Orientation-insensitive methodology for second harmonic generation. 2. Application to adsorption isotherm and...

In this work, the theory presented in part 1 for an experimental configuration that provides a means to isolate surface coverage measurements by SHG from coverage-dependent molecular orientation changes was tested. The adsorption isotherm for disperse red 1 on glass was found to be quite sensitive to the choice of excitation polarization rotation angle due to a coverage-dependent change in molecular orientation. Appropriate selection of the polarization rotation angle, as dictated by the theory presented in part 1, yielded reliable adsorption isotherm results that were essentially independent of changes in molecular orientation. Use of a standard experimental approach (e.g., probing the intensity of p-polarized second harmonic for p-polarized fundamental), resulted in large error (nearly 100%) in the calculated equilibrium constant. The adsorption isotherm for rhodamine 6G on glass was found to be relatively insensitive to the choice of polarization rotation angle since its apparent orientation angle did not change significantly as a function of coverage.