THE ADSORPTION OF CO ON IR(111) INVESTIGATED WITH FT-IRAS

The adsorption of CO on Ir(111) has been investigated with Fourier tra nsform infrared reflection-absorption spectroscopy, temperature progra mmed desorption, and low-energy electron diffraction. At sample temper atures between 90 and 350 K, only a single absorption band, above 2000 cm(-1), has been observed at ail CO coverages. For fractional coverag es above approximately 0.2, the bandwidth becomes as narrow as 5.5 cm( -1). The linewidth is attributed mainly to inhomogeneous broadening at low CO coverages and to the creation of electron-hole pairs at higher CO coverages, The coverage-dependent frequency shift of the IR band c an be described quantitatively using an improved dipolar coupling mode l. The contribution of the dipole shift and the chemical shift to the total frequency shift were separated using isotopic mixtures of CO. Th e chemical shift is positive with a constant value of approximately 12 cm(-1) for all coverages, whereas the dipole shift increases with cov erage up to a value of 36 cm(-1) at a coverage of 0.5 ML.