Optically induced band shifts in infrared spectra of mixed self-assembled monolayers of biphenyl thiols

Optically induced band shifts in reflection absorption infrared spectra (RA IR) of mixed self-assembled monolayers (SAMs) are reported. SAMs of 4-trifl uoromethyl-4-mercaptobiphenyl dispersed in 4'-methyl-4-mercaptobiphenyl on gold exhibit significant shifts of the spectral envelope of the symmetric C F3 stretch and the bl, skeletal mode. A linear relationship is observed bet ween the band positions, the band intensity, and the surface CF3 group conc entration. To rationalize the observed changes in band positions and shapes , a classical electromagnetic theory approach is used, providing an excelle nt agreement between theory and experiments. The simplified RAIR signature of 4'-trifluoromethyl-4-mercaptobiphenyl on gold is a unique example of the surface selection rule.