IR–visible sum-frequency vibrational spectroscopy of Biphenyl-3 methylene thiol monolayer on gold and silver: effect of the visible wavelength on the SFG spectrum

We measured IR–visible sum-frequency generation spectra of CH3–(C6H4)2–(CH2)3–S–H (Biphenyl-3) self-assembled monolayers on a silver and a gold substrate. For the latter substrate, we observed different interference patterns between the resonant signal of the CH vibration and the non-resonant contribution of the substrate as a function of the visible beam wavelength. The non-linear response of the gold substrate is enhanced around 480 nm corresponding to the s–d interband transition. Such effect is not observed for the silver substrate the interband transition of which is located out of the investigated visible spectral range of 450–700 nm.