ELECTROMAGNETIC MECHANISM OF ENHANCED INFRARED-ABSORPTION OF MOLECULES ADSORBED ON METAL ISLAND FILMS

Infrared absorption of molecules is enhanced when they are absorbed on island films of noble metals. An electromagnetic model which explains this phenomenon is proposed. The enhanced absorption is not of the absorbed molecules themselves but arises from the change in the effective dielectric function of the metal-island film at the frequencies of the molecular vibrations caused by the dipole interaction between the metal-islands and the adsorbed molecule. Using the Maxwell-Garnett effective medium theory, an enhancement factor of the order of 10 is calculated.