We have used SFG to study the C-H stretching region (2800-3000 cm^-1) of a variety of molecular adsorbates on both metal and dielectric surfaces. Internal reflection of the input IR and visible fields at a fused silica/air interface provides a two-orders of magnitude enhancement to the SFG intensity in comparison to an external reflection configuration. Analysis of the SFG dependence on the polarization of the input fields allows a determination of the average polar orientation of e.g. the terminal methyl groups in a stearic acid monolayer (Fig. 1) or of CH₃ and CO groups on metal surfaces. We will discuss results that show the ability of SFG to detect modifications in the monolayer induced by temperature, and by chemical and mechanical interactions.

© 1989 Optical Society of America