In situ visible-infrared sum and difference frequency generation at the electrochemical interface

We present a comparison between the sum frequency generation (SFG) and the difference frequency generation (DFG) at the electrochemical interface. We show that the phase matching conditions are not always satisfied for DFG an d analyse the experimental difficulties of DFG. We then present a modelling of the second order non linear susceptibilities (both in SFG and DFG) at t he electrochemical interface. As this modelling gives two mathematical solu tions of the fit of the experimental data, we demonstrate, thanks to experi mental results obtained for cyanide on platinum, that DFG could be the esse ntial complement of SFG for a quantitative analysis of the experimental dat a. In the particular case of noble metals for which the visible beam lies i n the spectral range of interband electronic transitions (case of CN- on go ld system), we show the possibility to overcome the obscuring of the signal by the non resonant contribution of the substrate, by switching from SFG t o DFG. (C) 1999 Elsevier Science S.A. All rights reserved.