Sum-frequency generation as a vibrational and electronic probe of the electrochemical interface and thin films

Standard vibrational sum-frequency generation (SFG) spectroscopy is performed to probe the electrochemical cyanoacetylene/Au(1 1 1) interface, enlightening a 20 cm −1/V Stark shift of the free CN vibration mode of this cyanopolyynes-class molecule. SFG data suggest an orientation for the adsorbed molecules with the CN moiety pointing out in a direction perpendicular to the metal surface. A newly developed two-colour SFG (2C-SFG) set-up based on the CLIO free electron laser synchronized with a tuneable visible laser source is used to probe simultaneously the vibrational and electronic fingerprint of a thiophenol/Ag(1 1 1) interface. Structural information on the adsorbed self-assembled monolayer is put in evidence as a function of the immersion time of the silver substrate in the thiophenol solution (24 h or 5 days). With a longer immersion time, the molecular packing is of better quality and more compact, with the carbon rings less tilted with respect to the surface normal. This new experimental set-up combines the advantages to have a higher spectral and temporal resolution with higher power energies at far infrared wavelengths than these presently available from tabletop infrared optical parametric oscillators.