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The surface electronic properties and the vibrational spectroscopy of the electrochemical interface are investigated by in situ Infrared‐Visible Sum (SFG) and Difference (DFG) Frequency Generation. We first present the theoretical background of SFG/DFG and their generation at the interface and calculate the interface second‐order susceptibility including the resonant contribution of the adsorbate and the non resonant contribution of the metal substrate electronic intraband and interband transitions. The effect of the latter is addressed by comparing the experimental SFG and DFG spectra recorded on silver, platinum and gold substrates. The experimental set‐up and procedure are then described, and some selected experimental examples are shown and discussed.

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