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Infrared transmission spectra of molecules adsorbed on silver island films evaporated on CaF2 have been investigated. The spectra are remarkably simple compared with those of the molecules in the solid state (KBr pellets). Only the vibrational modes which give dipole changes perpendicular to the metal surface are infrared active. In addition, their intensities are about 200 times larger than those of the free molecule. These results can be fully accounted for if the electric field which excites the surface molecule is perpendicular to the local surface of the metal islands and is stronger than the incident electric field. The origin of the absorption enhancement and the surface selection rule is discussed theoretically by using a classical electromagnetic model.

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Surface-enhanced infrared absorption spectroscopy (SEIRAS) in conjunction with cyclic voltammetry has been applied to in situ and real-time monitoring of the electrochemical redox reactions of heptylviologen at a silver electrode surface. The high sensitivity of SEIRAS enables the real-time spectral monitoring of the irreversible reaction processes on the millisecond time scale with rapid-scan interferometry. With the combination of the molecular information obtained from the spectra and the kinetic data obtained by cyclic voltammetry, the complex voltammetric behavior of the molecule can be discussed in detail at the molecular scale. The results demonstrate the utility of the real-time infrared monitoring in analyzing electrochemical reactions.

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