Factors Affecting Quantitative Studies of Surface Adsorbates Using Multiresonant Second-Order Nonlinear Spectroscopy

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Abstract
Fundamental quantitative studies of dyes (Nile Blue 690, Oxazine 725, Cresyl Violet 670) adsorbed to fused-silica surfaces were performed to determine the usefulness of multiresonant χ(2) spectroscopy as a surface-selective tool. Factors considered include laser-induced desorption, saturation of resonant transitions, choice of the most useful molecular resonances, molecular orientation, laser incidence angle, and laser polarization. In the most restrictive case (doubly resonant), elimination of desorption and saturation problems required laser power densities below 67 kW/cm2, while for a nonresonant case power densities of 2 GW/cm2 could be used. The resonant enhancement of χ(2) and limitations on usable power densities combine to yield similar detection limits for resonant and nonresonant studies. Detection limits of 0.037 to 0.17 monolayer were obtained for various multiresonant and nonresonant cases. The multiresonant case retains the important advantage of selecting a single mixture component for examination, as demonstrated by an initial study of an adsorbate mixture system.