THE USE OF OPTICS FOR UNDERSTANDING THE ELECTROCHEMICAL INTERFACE

We show that in situ linear and non-linear optics are very suitable and very accurate tools for the study of variations in electronic properties of the metal-electrolyte interface under the effect of electrode potential, of electrode charge, and of ionic adsorption processes. The determination by ellipsometry of the effective dielectric constant epsilon(m) of gold and silver single crystals at the pzc, shows that the plasma frequency omega(p) is dependent on surface orientation. Such anisotropy was also observed on Ag(hkl) electrodes by surface plasmon polariton excitation. The electrode charge induces an asymmetric effect on the optical parameters: the phase DELTA displays a strong decrease in positive charge which starts at the pzc, whereas a very weak signal is detected in the negative charge range. These results were analysed using a theoretical approach, in which the electron distribution at the interface was treated in a non-local field approximation. In the case of specific adsorption, interactions occurring at the interface are. much stronger and induce a strong enhancement of the optical response of the interface. A local field model was then sufficient to interpret the results. Finally, we used second-harmonic generation to improve theoretical work on charged metal surfaces in contact with aqueous solutions. Experiments carried out on Ag(111) in KClO4 solution enabled us to fit experimental and calculated data.