Study of Ag(111) and Au(111) electrodes by optical second-harmonic generation

We have measured the potential dependence of optical second-harmonic generation from Ag(111) and Au(111) electrodes at wavelengths below and above the free electron regime. In the free electron regime, the cubic nonlinear susceptibility is dominant, leading to similar effects for positive and negative charging. Above the free electron regime, the quadratic nonlinear susceptibility of the d-electrons is dominant, leading to different effects for positive and negative charging. We also followed the oxidoreduction of Au(111) and found that second-harmonic generation is very sensitive to the different steps of the reaction.