The average molecular orientation of methylene blue molecules adsorbed onto a silica substrate is obtained from measurements of the resonant molecular optical second harmonic generation (SHG) from the interface at several wavelengths. A r-electron nonlinear polarizability calculation is used to identify the primary molecular tensor elements responsible for the large nonlinear optical response of the adsorbed methylene blue. The polarization dependence of the SHG signal at three wavelengths is

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... en used to calculate the average dye molecule orientation. Inclusion of the Fresnel coefficients of the interface and the dielectric constant for the methylene blue monolayer at both the fundamental and second harmonic wavelengths is required for the orientation calculation. Phase measurements of the surface nonlinear susceptibility are performed in order to demonstrate the complex nature of the SHG from the interface. The average methylene blue orientation is then determined by using both the polarization dependence and phase measurements of the nonlinear response. The SHG measurements presented here yield an average angle of 5 8 O between the long axis of the methylene blue molecule and the surface normal. Possible methylene blue dimer geometries on the surface are discussed in light of the SHG measurements.