Optical studies of chemisorption on metals

Optical techniques, particularly ellipsometry, have been highly successful in the study of chemisorption on semiconductors. Such studies on metal surfaces are more difficult because the short screening lengths limit the chemisorption-induced perturbation to a surface-layer only a few ångströms thick. Some success has been achieved using differential reflectance spectroscopy. However this necessitates (1) two independent reflectance measurements or the use of KK analysis to obtain complex optical constants, and (2) assumed or independently-measured values for the thickness of the surface layer in order to completely specify the adsorption-induced changes. The advent of extremely precise in situ modulated ellipsometers has made the optical study of gas-metal interactions less difficult, in particular eliminating the necessity of KK analysis. We briefly describe such a system here, the automated polarization-modulated ellipsometer. When this instrument is coupled with a recently developed approximation technique for obtaining surface-layer thickness and optical constants from only two ellipsometric measurements, the complete optical characterization of many metal-absorbate systems is straightforward. Recent experimental results indicating a chemisorption-induced surface state within 2.4 eV of the Fermi level for oxygen on silver are discussed.