Sum and difference frequency generation at the electrochemical interface

The surface electronic properties and the vibrational spectroscopy of the e lectrochemical interface are investigated by in situ Infrared-Visible Sum ( SFG) and Difference (DFG) Frequency Generation. We first present the theore tical background of SFG/DFG and their generation at the interface and calcu late the interface second-order susceptibility including the resonant contr ibution of the adsorbate and the non resonant contribution of the metal sub strate electronic intraband and interband transitions. The effect of the la tter is addressed by comparing the experimental SFG and DFG spectra recorde d on silver, platinum and gold substrates. The experimental set-up and proc edure are then described, and some selected experimental examples are shown and discussed.