Abstract
We examine a quantum mechanical model for two photon absorption by a molecule near a spherical metal particle. The adsorbed molecule is idealized as a three level system and the metal particle is taken to represent a ‘‘rough surface.’’ Expressions for the second order nonlinear dipole moment of the adsorbed species are obtained. Since the nonlinear optical power absorption is directly proportional to the square of the respective nonlinear dipole moment, one is able to draw conclusions regarding the effect of the substrate on nonlinear absorption by the adsorbate. Peak enhancement of surface induced two photon absorption is predicted for a molecule–surface separation of ≈20 Å. Optimum conditions for maximum enhancement are investigated.
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