SURFACE-ENHANCED INFRARED-SPECTROSCOPY - THE ORIGIN OF THE ABSORPTIONENHANCEMENT AND BAND SELECTION RULE IN THE INFRARED-SPECTRA OF MOLECULES ADSORBED ON FINE METAL PARTICLES

Infrared transmission spectra of molecules adsorbed on silver island f ilms evaporated on CaF2 have been investigated. The spectra are remark ably simple compared with those of the molecules in the solid state (K Br pellets). Only the vibrational modes which give dipole changes perp endicular to the metal surface are infrared active. In addition, their intensities are about 200 times larger than those of the free molecul e. These results can be fully accounted for if the electric field whic h excites the surface molecule is perpendicular to the local surface o f the metal islands and is stronger than the incident electric field. The origin of the absorption enhancement and the surface selection rul e is discussed theoretically by using a classical electromagnetic mode l.