Effect of surface contamination on the heat of adsorption of gases on metals

In the infrared surface electromagnetic waves (SEW) can propagate several centimeters over a metal and some ferroelectrics. Therefore SEW can be used to integrate overlayer surface absorption over very much larger path lengths than in the usual transmission and reflection surface studies. Using a continuum model, we have calculated the surface electromagnetic wave modes for a dielectric slab on a metal against a third (weakly absorbing) medium. Unlike previous treatments, absorption has been included and the absorption expected using the two prism SEW technique has been calculated. For a thick slab (1 mm or more), we show that the absorption coefficient of the slab material can be obtained very simply. For very thin slabs both surface-plasmon-like modes and surface-phonon-like modes (including Fuchs-Kliewer modes) are found. A new parameter Δ₁ + iΔ₂ is introduced to describe the thin film absorption. It has the physical significance of an effective susceptibility for the metal-coating-overlayer system. Further, its experimental significance is that for the normalized transmission of SEW between two prisms, Δ₂ is a dominating parameter which can be measured. For the two prism experiments, numerical examples show 50 percent absorption at peak for a monolayer of CO on Pt and a 4 percent absorption for CuO on copper. Applications to the study of thin films, catalysis, corrosion and other surface problems are anticipated.