SPECIFIC ADSORPTION AT AU(111) ELECTRODES STUDIED BY 2ND HARMONIC-GENERATION

It will be shown that the main source of second harmonic generation (SHG) (including isotropic and anisotropic contributions) arises from an intermediate layer covering the same region where electrochemical processes take place. Therefore, SHG exhibits an enormous potential for surface studies and is very useful as an in situ spectroscopic technique in electrochemistry. For instance, the formation and lifting of surface reconstruction can be monitored in situ under the influence of both potential and adsorption of ions or small organic molecules. For the Au(111) surface, the Au(111)-(square-root 3 x 23) reconstruction leads to a symmetry change of the type C3v --> C(s). At the Au(100) electrode, reconstruction gives rise to a significant contribution of a three-fold symmetry to the observed anisotropy pattern and a complex potential dependence of the anisotropy is observed. In addition, adsorption and potential controlled changes of the symmetry pattern of the SHG intensity can be separated from the effect of (re)charging the electrode. The variation in the static, interfacial electric field alters the surface electron distribution and, hence, their net polarizability in the z direction. This effect can be used to study-during the course of increasing coverage-possible phase transitions within the adsorbate layer.