MOLECULAR 2ND HARMONIC-GENERATION STUDIES OF METHYLENE-BLUE CHEMISORBED ONTO A SULFUR-MODIFIED POLYCRYSTALLINE PLATINUM-ELECTRODE

The resonant molecular second harmonic generation (SHG) response of a monolayer of adsorbed methylene blue is studied in situ on a polycrystalline platinum electrode modified by the deposition of a monolayer of chemisorbed sulfur. The sulfur monolayer prevents the irreversible decomposition of the dye molecules on the platinum electrode and ensures that the contributions to the surface nonlinear susceptibility from the metal and from the methylene blue remain separable. At an incident angle of 40°, the resonant molecular SHG signal from the methylene blue monolayer dominates the nonlinear optical response of the interface, and the potential dependence of the resonant SHG signal matches that expected during the electrochemical reduction and reoxidation of the chemisorbed methylene blue. The polarization dependence of SHG signal provides a picture of the average molecular orientation of the dye molecules as a function of surface coverage; this calculation of the average molecule orientation must include the complex Fresnel coefficients for the metal-electrolyte interface at the fundamental and second harmonic wavelengths and a full accounting of the various molecular nonlinear polarizability tensor elements. © 1990 American Chemical Society.