Polarization characteristics from SFG spectra of clean and regulatively oxidized Ni(100) surfaces adsorbed by propionate and formate

Experiments have been carried out on propionate and formate adsorbed on clean and regulatively oxidized Ni(100) surfaces to scrutinize the notion that the polarization characteristics of infrared-visible sum frequency generation (SFG) signals of molecules on metallic surfaces are different from those due to molecules on insulating surfaces. Both propionate and formate give signals only for p-polarized visible pulses on clean metal and oxygen-saturated surfaces, but signals due to s-polarized visible pulses appeared from the propionate-covered surface as soon as the surface was covered with a monolayer of NiO(100). Vibrational peaks of propionate were located at 2887, 2945, and 2988 cm^-1 and were assigned to the symmetric stretching mode of CH₂ group, the symmetric stretching mode of CH₃ group, and the degenerate stretching mode of CH₃ group, respectively. The peak of the surface formate was located at 2948 cm^-1 on the clean metal surface, but another band appeared at 2860 cm^-1 on oxide-layered surfaces. The causes of the appearance of the (sp) polarization signals by the presence of the NiO layer are discussed.