Electromagnetic effect in enhanced infrared absorption of adsorbed molecules on thin metal films

The electric field strength within a homogeneous metal layer on a Ge hemi-cylindrical prism in the Kretschmann configuration has been calculated for infrared light using the Fresnel formula. To approximate the non-homogeneous island-like or porous nature of very thin metal films the dielectric constant of the metal layer was derived by the Bruggeman effective medium approximation. The dependence of the calculated intensity on the film thickness and the angle of incidence is in good agreement with the experimental observations of enhanced infrared absorption of adsorbed species. Combined with the observed short range effect of the enhancement this implies that the enhancement is due to actual high electric fields within the pores or between islands of the metal film which are proportional to the calculated effective field. © 1986.