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Zusammenfassung:
We have measured the optical second harmonic generation (SHG) at a mercury electrode. The influence of the applied potential and of the adsorption of aniline on the nonlinear optical response were studied by simultaneously measured differential capacitance. The results reflect the potential-dependent orientation of aniline molecules at the electrode. We proved that in the cathodic region where the aniline molecules are oriented perpendicularly to the electrode, the second harmonic signal is only an effect of the metal electrons. In the anodic region the SHG is markedly influenced by the interaction between the π electrons of the flatly oriented adsorbate and the metal.